DOT/FAA/AR-05/17 

Office of Aviation Research 

Washington, D.C. 20591

Fundamental Studies of Nickel 

Billet Materials—Engine Titanium 

Consortium Phase II 

June 2005 

Final Report 

This document is available to the U.S. public 

through the National Technical Information Service (NTIS), 

Springfield, VA 22161. 

U.S. Department of Transportation 

Federal Aviation Administration
NOTICE 

This document is disseminated under the sponsorship of the U.S. 

Department of Transportation in the interest of information exchange. The 

United States Government assumes no liability for the contents or use 

thereof. The United States Government does not endorse products or 

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because they are considered essential to the objective of this report. This 

document does not constitute FAA certification policy. Consult your local 

FAA aircraft certification office as to its use. 

This report is available at the Federal Aviation Administration William J. 

Hughes Technical Center’s Full-Text Technical Reports page: 

actlibrary.tc.faa.gov in Adobe Acrobat portable document format (PDF). 
 Technical Report Documentation Page

1. Report No.

DOT/FAA/AR-05/17

2. Government Accession No. 3. Recipient's Catalog No.

 4. Title and Subtitle 

FUNDAMENTAL STUDIES OF NICKEL BILLET MATERIALS—ENGINE 

5. Report Date 

June 2005 

TITANIUM CONSORTIUM PHASE II 6. Performing Organization Code

7. Author(s)

Frank Margetan1, Edward Nieters2, Pranaam Haldipur1, Lisa Brasche1, 

Thomas Chiou1

, Mike Keller2, Andrei Degtyar3, Jeff Umbach3, Waled Hassan4, 

Thadd Patton2

, Kevin Smith3

8. Performing Organization Report No. 

9. Performing Organization Name and Address

1

Iowa State University 2General Electric 

 Center for Nondestructive Evaluation Cincinnati, Ohio 

 Ames, IA 50011 

10. Work Unit No. (TRAIS) 

3

Pratt & Whitney 4Honeywell Engines 

 East Hartford, Connecticut Phoenix, Arizona

11. Contract or Grant No.

DTFA0398FIA029

12. Sponsoring Agency Name and Address

U.S. Department of Transportation 

Federal Aviation Administration

13. Type of Report and Period Covered

 Final Report

Office of Aviation Research 

Washington, DC 20591 

14. Sponsoring Agency Code

ANE-110

15. Supplementary Notes

The FAA William J. Hughes Technical Monitor was Cu Nguyen. 

16. Abstract

Nickel (Ni) alloys are used for their high-temperature strength properties in jet engine disks and other propulsion components. To 

ensure safe operation, typical Ni components are ultrasonically inspected to detect defects such as white spots, dirty white spots, 

and freckles. A fundamental understanding of the ultrasonic properties and their relationship to microstructural conditions is

useful in design of optimal inspection parameters. The report provides details of such studies for IN718 and Waspaloy, the two

most prevalent Ni alloys. 

17. Key Words 

Nickel billet, Ultrasonic inspection, Probability of detection

18. Distribution Statement 

This document is available to the public through the National 

Technical Information Service (NTIS), Springfield, Virginia 

22161.

19. Security Classif. (of this report)

 Unclassified 

20. Security Classif. (of this page)

 Unclassified

21. No. of Pages

 230

22. Price

Form DOT F1700.7 (8-72) Reproduction of completed page authorized
TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY xv 

1. INTRODUCTION 1-1 

1.1 Purpose 1-1 

1.2 Background 1-1 

1.3 Program Objectives 1-2 

1.4 Related Activities and Documents 1-2 

2. APPROACH 2-1 

2.1 Alloy Selection and Sample Fabrication 2-1 

2.2 Ultrasonic Property Measurements 2-1 

2.3 Synthetic Inclusion Samples 2-2 

2.4 Defect Characterization 2-2 

3. DISCUSSION 3-1 

3.1 Ultrasonic Property Measurements of Billets and Correlation 

With Microstructure 3-1 

3.1.1 Background and Specimen Selection 3-1 

3.1.2 Preliminary UT Examinations of the Billet Strip Coupons 3-11 

3.1.3 Methods Used for Detailed UT Property Measurements 3-25 

3.1.4 Results of UT Property Measurements 3-48 

3.1.5 Metallography Characterization and the Relationship Between UT 

Properties and Grain Size 3-64 

3.2 Summary 3-89 

4. SYNTHETIC INCLUSION SAMPLES 4-1 

4.1 Introduction 4-1 

4.2 Synthetic Defects 4-1 

4.3 Flat-Bottom Hole Block 4-6 

4.4 Trial Seed Block Design 4-8 

4.4.1 IN718-IN718 Interface 4-11 

4.4.2 Freckle-IN718 Interface Defects 4-11 

4.4.3 White Spot-IN718 Interface Defects 4-12 

iii
4.4.4 Dirty White Spot-IN718 Interface Defects 4-13 

4.5 Final Block Design 4-14 

4.6 Summary 4-20 

5. CONCLUSIONS 5-1 

6. REFERENCES 6-1 

APPENDICES 

A—Defect Characterization of Naturally Occurring Anomalies 

B—Ultrasonic Data Acquisition Procedure for ETC Real Defects

iv
LIST OF FIGURES 

Figure Page 

3-1 Typical Production of Ni Alloy Billets Used for Fabricating Rotating Jet Engine 

Components 3-2 

3-2 Noise Banding and UT Property Variations in a 6′′ Diameter Ti-6-4 Billet 3-3 

3-3 Coupon Layout for 10′′ Diameter Ni Alloy Billets 3-4 

3-4 C-Scan Image of Backscattered Noise for Zone 2 of the IN718 V-Die Billet 3-6 

3-5 Average Noise Levels in the Multizone Inspections of Four Ni Alloy Billets 3-7 

3-6 Degree of Noise Banding in the Multizone Inspections of Four Ni Alloy Billets 3-8 

3-7 C-Scan Image of Backscattered Noise for Zone 3 of the 12′′ Long IN718 GFM 

Billet 3-9 

3-8 C-Scan Image of Backscattered Noise for Zone 5 of the 40′′ Long Waspaloy Billet 3-9 

3-9 Stamped Markings Used to Label the Surfaces of the Ni Billet Strip Coupons 3-10 

3-10 Volume Determination for Ni Billet Strip Coupons 3-11 

3-11 Experimental Setups Used to Measure (a) Back-Wall Amplitude and TOF and 

(b) Backscattered Grain Noise Amplitude 3-12 

3-12 Back-Wall TOF C-Scan Images for the V-Die-B Strip Coupon 3-13 

3-13 Back-Wall Amplitude C-Scan Images for the Waspaloy Strip Coupon 3-14 

3-14 Back-Wall Amplitude C-Scan Images for the GFM-A Strip Coupon 3-14 

3-15 Back-Wall Amplitude C-Scan Images for the V-Die-A Strip Coupon 3-15 

3-16 Back-Wall Amplitude C-Scan Images for the V-Die-B Strip Coupon 3-15 

3-17 Rescaled C-Scan Images of Back-Wall Echo Amplitude for Axial (Side 2) and 

Hoop (Side 3) Inspections of the Four Ni Billet Strip Coupons 3-17 

3-18 Dependence of Average Back-Wall Amplitude on Radial Position in the Four Ni 

Billet Strip Coupons 3-17 

3-19 Backscattered Noise C-Scan Images for the Waspaloy Strip Coupon 3-19 

3-20 Backscattered Noise C-Scan Images for the GFM-A Strip Coupon 3-19 

v
3-21 Backscattered Noise C-Scan Images for the V-Die-A Strip Coupon 3-20 

3-22 Backscattered Noise C-Scan Images for the V-Die-B Strip Coupon 3-20 

3-23 Construction of Radial Profiles of Backscattered Grain Noise 3-21 

3-24 Dependence of Backscattered Noise on Radial Position for Each of the Four 

Strip Coupons, Relative to a #1 FBH at the Same Depth (0.5′′) in the IN100 

Reference Block 3-22 

3-25 Comparison of Backscattered Noise C-Scans and Back-Wall Amplitude C-Scans 

for the GFM-A Strip Coupon 3-23 

3-26 The Five Sites on a Given Strip Coupon Where Detailed UT Property 

Measurements Were Made 3-24 

3-27 Cutting and Labeling Protocols for the Ni Alloy Strip Coupons, Illustrated for the 

GFM-A Billet Strip 3-24 

3-28 Setup for UT Velocity Measurements Using the Successive Back-Surface 

Reverberations Observed at a Fixed Measurement Site 3-26 

3-29 (a)-(b) A-Scans Showing the Front-Wall (Truncated) and First Three Back-Wall 

Echoes for the Slice Coupons From the Centers of the Waspaloy and GFM-A 

Billets and (c)-(d) Overlap of the First Two Back-Wall Echoes After the Second 

Has Been Increased in Amplitude and Time Shifted 3-26 

3-30 Results of Sound Velocity Measurements Performed on the Waspaloy Strip 

Coupon Before Slicing 3-27 

3-31 The Two Methods Used for L-Wave Attenuation Versus Frequency 

Measurements 3-28 

3-32 Computed Radiation Patterns for the Sonic Beam From a 0.25′′ Diameter 

Transducer at Four Frequencies of Interest 3-30 

3-33 Attenuation Measurement for Axial Sound Propagation at the Center of the IN718 

GFM-A Billet Strip Coupon 3-31 

3-34 Attenuation Measurement for Axial Sound Propagation at the Center of the 

Waspaloy Billet Strip Coupon 3-32 

3-35 Measured Attenuation Values in the 5-15 MHz Range at the Five Sites in the 

GFM-A Billet Strip 3-34 

3-36 Deduced Attenuation Values at Various Sites for Axial and Hoop Propagation in 

the (a) Waspaloy and (b) IN718 GFM-A Strip Coupons 3-35 

vi
3-37 Power-Law Fits to Measured Attenuation Values at Two Locations in the IN718 

GFM-A Billet Specimen 3-36 

3-38 Power-Law Fits to Measured Attenuation Values at Two Locations in the 

Waspaloy Billet Specimen 3-36 

3-39 Immersion Setup for Grain Noise FOM Measurement 3-37 

3-40 (a) The Independent Scatterer Noise Model Neglects Grain Noise Contributions 

From Multiple-Scattering Events and (b) Relationship Between the Average 

Measured Noise Spectrum and Various Microstructure and Measurement System 

Factors in the Noise Model 3-39 

3-41 (a) Root Mean Square Noise Levels Measured at Five Sites in the GFM-A IN718 

Billet (Axial Propagation), and Time Gates Used for FOM Determination and 

(b) Deduced FOM Values for Selected GFM-A Cases Using the Principal Time 

Gate (G7) 3-40 

3-42 (a) Root Mean Square Noise Levels Measured at Five Sites in the Waspaloy Billet 

(Axial Propagation) and Time Gates Used for FOM Determination and 

(b) Deduced FOM Values for Selected Waspaloy Cases Using the Principal Time 

Gate (G7) 3-42 

3-43 Grain Noise FOM Values for Axial Propagation at the Center of the GFM-A 

IN718 Billet Deduced Using the Seven Time Gates Indicated in Figure 40(a) 3-43 

3-44 Measured and Predicted rms Grain Noise Levels for Axial Beam Propagation at 

the Center of the IN718 GFM-A Billet 3-44 

3-45 Additional FOM Measurements and Analyses at the Center of the IN718 GFM-A 

Billet Specimen 3-45 

3-46 Examples of Local Linear Fits Used to Determine the Best Estimate of the Grain 

Noise FOM at 7.5 MHz 3-46 

3-47 Comparison of Measured FOM and Attenuation Values at 7.5 MHz for the GFM 

IN718 Billet 3-47 

3-48 Comparison of Measured FOM and Attenuation Values at 7.5 MHz for the 

Waspaloy Billet 3-48 

3-49 Measured Longitudinal-Wave Sound Velocities as Functions of Radial Depth in 

the Billet 3-51 

3-50 Measured Attenuation Values at 7.5 MHz for the Ni Alloy Billet Specimens 3-57 

3-51 Measured Grain Noise FOM Values at 7.5 MHz for the Ni Alloy Billet Specimens 3-58 

vii
3-52 Correlation Between Backscattered Noise FOM and Attenuation 3-59 

3-53 Predicted Attenuation-Versus-Frequency Curves and Associated Power-Law Fits 

for Pure Ni Microstructures Having Average Grain Diameters of 10, 20, and 30 

Microns 3-61 

3-54 Predicted Relationship Between the Average Grain Diameter, the Attenuation at 

7.5 MHz, and the Exponent in the Power-Law Fit to the Attenuation-VersusFrequency Curve for Pure Ni Microstructures 3-61 

3-55 Predicted FOM-Versus-Frequency Curves for Pure Ni Microstructures Having 

Average Grain Diameters Ranging From 10 to 100 Microns 3-62 

3-56 Relationship Between FOM and Attenuation at 7.5 MHz 3-62 

3-57 Predicted Behavior of the Absolute Grain Noise Level Seen at Three Depths in 

Pure Ni as a Function of Mean Grain Diameter 3-63 

3-58 Metallography Coupons 3-65 

3-59 Axial View of Microstructure at the Center (0′′) of the Waspaloy Billet 3-65 

3-60 Axial, Radial, and Hoop Views of the Waspaloy Microstructure at Four 

Metallography Sites 3-66 

3-61 Radial Views of the IN718 GFM-A Microstructure at Two Sites 3-66 

3-62 Example of Processing a Grain Structure 3-67 

3-63 (a) Micrograph Subsection With Several Presumed Twin Boundaries Identified 

and (b) Boundaries Ignored in the Final Digitized Image 3-67 

3-64 Mean Chord Length Method for Determining Average Grain Diameter 3-69 

3-65 P(L) Method for Determining Average Grain Diameter 3-69 

3-66 Examples of Exponential Fits to Measured P(L) Curves for a Typical Case 3-70 

3-67 Effect of Twin Grain Treatment on Deduced Mean Diameter 3-77 

3-68 Comparisons of Average Grain Diameters in the X and Y Directions of Ni Alloy 

Micrographs Using the MCL Method 3-77 

3-69 Comparisons of Average Grain Diameters in the 12 V-Die-B Micrographs as 

Estimated Using the MCL and P(L) Methods 3-78 

3-70 Comparisons of Average Grain Diameters as Estimated by the MCL and P(L) 

Methods for Each of the 4 x 12 Micrographs Analyzed 3-79 

viii
3-71 (a) Locations and Designations of the 12 Original Metallography Coupons From 

Each Rough-Cut Specimen and (b) Locations and Shapes of the four New 

Metallography Coupons for V-Die-A 3-80 

3-72 Local Microstructures of the Four New Metallography Coupons From the 

V-Die-A Rough-Cut Slab 3-80 

3-73 Backscattered Grain Noise C-Scans of the GFM-A Strip Coupon and 

Metallography Slab 3-82 

3-74 Backscattered Grain Noise C-Scans of the V-Die-A Strip Coupon and 

Metallography Slab 3-83 

3-75 Backscattered Grain Noise C-Scans of the V-Die-B Strip Coupon and 

Metallography Slab 3-83 

3-76 Best Estimates of Grain Diameters Versus Radial Position 3-84 

3-77 Relationship Between Average Grain Diameter and Measured Attenuation 

(at 7.5 MHz) for Each of the Four Strip Coupons 3-85 

3-78 Relationship Between Average Grain Diameter and Measured Noise FOM 

(at 7.5 MHz) for Each of the Four Strip Coupons 3-86 

3-79 Example of Determining IN718 Single-Crystal Elastic Constants by Fitting to 

Measured Velocity and Attenuation Data 3-88 

4-1 Microstructure of White Spot Casting 4-3 

4-2 Microstructure of White Spot Forging 4-3 

4-3 Microstructure of Freckle Ingot 4-4 

4-4 Microstructure of Freckle Forging, Exhibiting Refinement Due to Forging 4-4 

4-5 Microstructure of Dirty White Spot Button 4-5 

4-6 Microstructure of Dirty White Spot Forging 4-5 

4-7 #1 FBH IN718 Block Design 4-7 

4-8 Ultrasonic C-Scan of #1 FBH IN718 Block 4-8 

4-9 Trial Block Design 4-9 

4-10 Ultrasonic C-Scans of Trial Blocks 4-10 

4-11 IN718 to IN718 Bond Line in the Dirty White Spot Test Block 4-11 

ix
4-12 Bond Between Freckle Seed 1 and IN718 4-12 

4-13 Detail of Bond Line Between Freckle Seed 2 and IN718 Showing Small 

Discontinuities 4-12 

4-14 Large Grain Structure of White Spot Seed 1 4-13 

4-15 IN718 and White Spot Seed 2 Interface 4-13 

4-16 IN718 Dirty White Spot 6 Bond Line in Which Large Ceramic Particles Create a 

Nonbonded Region Greater Than 0.5 mm Long 4-14 

4-17 Dimensions and Seed Map for the Dirty White Spot Synthetic Seed Block 4-15 

4-18 Dimensions and Seed Map for the Freckle Synthetic Seed Block 4-16 

4-19 Dimensions and Seed Map for the White Spot Synthetic Seed Block 4-17 

4-20 A 10-MHz F/7 C-Scan of Blocks Containing Synthetic Defects 4-19 

4-21 A 10-MHz C-Scan of Blocks With Synthetic Defects After Second HIP 4-20 

x
LIST OF TABLES 

Table Page 

1-1 Typical Ni-Based Alloys Used in Rotating Components of Aircraft Engines 1-1 

3-1 Multizone Inspection Parameters for 10′′ Diameter Billet 3-5 

3-2 Dimension, Mass, and Density Values for Ni Billet Strip Coupons 3-11 

3-3 Approximate Upper Frequency Limit for Trustworthy Attenuation Measurements 

as a Function of the Material Attenuation Near 8 MHz 3-33 

3-4 Results of Longitudinal-Wave Sound Velocity Measurements 3-50 

3-5 Results of Shear-Wave Sound Velocity Measurements 3-52 

3-6 Summary of Measured Attenuation Values for the Ni Alloy Billet Specimens 3-53 

3-7 Summary of Measured Grain Noise FOM Values for the Ni Alloy Billet 

Specimens 3-55 

3-8 Ranges of Attenuation and FOM Values Measured Within Each of the Ni Alloy 

Billet Strip Coupons 3-58 

3-9 Summary of Grain-Sizing Analyses for One Micrograph of Metallography 

Coupon C of the GFM-A Specimen 3-71 

3-10 Average Grain Diameters Deduced From Waspaloy Metallography 3-72 

3-11 Average Grain Diameters Deduced From GFM-A Metallography 3-73 

3-12 Average Grain Diameters Deduced From V-Die-A Metallography 3-74 

3-13 Average Grain Diameters Deduced From V-Die-B Metallography 3-75 

3-14 Best Estimates of Average Grain Diameters at Radial Sites in the Four Ni Alloy 

Metallography Slabs 3-79 

3-15 Best Estimates of Average Grain Diameters at Radial Sites in the V-Die-A 

Metallography Slab 3-81 

3-16 Preliminary Values of Deduced Single-Crystal Elastic Constants for the V-Die-B, 

GFM-A, and Waspaloy Billet Specimens 3-89 

4-1 Compositions, in Weight Percent, of Synthetic Defects 4-2 

4-2 Results of UT Velocity Measurements on Rectangular Coupons of Synthetic Ni 

Billet Defect Material 4-6 

xi
4-3 Anomalous Indications From IN718 Trial Blocks 4-10 

4-4 Seed Types and Dimensions for Synthetic Blocks 4-18 

4-5 Synthetic Seed Dimensions and Sound Velocities 4-19 

xii
LIST OF ACRONYMS 

2D Two-dimensional 

EB Electric beam 

EDM Electron discharge machining 

ESR Electroslag remelting 

ETC Engine Titanium Consortium 

FAA Federal Aviation Administration 

FBH Flat-bottom hole 

FOM Figure of merit 

FQ Fused quart 

FSH Full-screen height 

GE General Electric 

GFM Gesellschaft fur Maschinenbau-und-Fertigungstechnik 

IFPP Ideal focus piston probe 

ISU Iowa State University 

MCL Mean chord length 

N/cm Nepers per centimeter 

Ni Nickel 

OD Outer diameter 

P&W Pratt & Whitney 

PoD Probability of Detection 

RF Radio frequency 

rms Root mean square 

SHA Synthetic hard alpha 

SMPC Specialty Metals Processing Consortium 

SNR Signal-to-noise ratio 

Ti Titanium 

TOF Time-of-flight 

UT Ultrasonic 

VAR Vacuum arc melting 

xiii/xiv
EXECUTIVE SUMMARY

Selection of engine materials is a balance between the ability to operate at temperature, adequate 

creep, fatigue, and strength properties, and acceptable weight. The primary materials considered 

for modern-day rotating components are titanium (Ti) alloys, valued for their lower weight but 

limited to intermediate temperature ranges, and nickel (Ni) alloys, higher-temperature materials 

with better damage tolerance properties but also heavier. Phase I of the Engine Titanium 

Consortium (ETC) research focused on Ti, particularly Ti-6Al-4V, and the detection of hard 

alpha defects. In Phase II, ETC extended this focus to include Ni-based alloys, which serve as 

the primary alloy in turbine and compressor disks. Extensive materials property data exists for 

Ni alloys, as is also the case for Ti materials. However, fundamental data relating 

microstructural features to inspectability are not available. Given the importance of 

understanding the relationship between material properties and inspectability, this program set 

about generating data for typical alloys used in commercial jet engines. 

The steps to inspection implementation include gathering base material property data, defect 

characterization, and application of this knowledge to inspection development. Several meltprocessing approaches and billet conversion practices are used by the industry, and samples were 

selected that are representative of these variations. Ni alloys such as IN718 (cast/wrought), 

IN901 (cast/wrought), Waspaloy (cast/wrought), IN100 (powder metal), and R95 (powder metal) 

are used extensively in high-temperature jet engine applications. Multiple defect types can exist 

in forged alloys, including dirty white spots, clean white spots, segregation, and slag. Defect 

types found in powder alloys include ceramic particles, porosity, and powder lot anomalies. This 

program generated inspection property data for the two most common alloys, IN718 and 

Waspaloy. 

In cooperation with the Specialty Metals Processing Consortium, several naturally occurring 

defects were characterized using ultrasonics and metallography. These results assist the 

inspection community in understanding flaw morphology and its relationship to inspectability. 

They also provide valuable information to the lifing community for consideration in design and 

life management. While information about natural defects is valuable, benefit also comes from 

the ability to measure signal response from control samples. Processes to develop synthetic 

defects in Ni alloys were also completed and representative samples were fabricated. The key 

accomplishments from the program are listed below. 

• Ultrasonic velocity was found to be relatively uniform for a given alloy. Since sonic 

velocity influences the ultrasonic beam radiation pattern, sizeable velocity variations can 

shift the focal depth and impact detectability. Given the uniformity of velocity 

measurements for IN718 and Waspaloy, it is unlikely that velocity variations will cause 

noticeable affects on detectability for these alloys. 

• In contrast to velocity, attenuation and backscattered grain noise capacity were found to 

vary significantly with frequency and position. Attenuation and grain noises were highly 

correlated, i.e., billet regions of high attenuation also were found to have high grain noise 

capacity. Attenuation and noise capacity values were generally found to be larger in 

Waspaloy than in IN718. 

xv
• Grain size is the predominant factor controlling noise and attenuation in the Ni alloys 

studied in this program. In Ni processing, there is a significant amount of 

recrystallization that occurs due to the mechanical working of the billet during the 

conversion process. This leads to approximately equiaxed grains as determined by 

metallographic characterization performed during the program. As the mean grain size 

increased, the attenuation and grain noise capacity were also found to increase. 

• Billet conversion practice was also found to affect the attenuation and grain noise 

characteristics of the alloys studied. Values of both properties were found to be lower for 

V-Die processing than for GFM (Gesellschaft fur Maschinenbau-und-Fertigungstechnik) 

processing. 

• At a given location in a billet, attenuation and grain noise were found to be approximately 

isotropic with little difference between the axial, radial, and hoop directions. 

• Methods to produce synthetic defects with similar chemistries to naturally occurring 

defects were developed, and several specimens were fabricated that contain so-called 

white spot, dirty white spot, and freckle defects. 

xvi
1. INTRODUCTION. 

1.1 PURPOSE. 

Nickel (Ni) alloys such as IN718 (cast/wrought), IN901 (cast/wrought), Waspaloy 

(cast/wrought), IN100 (powder metal), and R95 (powder metal) are used extensively in hightemperature jet engine applications. Multiple defect types exist in forged alloys, including dirty 

white spots, clean white spots, segregation, and slag. In powder alloys, defect types include 

ceramic particles, porosity, and powder lot anomalies [1]. Typically, dirty white spots contain a 

microstructure of Nb-lean alloy with fine oxides and nitrides. Typical freckle segregation 

consists of Nb-rich regions in billets that produce undesirable, brittle phases. Slag defects are 

Ca-rich ceramic phases originating from the electroslag remelting (ESR) melting step and result 

from poor ingot surface preparation. An analysis of the acoustic properties of these materials 

and defect types will facilitate improvements to current Ni billet inspections as well as provide 

data for analysis of the probability of detection (PoD) of the inspections for various defect types. 

1.2 BACKGROUND. 

Selection of engine materials is a balance between the ability to operate at temperature, adequate 

creep, fatigue, and strength properties, and acceptable weight. For the fabrication of modern-day 

rotating components, two classes of materials are principally used: (1) titanium (Ti) alloys, 

which are valued for their lower weight but have limited intermediate temperature ranges and (2) 

Ni alloys, which are heavier but can operate at higher temperatures, and have operated at these 

higher temperatures, with better damage tolerance properties. Phase I of the Engine Titanium 

Consortium (ETC) focused on Ti, particularly Ti-6Al-4V, and the detection of hard alpha 

defects. In Phase II, ETC extended this focus to include Ni-based alloys. Typical Ni-based 

alloys are listed in table 1-1. 

TABLE 1-1. TYPICAL Ni-BASED ALLOYS USED IN ROTATING COMPONENTS OF 

AIRCRAFT ENGINES 

Alloy Composition Typical Usage 

IN718 Ni-19Fe-19Cr-5Nb-3Mo-0.5Al-1Ti-0.1C-0.

006B 

Turbine disks, final stage 

compressor disks 

IN901 43Ni-35Fe-13Cr-6Mo High-pressure compressor 

Waspaloy 56Ni-19Cr-14Co-4Mo High-pressure compressor 

IN100 60Ni-15Co-10Cr-5Al-5Ti Turbine disks 

Rene 95 Ni-13Cr-8 Co-3.5Mo-2.5Ti-3.5 Al-3.5 

W-3.5 Nb-.06 C-.05 Zr-.01 B 

Turbine disks 

Extensive materials property data exists for these Ni alloys as it does for Ti materials. However, 

fundamental data relating microstructural features to inspectability are not available. The steps 

to inspection implementation include gathering base material property data, defect 

characterization, and application of this knowledge to inspection development. Several meltprocessing approaches are used by the industry and consideration was given to choosing samples 

that address the broad classes of material processing in the industry. 

1-1
1.3 PROGRAM OBJECTIVES. 

• To establish the basic ultrasonic (UT) properties of Ni-based alloy billet materials and 

relevant inclusions as an appropriate foundation for selection of UT inspection 

approaches. 

• To manufacture and characterize flat-bottom hole (FBH), synthetic inclusion, and real 

defect standards to provide data for determining defect detectability and developing 

improved inspections. 

• To improve the understanding of the relationship of defect size, shape, and composition 

on defect detectability in Ni alloys. 

1.4 RELATED ACTIVITIES AND DOCUMENTS. 

The ETC was established in 1993 and includes Iowa State University (ISU), General Electric 

Aircraft Engines, Honeywell Engines Systems and Services, and Pratt & Whitney (P&W) in a 

partnership to perform research that contributes to improvements in flight safety. The Phase I 

program, which was completed in 1998, led to improvements in production inspection of Ti 

billet [2], improved physics models for UTs [3 and 4], and a feasibility study for phased array for 

UT inspection of billets [5]. In-service inspection efforts led to a commercially available 

portable scanner [6] and eddy-current probes [7] as well as improved probe designs [8] and 

eddy-current probe design tools [9]. Considerable progress was also made in developing a new 

approach [10] to quantifying inspection performance as reported in the Federal Aviation 

Administration (FAA) report “A Methodology for the Assessment of the Capability of Inspection 

Systems for Detection of Subsurface Flaws in Engine Components” [11]. 

The technical approach of the ETC Phase I program included the characterization of material 

properties, which impact inspectability and the subsequent use of that information in inspection 

design. Ti alloys typically suffer from UT noise, which implies a lower signal-to-noise ratio 

(SNR) and can lead to false indications or missed flaws. Full details of this study are provided in 

the final report for the program. Key accomplishments of the ETC Phase I program were as 

follows: 

• Synthetic hard alpha (SHA) material, essentially Ti-N-O alloys, can be manufactured by 

either powder metallurgical or arc-melting techniques. Batches of such SHA material 

with nitrogen contents ranging from 1.5 to 16.2 wt.% were made for basic property 

measurements and for insertion as defects into HIPped test specimens. 

• The UT reflectance of a hard alpha inclusion in a Ti alloy host depends on the difference 

in impedance (product of sound velocity and density) between the two materials. For 

each 1 wt.% increase in nitrogen content, the longitudinal sound speed of SHA increases 

by about 3% and the density increases by 1%. The reflectance of hard alpha in a Ti alloy 

host rises approximately linearly from zero at 0 wt.% N to 0.14 at 10 wt.% N. 

1-2
• Uncracked, cylindrically shaped SHA inclusions in Ti alloy host material could be 

imaged and detected with compositions as low as 1.5 wt.% N. The SNR for such 

inclusions rose linearly as the nitrogen content was increased. 

• Ti alloys are ultrasonically anisotropic within a single metal grain or within a colony of 

aligned grains. A plate of Ti-6-4 alloy with very large alpha-grain colonies was produced 

to study this anisotropy. The minimum longitudinal sound velocity (0.6063 cm/μs) was 

measured in a colony whose hexagonal symmetry axis was perpendicular to the direction 

of sound propagation. The maximum sound velocity (0.6712 cm/μs) was measured for 

propagation parallel to the symmetry axis. The anisotropy in longitudinal sound velocity 

in Ti-6-4 was roughly twice that reported for pure Ti. 

• The above study of velocity anisotropy in the enlarged grain block suggests that 

uncracked hard alpha inclusions with compositions less than 3 wt.% N might not be 

detectable if present in a region of large alpha colonies. Based on the measured 

velocities, the UT reflectance at the boundary between two alpha-Ti colonies can be as 

high 0.051. This value exceeds the typical reflectance of uncracked hard alpha inclusions 

with less than 3 wt.% N (see bullet 2 above). Thus, signals from such inclusions could be 

masked by UT grain-boundary echoes. 

• UT property measurements were carried out on rectangular coupons cut from 

representative Ti-17 and Ti-6-4 billets and forgings. Longitudinal wave velocity, 

attenuation, and backscattered noise figure of merit (FOM) were systematically measured 

for the radial, axial, and hoop propagation directions in 14 coupons labeled A-N. 

Additional measurements using these and a dozen other specimens were then made to 

further investigate the effects of Ti microstructure on UT inspections. 

• For a given coupon, all measured properties showed a dependence on propagation 

direction. This dependence was generally greatest for backscattered noise, less (but still 

large) for attenuation, and smallest for velocity. There were also large noise and 

attenuation differences between coupons from different sources. 

• Attenuation and backscattered noise depended strongly on frequency (f), while velocity 

did not. If the dependence on frequency is approximated as a power function, cfp

, then 

for attenuation, p typically ranged from 1 to 2, and for noise FOM, p typically ranged 

from 0.5 to 1.5. 

• Attenuation and backscattered noise exhibited opposite dependencies on propagation 

direction within a given coupon. Attenuation was highest and noise FOM was lowest 

when sound propagated parallel to macrograin elongation (billets) or flow lines 

(forgings). Conversely, attenuation was lowest and noise highest for propagation 

perpendicular to macrograins or flow lines. 

• Beam-mapping measurements revealed that the Ti microstructure could cause significant 

distortion of the sonic amplitude and phase profiles. These distortions were worse for 

propagation along macrograins and worse at higher frequencies. They affected all types 

of UT measurements. 

1-3
• When beam distortions were large, there were significant fluctuations in the UT signal 

amplitudes of nominally identical reflectors. In addition, different methods for 

measuring attenuation yielded different results. For example, the average attenuation of 

back-surface echoes was larger than that expected due to energy loss alone and different 

from the attenuation deduced from FBH echoes. 

• In current billet inspection procedures, 5-MHz transducers are used and the sonic beam 

travels radially inward, perpendicular to the columnar macrograins. Thus, the effects of 

beam distortion on the inspection are generally modest. However, beam distortion effects 

may significantly degrade inspections at higher frequencies, particularly Ti forging 

inspections at locations where the sound beam is not perpendicular to the flow lines. 

• Some models for predicting backscattered noise attributes assume Gaussian noise 

statistics at each fixed observation time. The noise measurements indicate that these 

models should be applicable to 5-MHz Ti billet inspections. One such model was in fact 

used for detection reliability studies by the ETC PoD subtask. 

Alloy selection for jet engine applications is dictated by operational parameters such as 

temperature, stress, and environment. Operational requirements for jet engine alloys are some of 

the most stringent, placing critical demands on the integrity and purity of both Ni and Ti alloys. 

The Specialty Metals Processing Consortium (SMPC) was established to improve melting 

practices and other manufacturing processes associated with the production of jet engine 

materials. Details of their work are available from Sandia National Laboratories [12]. 

1-4
2. APPROACH. 

The approach for this research effort included the following: 

• Alloy selection and sample fabrication 

• UT property measurements 

• Synthetic inclusion samples 

• Defect characterization 

2.1 ALLOY SELECTION AND SAMPLE FABRICATION. 

Two alloys were selected for fundamental property measurements. The most widely used alloy, 

IN718, was selected to receive the primary focus, and it was decided that a more limited sample 

set would be generated for Waspaloy. Waspaloy was selected because it tends to exhibit higher 

levels of UT backscattered noise and, therefore, would offer an opportunity to study a material 

that is traditionally more difficult to inspect. The sequence of manufacturing the properties 

specimens and the specimen configuration was planned to yield as much data as possible on 

properties as a function of depth and orientation. 

2.2 ULTRASONIC PROPERTY MEASUREMENTS. 

Using rectangular coupons cut from Ni alloy billets, baseline UT properties (velocity, 

attenuation, and backscattered noise) were measured to determine the impact of properties on 

inspectability. In Phase I, the UT properties of Ti billets were found to vary with position, and 

similar variations were expected in Ni alloy billets and forgings. The selection of representative 

Ni billets to serve as sources for property measurement specimens was based on an initial 

screening inspection. The grain noise-banding patterns observed in the screening inspections 

were then used to select the locations where rectangular property measurement coupons were 

cut. This guaranteed that billet coupons with a range of representative microstructures would be 

studied. 

The sequence for manufacturing and using the property measurement coupons was planned to 

yield as much data as possible on the variation of UT properties with radial depth and 

orientation. Strip-like coupons were initially cut along billet diameters and used to survey UT 

properties for beam propagation in the axial and hoop directions. The strip-like coupons were 

later cut into smaller slices to facilitate measurements in the radial direction. UT velocity, 

attenuation, and backscattered noise capacity were measured using the same methods developed 

in Phase I for Ti specimens. These measurements on billet coupons allowed the team to 

assemble a more comprehensive picture of UT property variations within representative billets 

and to better understand the effect of these variations on inspectability. In addition to the UT 

measurements, metallographic studies were performed and average grain diameters were 

deduced from optical micrographs. The grain size data were then correlated with the measured 

UT properties. 

2-1
2.3 SYNTHETIC INCLUSION SAMPLES. 

Synthetic inclusions were embedded into Ni test standards for the purpose of evaluating the 

inspection sensitivity of UTs on melt-related defects. Public literature [1] of metallurgical 

analysis on the naturally occurring defects was used to identify the candidate inclusion types for 

sample manufacture. Construction methods were developed at General Electric (GE) Corporate 

Research and Development to chemically manufacture the synthetic inclusions and to embed the 

inclusions into Ni alloy test blocks. After the successful development of synthetic inclusion 

manufacturing methods, three blocks containing synthetic inclusions of different geometry and 

composition were manufactured. The types of inclusions were determined by the team based on 

the ability to manufacture the synthetic inclusions, the criticality of the defect to part life, and the 

sensitivity of the inspection to the composition and geometry of the defect type. The team 

ultrasonically evaluated the synthetic inclusion samples to determine the sensitivity of the 

inspections for detecting and characterizing melt-related defects. 

2.4 DEFECT CHARACTERIZATION. 

Samples were also acquired with real defects in cooperation with SMPC, reflecting both vacuum 

induction melting (VIM)/vacuum arc melting (VAR) and VIM/ESR/VAR material defects in 

IN718 and Waspaloy. The initial effort will focus on evaluation of natural defects to establish 

typical compositions and properties and their detectability. Five samples were evaluated using a 

limited UT characterization and a simplified metallographic process. UT measurements were 

performed at two stages: original samples prior to sectioning and defects machined to regular 

shapes. 

2-2
3. DISCUSSION. 

3.1 ULTRASONIC PROPERTY MEASUREMENTS OF BILLETS AND CORRELATION

WITH MICROSTRUCTURE. 

3.1.1 Background and Specimen Selection. 

3.1.1.1 Background. 

As part of the ongoing effort to quantify and improve inspection practices, the ETC investigated 

the UT properties of Ni alloy, cylindrical billets used in the fabrication of rotating jet engine 

components. A primary goal of the Fundamental Studies was to determine, for representative 

10′′ diameter billets, the manner in which the UT inspection properties depended on position, 

inspection direction, and frequency. The inspection properties of interest were the density and 

the sonic velocity, attenuation, and backscattered grain noise capacity. These properties 

influence UT inspections in a direct manner. For a given transducer and billet diameter, the 

density, sonic velocity, and attenuation determine the shape and strength of the incident pressure 

field within the billet and, thus, directly influence the amplitude of the echo from an internal 

defect. The backscattered noise capacity then determines the average strength of the grainboundary echoes that can mask or obscure the defect echo. 

This section summarizes the methods and results of the Ni billet survey study. The review of the 

specimen selection begins with the cutup procedures used to obtain rectangular coupons for 

property measurements. The various UT measurement techniques are then reviewed and 

representative results are shown. The UT measurements themselves fall into two classes. In 

preliminary work, C-scans of back-surface amplitude, back-surface time-of-flight (TOF), and 

backscattered grain noise amplitude were performed to determine the degree of property 

variability with position and inspection direction. Careful measurements of velocity, attenuation, 

and grain noise capacity were then made at selected sites, with site locations based on the earlier 

C-scans. After summarizing the results of these more-detailed UT property measurements, two 

complementary metallographic analyses used to estimate average grain diameters at the UT 

measurement sites are discussed. The correlations between mean grain diameter, UT 

attenuation, and UT backscattering are then discussed. Experimental results are also compared 

to model predictions for simple microstructures consisting of equiaxed, randomly oriented, pure 

Ni crystallites. An approach for using the combined UT and metallographic data to estimate the 

single-crystal elastic constants of IN718 and Waspaloy is discussed next. Finally, a summary of 

the findings and their implications for billet inspections is presented. 

Currently, most Ni alloy billet inspections are performed using 5-MHz transducers. In the 

future, if greater inspection sensitivity is desired, it may be advisable to increase the frequency to 

take advantage of the greater focusing ability of higher-frequency transducers. UT attenuation 

and backscattered noise capacity are strongly frequency-dependent, with both quantities tending 

to rise as the frequency is increased. In the Ni alloy survey study, it was endeavored to measure 

these quantities within the frequency range of greatest present and future interest, namely, from 

about 5 to 12.5 MHz. The manner in which attenuation and noise capacity depend on frequency 

will be shown explicitly for selected cases. To illustrate the manner in which attenuation and 

noise capacity vary with billet position and inspection direction, graphical results will be shown 

3-1
for a fixed inspection frequency, typically 7.5 MHz. Results for other frequencies (5, 10, and 

12.5 MHz) are presented in tabular form and have similar dependencies on billet position and 

inspection direction as the 7.5 MHz results (see section 3.1.4.2). 

3.1.1.2 Specimen Selection. 

Ni alloy material intended for the fabrication of rotating jet engine components generally begins 

as a cast ingot. As illustrated in figure 3-1, the initial ingot, which may have a rectangular or 

cylindrical cross section, was subjected to mechanical-working processes that result in a reduced 

diameter cylindrical billet. Although the final billet shape has cylindrical symmetry, the UT 

properties of the billet do not share this symmetry even if the cast ingot is cylindrical. This is 

because the billet is generally rotated in fixed increments, rather than continuously, when 

mechanical working is done. The UT properties are dependent, to some extent, on the 

mechanical-working history, and the lack of cylindrical symmetry during working leads to 

noncylindrically symmetric UT properties. 

Cast Ingot

Heat and 

Beat

GFM or V-die 

method

Billet

UT

Inspection

Cast Ingot

Heat and 

Beat

GFM or V-die 

method

Billet

UT

Inspection

FIGURE 3-1. TYPICAL PRODUCTION OF Ni ALLOY BILLETS USED 

FOR FABRICATING ROTATING JET ENGINE COMPONENTS 

UT property measurements are most easily carried out on rectangular specimens. The Ti billet 

research conducted during ETC Phase I provided guidance for choosing a scheme for cutting 

rectangular coupons from Ni alloy billets [13-19]. An example of UT property variations in 

cylindrical Ti billets is illustrated in figure 3-2, in this case, for a 6′′ diameter Ti-6-4 billet. Panel 

3-2(a) displays the backscattered grain noise pattern that was seen when an axial-cylindrical scan 

of the specimen was made using a 5-MHz, bicylindrically focused (multizone) transducer, and 

the resulting gated-peak noise amplitude was displayed as a function of scan position. Prominent 

bands of high and low backscattered noise amplitude are seen. As shown in panel 3-2(b), two 

sets of coupons were cut from the billet for property measurements. Each set of three coupons 

was cut along a radius of the billet, one set from a region having a low-noise amplitude near the 

outer diameter (OD) (at 1′′ depth) and the other set having a high-noise amplitude near the OD. 

For sonic beam propagation in the radial direction, figure 3-2(c) shows measured longitudinal 

wave speeds, attenuations, and FOM values for each coupon. The FOM is a measure of the 

noise generation capacity of the microstructure that (ideally) is a property of the microstructure 

alone and independent of the details of the measurement procedure [20-22]. One sees that the 

UT properties vary significantly throughout the billet, and that the differences are greatest near 

the OD. It is important to note that changes in velocity and attenuation values are well correlated 

3-2
with changes in backscattered noise capacity. This indicates that the backscattered noise patterns 

that are seen in routine inspections can be used to identify coupon sites with differing UT 

properties. 

Mask

Scan Region : 6” Diameter

11” Height

(Ti 6-4 billet) 

Probe : 5-MHz GE Multizone ;

1” diameter

“High Noise”

Region

“Low Noise”

Region

Gated Peak Noise in a 4-μs-wide gate centered 1” deep (a)

Mask

Scan Region : 6” Diameter

11” Height

(Ti 6-4 billet) 

Probe : 5-MHz GE Multizone ;

1” diameter

“High Noise”

Region

“Low Noise”

Region

Gated Peak Noise in a 4-μs-wide gate centered 1” deep (a)

(b)

883588 B3W3B

Billet

Axis

“Low

Noise”

“High

Noise”

1”

2”

Coupons

2.25” thick

along billet

axis.

LN3 LN2 LN1

Serial Nos. for low-noise coupons are LN1, LN2, LN3.

Serial Nos. for high noise coupons will have HN3, HN2, HN1.

(On the axial furthest from the billet serial number).

Three coupons from each region. Each about 1” x 2” x 2.25”

“CL” “Mid” “OD”

(b)

883588 B3W3B

Billet

Axis

“Low

Noise”

“High

Noise”

883588 B3W3B

Billet

Axis

“Low

Noise”

“High

Noise”

1”

2”

Coupons

2.25” thick

along billet

axis.

LN3 LN2 LN1

Serial Nos. for low-noise coupons are LN1, LN2, LN3.

Serial Nos. for high noise coupons will have HN3, HN2, HN1.

(On the axial furthest from the billet serial number).

Three coupons from each region. Each about 1” x 2” x 2.25”

“CL” “Mid” “OD”

1”

2”

Coupons

2.25” thick

along billet

axis.

LN3 LN2 LN1

Serial Nos. for low-noise coupons are LN1, LN2, LN3.

Serial Nos. for high noise coupons will have HN3, HN2, HN1.

(On the axial furthest from the billet serial number).

Three coupons from each region. Each about 1” x 2” x 2.25”

“CL” “Mid” “OD”

(c)

0.000

0.020

0.040

0.060

0.080

0.100

OD Mid CL

Coupon Location

 Noise FOM (cm**-0.5) 

At 15 MHz using

0.5" F=90mm probe

(1/2 of measured α)

"Low Noise"

Region

"High Noise"

Region

0.618

0.620

0.622

0.624

0.626

0.628

0.630

0.632

OD Mid CL

Coupon Location

L-wave speed (cm/μs)

"Low Noise"

Region

"High Noise"

Region

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

OD Mid CL

Coupon Location

Attenuation (N/cm)

Average α at 

15.2 MHz.

Planar probe.

"Low Noise"

Region

"High Noise"

Region

10

 9

 8

 7

 6

 5

 4

 3

 2

dB/in

L-wave Speed

Varied by ~ 2 %

Attenuation

Varied by factor of ~ 2.5

Noise FOM

Varied by factor of ~ 2

(c)

0.000

0.020

0.040

0.060

0.080

0.100

OD Mid CL

Coupon Location

 Noise FOM (cm**-0.5) 

At 15 MHz using

0.5" F=90mm probe

(1/2 of measured α)

"Low Noise"

Region

"High Noise"

Region

0.618

0.620

0.622

0.624

0.626

0.628

0.630

0.632

OD Mid CL

Coupon Location

L-wave speed (cm/μs)

"Low Noise"

Region

"High Noise"

Region

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

OD Mid CL

Coupon Location

Attenuation (N/cm)

Average α at 

15.2 MHz.

Planar probe.

"Low Noise"

Region

"High Noise"

Region

10

 9

 8

 7

 6

 5

 4

 3

 2

dB/in

0.000

0.020

0.040

0.060

0.080

0.100

OD Mid CL

Coupon Location

 Noise FOM (cm**-0.5) 

At 15 MHz using

0.5" F=90mm probe

(1/2 of measured α)

"Low Noise"

Region

"High Noise"

Region

0.618

0.620

0.622

0.624

0.626

0.628

0.630

0.632

OD Mid CL

Coupon Location

L-wave speed (cm/μs)

"Low Noise"

Region

"High Noise"

Region

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

OD Mid CL

Coupon Location

Attenuation (N/cm)

Average α at 

15.2 MHz.

Planar probe.

"Low Noise"

Region

"High Noise"

Region

10

 9

 8

 7

 6

 5

 4

 3

 2

dB/in

L-wave Speed

Varied by ~ 2 %

Attenuation

Varied by factor of ~ 2.5

Noise FOM

Varied by factor of ~ 2

FIGURE 3-2. NOISE BANDING AND UT PROPERTY VARIATIONS IN A 6′′ DIAMETER 

Ti-6-4 BILLET (a) NOISE-BANDING PATTERN, (b) RECTANGULAR COUPON 

LOCATIONS, AND (c) UT PROPERTY VARIATIONS FOR 

RADIAL SOUND PROPAGATION 

3-3
To construct a coherent picture of how UT velocity, attenuation, and backscattered noise vary 

with position and sound propagation, the coupon-cutting scheme shown in figure 3-3 was used in 

this study of Ni billets. Rectangular strip coupons with approximate dimensions of 2′′ x 2′′ x 10′′

were cut along billet diameters, as illustrated in figure 3-3(a), and used for UT measurements in 

the axial and hoop directions. The 2′′ thickness in the axial and hoop directions was chosen 

because: (1) it provided ample lateral width so that the incident sonic beams of the transducers 

being used did not appreciably interact with the sides of the specimen during measurements of 

back-wall echoes and (2) a 2′′ thick fused-quartz (FQ) reference block (used for comparative 

attenuation measurements) was already in hand. The strip coupons would be later cut into slices 

for UT measurements in the radial direction, figure 3-3(b), with the number and thickness of the 

slices dependent upon the radial variation of UT properties observed in the strip coupon 

themselves. If the perceived radial variation was small, thicker slices would be used; if the radial 

variation was large, thinner slices would be used. As will be discussed in section 3.1.2.3, the 

slice thickness was eventually chosen to be 1.0′′ for all the slice coupons studied. In addition to 

the property measurement coupons, which had smooth (ground) surfaces, a rough-cut slice of 

adjacent material was obtained for use in metallographic studies. As shown in figure 3-3(a), this 

measured approximately 0.5′′ x 2′′ x 10′′ and abutted the strip coupon in the axial direction. 

Small coupons were obtained from the rough-cut slice at selected radial positions, and these were 

polished and etched to reveal the metal grain structure. 

FIGURE 3-3. COUPON LAYOUT FOR 10′′ DIAMETER Ni ALLOY BILLETS 

(a) STRIP COUPON FOR AXIAL AND HOOP MEASUREMENTS, AND ITS 

ASSOCIATED ROUGH-CUT (METALLOGRAPHY) COUPON AND 

(b) SLICE COUPONS FOR RADIAL PROPERTY MEASUREMENTS 

Four strip coupons and their associated slices were studied. It was decided by consensus that 

two Ni alloys would be studied: IN718 and Waspaloy. Their nominal chemical compositions 

were listed in table 1-1. Two different forging methods known as GFM (Gesellschaft fur 

Maschinenbau-und-Fertigungstechnik) and V-Die, respectively, are commonly used to transform 

IN718 cast ingot into finished billet. GFM employed multiple hammers oriented at 90 degrees to 

one another, which pound the ingot while it rotates, while V-die used a single hammer to strike 

the ingot while it rotates within in a V-shaped trough. IN718 billets produced by both methods 

were studied. 

Radial

“strip coupon” for UT

property measurements

Hoop

“rough cut” coupon for 

use in metallography

possible sites for

metallography

Axial

“Strip”

Coupon

(~ 2” x 2” x 10”)

Billet

1”-thick “slice”

coupons

(a) (b)

Radial

“strip coupon” for UT

property measurements

Hoop

“rough cut” coupon for 

use in metallography

possible sites for

metallography

Axial

“Strip”

Coupon

(~ 2” x 2” x 10”)

Billet

1”-thick “slice”

coupons

Radial

“strip coupon” for UT

property measurements

Hoop

“rough cut” coupon for 

use in metallography

possible sites for

metallography

Axial

Hoop

“rough cut” coupon for 

use in metallography

possible sites for

metallography

Axial

“Strip”

Coupon

(~ 2” x 2” x 10”)

Billet

1”-thick “slice”

coupons

“Strip”

Coupon

(~ 2” x 2” x 10”)

Billet

1”-thick “slice”

coupons

(a) (b)

3-4
The precise locations of the strip coupons within the candidate billets were determined from 

measured backscattered grain noise patterns. Four 10′′ diameter billets having typical 

backscattered grain noise levels were selected by the original engine manufacturers (OEM): 

• A 30′′ length of V-Die-processed IN718 billet 

• A 30′′ length of GFM-processed IN718 billet 

• A 12′′ length of GFM-processed IN718 billet 

• A 40′′ length of Waspaloy billet 

These candidate billets were then inspected in the radial direction using a multizone inspection 

system [23] for 10′′ diameter billets located at GE in Cincinnati. That system contains six 

focused 5-MHz transducers each designed to inspect a different depth zone. The depth zones 

and associated transducer properties are listed in table 3-1. Except for the shallowest inspection 

zone, the multizone transducers have bicylindrically curved lenses with different focal lengths 

(in water) in the radial/axial and radial/hoop planes. The transducer diameters and focal lengths 

are chosen to produce focal spots of similar size at the centers of each of the six inspection 

zones. Each transducer is nominally operated at a 3.0′′ water path, although small adjustments 

are sometimes made to better balance the beam focus within the inspection zone. 

TABLE 3-1. MULTIZONE INSPECTION PARAMETERS FOR 10′′ DIAMETER BILLET 

Zone 

Number 

Zone Depth Range 

(inches) 

Transducer Diameter

(nominal, inches) 

Transducer Focal 

Depth in Metal 

(nominal, inches) 

1 0.2-0.9 0.75 0.55 

2 0.9-1.8 1.25 1.35 

3 1.8-2.7 1.50 2.25 

4 2.7-3.6 1.75 3.15 

5 3.6-4.5 2.00 4.05 

6 4.5-5.5 2.35 5.00 

For the Ni alloy billet inspections, the usual multizone setup and calibration procedure was 

generally followed [23], although the gain settings were increased to emphasize the grain noise. 

The calibration specimen was a 10′′ diameter Ti-6-4 billet containing #2 FBHs at various depths. 

The gain setting for each transducer was adjusted to bring the amplitudes of FBH signals in the 

associated zone of the standard to 80% full-screen height (FSH). Then, prior to scanning a given 

Ni billet, an additional gain was applied to boost the average gated peak-to-peak noise level in 

the highest-noise zone to approximately 50% FSH. The amount of added gain was different for 

each Ni billet inspected and ranged from 20 to 31.5 dB. The electronic noise level of the 

measurement system was determined by performing a similar rotational/axial scan beyond the 

end of the billet (i.e., in water only). 

3-5
For each billet inspected, six gated-peak noise C-scan images were obtained, one for each depth 

zone. Some degree of grain noise banding was seen in all C-scans. An example is shown in 

figure 3-4 for the IN718 V-Die billet. For the full useable area of each C-scan image, the 

average and peak values of the observed noise were measured and tabulated. Small subregions 

with areas of several square inches on the billet OD were identified, where the average noise 

level was approximately highest and lowest, respectively. Gated-peak noise statistics (mean 

noise; peak noise) were then computed for these subregions as well. For example, in figure 3-4, 

the high- and low-noise subregions are denoted by A and B, respectively, and have average noise 

levels of 45% and 17% FSH, respectively, at the gain used for this particular inspection (31.5 dB 

above standard). 

System Peak Noise = 37%

System Mean Noise = 3%

30”

A

B

Overall: mean=27%, max=91%

A: mean=45%, max=87%

B = mean=17%, max=38%

centers of

strip coupons

System Peak Noise = 37%

System Mean Noise = 3%

30”

A

B

Overall: mean=27%, max=91%

A: mean=45%, max=87%

B = mean=17%, max=38%

centers of

strip coupons

FIGURE 3-4. C-SCAN IMAGE OF BACKSCATTERED NOISE FOR ZONE 

2 OF THE IN718 V-DIE BILLET 

(Locations are shown where two strip coupons were later cut.) 

Average noise levels seen in the four Ni alloy billets are compared in figure 3-5. The measured 

noise values have been renormalized to a common gain setting, namely, the gain needed to raise 

#2 FBH signals in the calibration standard to 80% FSH plus an additional 30 dB. At this gain, 

the noise is shown as a percentage of FSH. The measured noise values are seen to be well above 

the electronic noise level of the inspection system, which tends to increase with inspection depth. 

Note that the observed system noise is expected to increase linearly with the absolute voltage 

amplification applied by the receiver. Also, because of material attenuation in the calibration 

standard, the absolute gain required to bring a calibration FBH echo to 80% FSH tends to 

increase with inspection depth. In figure 3-5, results are shown for inspection zones 2-6 only. 

At the large gain settings used, ring-down of the front-surface echo affected the zone 1 noise 

level in a manner which was very sensitive to probe alignment. Since the measured noise levels 

in zone 1 were generally not reproducible, they have been excluded from the figure. 

3-6
Average Noise Levels For Inspection

of 10"-Diameter Nickel-Alloy Billets

0

20

40

60

80

100

120

140

1234567

MZ Inspection Zone

Mean Gated-Peak Noise Level .

(% FSH) 

Waspalloy, 40"long

IN718 GFM, 12"long

IN718 GFM, 30"long

IN718 V-die,30"long

System

Gain Setting: #2 FBH

at 80% FSH in 10" Ti 

Standard + 30 dB

 Average Gated-Peak Noise Level (%FSH at 30 dB extra gain) 

Zone System Noise GFM (12"-long) GFM (30"-long) V-die (30"-long) Wasp. (40"-long)

2 2 29 34 23 79

3 3 30 32 25 101

4 3 39 37 30 104

5 10 57 56 44 136

6 11 52 50 39 108

Average Noise Levels For Inspection

of 10"-Diameter Nickel-Alloy Billets

0

20

40

60

80

100

120

140

1234567

MZ Inspection Zone

Mean Gated-Peak Noise Level .

(% FSH) 

Waspalloy, 40"long

IN718 GFM, 12"long

IN718 GFM, 30"long

IN718 V-die,30"long

System

Gain Setting: #2 FBH

at 80% FSH in 10" Ti 

Standard + 30 dB

 Average Gated-Peak Noise Level (%FSH at 30 dB extra gain) 

Zone System Noise GFM (12"-long) GFM (30"-long) V-die (30"-long) Wasp. (40"-long)

2 2 29 34 23 79

3 3 30 32 25 101

4 3 39 37 30 104

5 10 57 56 44 136

6 11 52 50 39 108

FIGURE 3-5. AVERAGE NOISE LEVELS IN THE MULTIZONE INSPECTIONS OF 

FOUR Ni ALLOY BILLETS 

Note in figure 3-5 that the Waspaloy billet has a mean noise level approximately 2-3 times 

higher than those of the three IN718 billets. This will later be traced to the significantly larger 

mean grain diameters in the Waspaloy billet. Also note in figure 3-5 that the two IN718 GFM 

billets have very similar noise levels. These two billets had similar noise-banding patterns as 

well. It was decided that the shorter of the two billets would be used for UT property specimens 

and the longer would be used to construct an FBH calibration standard for 10′′ diameter Ni alloy 

billets. Finally, note in figure 3-5 that the average noise level of each Ni billet (relative to the #2 

FBH standard) is always greatest in zone 5. Since noise-to-signal ratios tend to increase as the 

UT pulse volume increases [24], this may indicate that the zone 5 transducer has a broader focal 

spot than the transducers used for the other inspection zones. Alternatively, it may indicate that 

for all four billets, average grain diameters are largest near the billet center. 

Figure 3-6 displays the ratio of the average noise levels seen in the high- and low-noise 

subregions of the Ni alloy billets. This ratio serves as a measure of the severity of noise banding. 

For the Waspaloy and IN718-GFM billets, noise banding was relatively moderate with the 

average noise level in high-noise bands about 1.5 times that in low-noise bands. For the IN718 

V-Die billet, however, noise banding in the shallower depth zones was much more severe, and it 

was decided that strip coupons would be cut from both high- and low-noise bands of that billet. 

3-7
It was also decided that a single strip coupon would be cut from each of the Waspaloy and 

IN718-GFM billets, with that single coupon taken from a high-noise band because high-noise 

regions generally present more challenging inspection difficulties. Thus, the four strip coupons 

that the budget permitted, together with their short-hand designations, were: 

• V-Die-A: Strip coupon from a high-noise band in an IN718 V-Die billet 

: 

FIGURE 3-6. DEGREE OF NOISE BANDING IN THE MULTIZONE INSPECTIONS OF 

or the IN718 V-Die billet, the locations of the two strip coupons within the noise-banding 

• V-Die-B: Strip coupon from a low-noise band in the above V-Die billet 

• GFM-A: Strip coupon from a high-noise band in an IN718-GFM billet 

• Waspaloy Strip coupon from a high-noise band in a Waspaloy billet 

FOUR Ni ALLOY BILLETS 

F

pattern were shown in figure 3-4. Noise-banding patterns and strip coupon locations for the 

other two billets are displayed in figures 3-7 and 3-8. The single strip coupon for the Waspaloy 

 Average Gated-Peak Noise Levels in "Low-Noise" and "High-Noise" Subregions

 (%FSH at 30 dB extra gain) 

Zone GFM (12"-long) V-die (30"-long) Wasp. (40"-long)

2 22, 36 14, 38 60, 85

3 26, 34 18, 36 73, 111

4 34, 45 26, 35 85, 114

5 49, 62 41, 49 104, 142

6 47, 57 39. 42 79, 114

Ratio of Average Noise Levels in "High Noise" 

and "Low Noise" Subregions of Ni Alloy Billets

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1234567

MZ Inspection Zone

 High Noise / Low Noise .

IN718 V-die (30" long)

IN718 GFM (12" long)

Waspaloy (40" long)

Gain Setting: #2 FBH

at 80% FSH in 10" Ti 

Standard + 30 dB

 Average Gated-Peak Noise Levels in "Low-Noise" and "High-Noise" Subregions

 (%FSH at 30 dB extra gain) 

Zone GFM (12"-long) V-die (30"-long) Wasp. (40"-long)

2 22, 36 14, 38 60, 85

3 26, 34 18, 36 73, 111

4 34, 45 26, 35 85, 114

5 49, 62 41, 49 104, 142

6 47, 57 39. 42 79, 114

Ratio of Average Noise Levels in "High Noise" 

and "Low Noise" Subregions of Ni Alloy Billets

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1234567

MZ Inspection Zone

 High Noise / Low Noise .

IN718 V-die (30" long)

IN718 GFM (12" long)

Waspaloy (40" long)

Gain Setting: #2 FBH

at 80% FSH in 10" Ti 

Standard + 30 dB

3-8
billet was cut near an end of the 40′′ length, allowing the remainder to be used for the 

construction of a full-round calibration standard containing FBHs at selected depths. 

Calibration: 

Ti #2 FBH 80% + 30dB 

Overall Peak Noise 88%

Overall Mean Noise 30%

12”

A

C B

A: mean = 34%, max = 65%

B: mean = 26%, max = 46%

C: mean = 34%, max = 65%

center of

strip coupon

Calibration: 

Ti #2 FBH 80% + 30dB 

Overall Peak Noise 88%

Overall Mean Noise 30%

12”

A

C B

A: mean = 34%, max = 65%

B: mean = 26%, max = 46%

C: mean = 34%, max = 65%

center of

strip coupon

FIGURE 3-7. C-SCAN IMAGE OF BACKSCATTERED NOISE FOR ZONE 3 

OF THE 12′′ LONG IN718 GFM BILLET 

(A strip coupon was cut at site A within a high-noise band.) 

Overall Peak Noise = 91%

Overall Mean Noise = 43%

Calibration: 

Ti #2 FBH 80%+ 20dB 

Peak System Noise 100% 

Mean System Noise 5%

A: mean = 45%, max = 91%

B: mean = 33%, max = 60%

C: mean = 45%, max = 73%

40”

A

B

C

associated with 

surface imperfections center of

strip coupon

Overall Peak Noise = 91%

Overall Mean Noise = 43%

Calibration: 

Ti #2 FBH 80%+ 20dB 

Peak System Noise 100% 

Mean System Noise 5%

A: mean = 45%, max = 91%

B: mean = 33%, max = 60%

C: mean = 45%, max = 73%

40”

A

B

C

associated with 

surface imperfections center of

strip coupon

40”

A

B

C

associated with 

surface imperfections

40”

A

B

C

associated with 

surface imperfections center of

strip coupon

FIGURE 3-8. C-SCAN IMAGE OF BACKSCATTERED NOISE FOR ZONE 5 

OF THE 40′′ LONG WASPALOY BILLET 

(A strip coupon was cut at site A within a high-noise band.) 

3-9
The four strip coupons were cut from their respective billets using electron discharge-machining 

(EDM) techniques. The four large (2′′ x 10′′) surfaces of each coupon having normal vectors 

pointing in the axial or hoop directions were then ground smooth to facilitate accurate UT 

measurements. The six surfaces of each specimen were labeled 1-6 in the manner shown in 

figure 3-9. Note that the outward normal vectors to sides 1, 2, and 3 point in the radial, axial, 

and hoop directions of the billet respectively. Sides 4, 5, and 6 are opposite to sides 1, 2, and 3 

respectively. Note that both sides 1 and 4 are slightly curved before finish machining, and both 

are portions of the OD billet surface. In all cases, side 1 denotes the OD face that was the sound 

entry surface for the backscattered noise C-scan used to select the coupon location. 

3

101-S1

waspaloy

Billet 10”

5

2

6

G1 3 1 4

5

2

6

INCO 718 GFM SITE A HIGH NOISE 4

IN718 Billet Coupon Waspaloy Billet Coupon

Markings for the IN718 GFM billet coupon are shown above at left. Markings for the IN718 V-die coupons are 

similar except:

V-die “low noise “ coupon has “INCO718 V DIE SITE B LOW NOISE” stamped on side 3, 

and “V1” stamped on side 1 in place of G1.

V-die “high noise “ coupon has “INCO718 V DIE SITE A HIGH NOISE” stamped on side 3, 

and “VV1” stamped on side 1 in place of G1.

numeric 

“side” 

labels

3

101-S1

waspaloy

Billet 10”

5

2

6

1 4 3

101-S1

waspaloy

Billet 10”

5

2

6

G1 3 1 4

5

2

6

G1 3 INCO 718 GFM SITE A HIGH NOISE 4

5

2

6

INCO 718 GFM SITE A HIGH NOISE 4

IN718 Billet Coupon Waspaloy Billet Coupon

Markings for the IN718 GFM billet coupon are shown above at left. Markings for the IN718 V-die coupons are 

similar except:

V-die “low noise “ coupon has “INCO718 V DIE SITE B LOW NOISE” stamped on side 3, 

and “V1” stamped on side 1 in place of G1.

V-die “high noise “ coupon has “INCO718 V DIE SITE A HIGH NOISE” stamped on side 3, 

and “VV1” stamped on side 1 in place of G1.

numeric 

“side” 

labels

FIGURE 3-9. STAMPED MARKINGS USED TO LABEL THE SURFACES OF 

THE Ni BILLET STRIP COUPONS 

(Here, the six faces of a given coupon have been unfolded into a plane.) 

For each of the four strips, the density was determined in the standard manner by dividing the 

measured mass by the computed volume. The three strips cut from the IN718 billets had curved 

ends (sides 1 and 4) whose cylindrical curvatures were those of the billet OD surfaces. For the 

Waspaloy strip coupon, the two curved ends had been machined flat, resulting in a rectangular 

prism. Formulas used to compute the volumes of the strip coupons from the measured 

dimensions are shown in figure 3-10. The measured dimensions and masses are listed in 

table 3-2, together with the deduced density values. For the three IN718 strip coupons, the 

measured densities were the same to within 1 part in 2000, averaging 8.259 gm/cm. The 

measured density for the Waspaloy specimen, which has a different alloy chemistry, was slightly 

lower at 8.209 gm/cm. 

3-10
Strip coupon volume was computed using:

for the rectangular Waspalloy coupon, and

for the IN718 coupons which each had two cylindrically-curved surfaces.

Here, Lxxx denotes the full length in the xxx direction. For the IN718 coupons 

Lradial is effectively the billet diameter:

( )( )( ) VL L L = hoop axial radial

2 2 21 ( )( ) ( ) 2

axial hoop

hoop radial hoop radial

radial

L L

V LL L LSin

L

− ⎡ ⎤ ⎧ ⎫ ⎪ ⎪ ⎛ ⎞ = − ⎢ ⎥ ⎨ ⎬ ⎡ ⎤ + ⎜ ⎟ ⎣ ⎦ ⎣ ⎦ ⎪ ⎪ ⎩ ⎭ ⎝ ⎠

Side 2

( ) Lradial

( ) Lhoop

Strip coupon volume was computed using:

for the rectangular Waspalloy coupon, and

for the IN718 coupons which each had two cylindrically-curved surfaces.

Here, Lxxx denotes the full length in the xxx direction. For the IN718 coupons 

Lradial is effectively the billet diameter:

( )( )( ) VL L L = hoop axial radial

2 2 21 ( )( ) ( ) 2

axial hoop

hoop radial hoop radial

radial

L L

V LL L LSin

L

− ⎡ ⎤ ⎧ ⎫ ⎪ ⎪ ⎛ ⎞ = − ⎢ ⎥ ⎨ ⎬ ⎡ ⎤ + ⎜ ⎟ ⎣ ⎦ ⎣ ⎦ ⎪ ⎪ ⎩ ⎭ ⎝ ⎠

Side 2

( ) Lradial

( ) Lhoop

Side 2

( ) Lradial

( ) Lhoop

FIGURE 3-10. VOLUME DETERMINATION FOR Ni BILLET STRIP COUPONS 

TABLE 3-2. DIMENSION, MASS, AND DENSITY VALUES FOR Ni BILLET 

STRIP COUPONS 

Specimen 

L_radial 

(inches) 

L_axial 

(inches) (ave.) 

L_hoop 

(inches) (ave.) 

Mass 

(grams) 

Density 

(gm/cc) 

Waspaloy-A 10.000 1.9975 2.0000 5374 8.209 

GFM-A 10.025 1.9645 1.9905 5272 8.262 

V-Die-A 9.983 2.0050 1.9945 5366 8.258 

V-Die-B 9.980 2.0080 2.0070 5406 8.258 

A = high-noise site 

B = low-noise site 

3.1.2 Preliminary UT Examinations of the Billet Strip Coupons. 

Before performing detailed UT property measurements, the strip coupons were scanned at 

normal incidence using the two experimental setups shown in figure 3-11. The object was to 

produce C-scan images of back-wall TOF, back-wall amplitude, and backscattered grain noise 

amplitude. To produce C-scan images of back-wall amplitude and TOF, a 10-MHz, 0.25′′

diameter planar transducer was used, and a 2′′ thick FQ block served as a low-attenuation 

reference. To produce C-scan images of backscattered noise, a 15-MHz focused transducer 

having a 0.5′′ diameter and a 3.5′′ focal length in water was used. The beam was focused 0.5′′

deep in metal, i.e., midway between the front surface and the center plane. For the noise 

3-11
measurements, a 1/64′′ diameter (#1 FBH) in a low-attenuation Ni alloy step block (IN100) 

served as the reference. For producing the various C-scan images of a given strip coupon, each 

of the four 2′′ x 10′′ surfaces served in turn as the sound entry surface. All C-scan images were 

produced using a SONIX scanning system with the digitization rate set at 100 MHz. For 

backscattered noise measurements, local signal averaging (64 averages at each transducer 

position) was used to reduce the effect of electronic noise. No signal averaging was required 

when digitizing the much stronger back-wall echoes. The SONIX system had an eight-bit 

analog-to-digital voltage converter, resulting in 256 amplitude levels. 

Setup for Backscattered Grain Noise C-Scan: 

0.5” diameter, 15-MHz, focused probe used.

#1 FBH 

Ni coupon Reference

Scan 

Grains

Fused Quartz 

Reference

Scan 

Ni coupon

Setup for Back-wall Echo C-Scan: 

10-MHz, 1/4”, planar probe used.

(a) (b)

Setup for Backscattered Grain Noise C-Scan: 

0.5” diameter, 15-MHz, focused probe used.

#1 FBH 

Ni coupon Reference

Scan 

Grains

Fused Quartz 

Reference

Scan 

Ni coupon

Setup for Back-wall Echo C-Scan: 

10-MHz, 1/4”, planar probe used.

(a) (b)

FIGURE 3-11. EXPERIMENTAL SETUPS USED TO MEASURE (a) BACK-WALL 

AMPLITUDE AND TOF AND (b) BACKSCATTERED GRAIN NOISE AMPLITUDE 

3.1.2.1 Back-Wall TOF and Amplitude C-Scans. 

For the back-wall echo measurements using the 10-MHz planar transducer, the water path was 

fixed at 5.0 cm (1.97′′) for both the strip coupons and the FQ reference block, guaranteeing that 

the specimen back walls were located well within the far field of the sonic beam. If a specimen 

block is simply laid upon a flat support and back-wall echoes are acquired, the echoes will 

usually be affected by reverberations of sound within a thin water layer trapped between the 

specimen and its support. For these measurements, the strip coupons and reference block were 

each supported only by their far edges so that a thick layer of water was immediately below each 

back wall. Thus, the reflecting back-wall surface was a simple water/metal or water/FQ interface 

without complicating factors. For radio frequency (RF) waveform digitizations, a time gate of 2 

μsec duration was used, which was approximately centered at the arrival time of the back-wall 

echo. Amplitude C-scans were constructed, which displayed the peak rectified back-wall 

amplitude within the time gate. TOF C-scans were also constructed, which displayed the time at 

which the leading edge of the back-wall echo crossed a preset threshold level. For the strip 

coupons, the scan area typically measured 10.4′′ (radial) by 2.4′′ (axial or hoop) with a step size 

of 0.04′′ in each direction. A smaller scan region of 2.2′′ x 2.2′′ was used for the FQ block, but 

3-12
the same 0.04′′ step size was used. As illustrated in figure 3-12 for the V-Die-B case, the backwall TOF images showed very little variation. Note that for figures in the style of figure 3-12, 

the sound entry surface is indicated below each separate C-scan image. The four numbers listed 

immediately above, below, left, and right of an image are the labels of the four adjacent lateral 

surfaces, which are parallel to the insonification direction, following the earlier notational 

scheme of figure 3-9. The TOF images were found to be consistent with having velocity 

variations of less than about 0.2% throughout each coupon. Longitudinal velocities measured in 

the axial and hoop directions of the strip coupons were found to range over 0.5808 ±0.0008 

cm/μsec (or ±0.14%) for the group of three IN718 coupons, and over 0.6012 ±0.0003 cm/μsec 

(or ±0.05%) within the Waspaloy coupon. 

( )

FIGURE 3-12. BACK-WALL TOF C-SCAN IMAGES FOR THE V-DIE-B STRIP COUPON 

Although the sound velocities were quite uniform, a strong dependence of back-wall echo 

amplitude on radial position was typically seen. Back-wall amplitude C-scan images for the four 

strip coupons are shown in figures 3-13 through 3-16, respectively. Note that back-wall 

amplitude images made from opposite sides of the same strip coupon (e.g., sides 2 and 5) are 

essentially mirror versions of each other, as expected. Also note that for a given strip coupon, 

the images for beam propagation in the axial direction (through sides 2 and 5) tend to closely 

resemble those for propagation in the hoop direction (through sides 3 and 6). 

Insp. thru Side 2

at 15 dB gain

Insp. thru Side 5

at 15 dB gain

Insp. thru Side 3

at 15 dB gain

Insp. thru Side 6

at 15 dB gain

g

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

0% 100%

Scan step size 0.04”

Scan area 2.4” x 10.4” 

for all four sides.

Label for adjacent side 

whose face is parallel to 

the sound propagation 

direction.

sound propagation direction.

Insp. thru Side 2

at 15 dB gain

Insp. thru Side 5

at 15 dB gain

Insp. thru Side 3

at 15 dB gain

Insp. thru Side 6

at 15 dB gain

( ) g

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

0% 100%

Scan step size 0.04”

Scan area 2.4” x 10.4” 

for all four sides.

Label for adjacent side 

whose face is parallel to 

the sound propagation 

direction.

sound propagation direction.

3-13
Insp. thru Side 2

at 18 dB gain

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

Insp. thru Side 5

at 18 dB gain

Insp. thru Side 3

at 18 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 6

at 18 dB gain

0% 100%

Scan step size 0.04”

Scan area 2.4” x 10.4” 

for all four sides.

Insp. thru Side 2

at 18 dB gain

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

Insp. thru Side 5

at 18 dB gain

Insp. thru Side 3

at 18 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 6

at 18 dB gain

0% 100%

Scan step size 0.04”

Scan area 2.4” x 10.4” 

for all four sides.

FIGURE 3-13. BACK-WALL AMPLITUDE C-SCAN IMAGES FOR THE 

WASPALOY STRIP COUPON 

Insp. thru Side 2

at 15 dB gain

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

Insp. thru Side 5

at 15 dB gain

Insp. thru Side 3

at 15 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 6

at 15 dB gain

0% 100%

Scan step size 0.04”

Scan area 2.4” x 10.4” 

for all four sides.

Insp. thru Side 2

at 15 dB gain

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

Insp. thru Side 5

at 15 dB gain

Insp. thru Side 3

at 15 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 6

at 15 dB gain

0% 100%

Scan step size 0.04”

Scan area 2.4” x 10.4” 

for all four sides.

FIGURE 3-14. BACK-WALL AMPLITUDE C-SCAN IMAGES FOR THE 

GFM-A STRIP COUPON 

3-14
Insp. thru Side 2

at 16 dB gain

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

Insp. thru Side 5

at 16 dB gain

Insp. thru Side 3

at 16 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 6

at 16 dB gain

0% 100%

Scan step size 0.04”

Scan area 2.52” x 10.8”

for Side 2; and

2.4” x 10.4” for other

three sides

Insp. thru Side 2

at 16 dB gain

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

Insp. thru Side 5

at 16 dB gain

Insp. thru Side 3

at 16 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 6

at 16 dB gain

0% 100%

Scan step size 0.04”

Scan area 2.52” x 10.8”

for Side 2; and

2.4” x 10.4” for other

three sides

FIGURE 3-15. BACK-WALL AMPLITUDE C-SCAN IMAGES FOR THE 

V-DIE-A STRIP COUPON 

Insp. thru Side 2

at 15 dB gain

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

Fq.cs1

Insp. thru Side 5

at 15 dB gain

Insp. thru Side 3

at 15 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 6

at 15 dB gain

0% 100%

Scan step size 0.04”

Scan area 2.4” x 10.4” 

for all four sides.

Insp. thru Side 2

at 15 dB gain

1

3 6

4

1

2 5

4

Insp. of 2” FQ block

at 6 dB gain

Fq.cs1

Insp. thru Side 5

at 15 dB gain

Insp. thru Side 3

at 15 dB gain

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 6

at 15 dB gain

0% 100%

Scan step size 0.04”

Scan area 2.4” x 10.4” 

for all four sides.

FIGURE 3-16. BACK-WALL AMPLITUDE C-SCAN IMAGES FOR THE 

V-DIE-B STRIP COUPON 

3-15
The back-wall amplitude C-scan images for the four strip coupons are directly compared to one 

another in figure 3-17. There, the images have been rescaled so that the coloration is related to 

the average attenuation in dB/inch units. The rescaling process assumed that the FQ reference 

block had negligible attenuation, and corrected for the different gain settings used during the 

scans and for the differences in interface transmission/reflection losses. The rescaling process 

involved four major steps: 

1. All measured back-wall amplitudes were first multiplied by a constant scale factor chosen 

such that the rescaled FQ image had a mean amplitude of 100% FSH. Since FQ was 

assumed to have zero attenuation, an amplitude of 100% FSH was then associated with 

zero attenuation. 

2. The Ni alloy images were then rescaled to correct for gain differences between the Ni and 

FQ scans. These gain differences ranged from 9 to 12 dB. 

3. The Ni images were then rescaled to correct for differences (between Ni and FQ) in the 

reflection and transmission losses at water/solid interfaces. These losses were quantified 

by the product of one reflection and two transmission coefficients, denoted here by 

T01R11T10, where subscripts 0 and 1 refer to water and solid, respectively. Plane-wave 

transmission and reflection coefficients were used; these can be easily calculated from the 

measured densities and sound velocities [22]. The product T01R11T10 was computed to be 

0.291, 0.109, and 0.107 for FQ, IN718, and Waspaloy respectively. 

4. After the above rescalings, all Ni back-wall amplitudes were found to be less that 100%, 

and the reduction below 100% was assumed to result from UT attenuation. The 

attenuation value (α) was then estimated using 

Amplitude/100% = exp(-2αz) ; z = 2 inches. 

The resulting rescaled back-wall amplitude images are shown in figure 3-17. 

Figure 3-18 provides a second depiction of the same rescaled amplitude data. There, for both 

axial and hoop sound propagation, the rescaled amplitude and estimated attenuation of each 

coupon are shown as functions of radial position. Each radial profile shown is an average (in the 

axial or hoop direction) over the central 1/3 of the coupon surface. The coupon region used in 

the averaging and the sign choice for the radial coordinate axis are both indicated in the righthand portion of figure 3-18. Note that the attenuation estimates shown in figures 3-17 and 3-18 

are only approximate. This is because UT attenuation tends to increase with frequency, and a 

broadband pulse is being used. The center frequency of the sonic pulse is near 10 MHz when the 

pulse initially leaves the transducer, but that center frequency shifts downward during 

propagation since the high-frequency components of the beam are attenuated more that the lowfrequency ones. Figures 3-17 and 3-18 may be thought of as providing estimates of the material 

attenuation averaged within the bandwidth of the back-wall echo. These estimates are useful for 

quick visual comparisons of the relative attenuations of the four specimens. 

3-16
IN718

V-die A

IN718

V-die B

IN718

GFM A

Waspaloy

A

Axial Hoop Axial Hoop Axial Hoop Axial Hoop

2 3 2 3 2 3 2 3

11 11 1 1 11

44 44 4 4 44

3.5 2.0 1.1 0.5 0 dB/in

0 20 40 60 80 100 %

Estimated attenuation

(Average over bandwidth

of 10-MHz broadband pulse) :

FQ

(100%) :

IN718

V-die A

IN718

V-die B

IN718

GFM A

Waspaloy

A

Axial Hoop Axial Hoop Axial Hoop Axial Hoop

2 3 2 3 2 3 2 3

11 11 1 1 11

44 44 4 4 44

IN718

V-die A

IN718

V-die B

IN718

GFM A

Waspaloy

A

Axial Hoop Axial Hoop Axial Hoop Axial Hoop

2 3 2 3 2 3 2 3

11 11 1 1 11

44 44 4 4 44

3.5 2.0 1.1 0.5 0 dB/in

0 20 40 60 80 100 %

Estimated attenuation

(Average over bandwidth

of 10-MHz broadband pulse) :

FQ

(100%) :

FIGURE 3-17. RESCALED C-SCAN IMAGES OF BACK-WALL ECHO AMPLITUDE 

FOR AXIAL (SIDE 2) AND HOOP (SIDE 3) INSPECTIONS OF THE 

FOUR Ni BILLET STRIP COUPONS 

p g

FIGURE 3-18. DEPENDENCE OF AVERAGE BACK-WALL AMPLITUDE ON RADIAL 

POSITION IN THE FOUR Ni BILLET STRIP COUPONS 

-5”

+5”

0”

Side 1

Side 4

Radial profile 

shown is an 

average over the 

central 1/3 of the 

coupon surface.

p

Billet Strip Coupons - Axial & Hoop Propagation

Amplitude of 10 MHz Broadband BW Echo

0

50

100

150

200

250

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Distance from Billet Center (inches)

Backwall Amplitude

VdieA-axial

VdieB-axial

GFMA-axial

WaspA-axial

VdieA-hoop

VdieB-hoop

GFMA-hoop

WaspA-hoop

0.0

0.5

1.1

2.0

3.5

5.0

Effective

attenuation

in dB/inch

-5”

+5”

0”

Side 1

Side 4

Radial profile 

shown is an 

average over the 

central 1/3 of the 

coupon surface.

-5”

+5”

0”

Side 1

Side 4

Radial profile 

shown is an 

average over the 

central 1/3 of the 

coupon surface.

p g p

Billet Strip Coupons - Axial & Hoop Propagation

Amplitude of 10 MHz Broadband BW Echo

0

50

100

150

200

250

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Distance from Billet Center (inches)

Backwall Amplitude

VdieA-axial

VdieB-axial

GFMA-axial

WaspA-axial

VdieA-hoop

VdieB-hoop

GFMA-hoop

WaspA-hoop

0.0

0.5

1.1

2.0

3.5

5.0

Effective

attenuation

in dB/inch

Billet Strip Coupons - Axial & Hoop Propagation

Amplitude of 10 MHz Broadband BW Echo

0

50

100

150

200

250

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Distance from Billet Center (inches)

Backwall Amplitude

VdieA-axial

VdieB-axial

GFMA-axial

WaspA-axial

VdieA-hoop

VdieB-hoop

GFMA-hoop

WaspA-hoop

0.0

0.5

1.1

2.0

3.5

5.0

0.0

0.5

1.1

2.0

3.5

5.0

Effective

attenuation

in dB/inch

3-17
As shown in figures 3-17 and 3-18, there are significant differences in the back-wall amplitude 

(or attenuation) values of the four strip coupons. Note that for each coupon there is a clear 

dependence of back-wall amplitude on radial depth, but at a given radial depth, there is generally 

little difference between the results for the axial and hoop directions. Such a behavior is 

consistent with strip coupon microstructures that have equiaxed grains whose average grain 

diameter varies with radial depth. It is also interesting to note that the V-Die-A and V-Die-B 

strip coupons, which were cut from high- and low-noise bands of the same IN718 billet, differ 

significantly in their effective attenuation values near the coupon ends (OD regions) but have 

very similar attenuation values near their centers. Such a coalescing of attenuation values at the 

billet center has also been seen for coupons cut from high- and low-noise bands of Ti-6-4 billets. 

One instance of this behavior was shown in figure 3-2. As might be expected, the asymmetries 

in the mechanical working of the billet, which led to noise-banding and other property variations 

in the hoop direction, evidently have a diminished effect near the billet center. 

3.1.2.2 C-Scans of Backscattered Noise Amplitude. 

The measurement setup used to produce backscattered grain noise C-scan images was shown in 

figure 3-11. The 15-MHz, 0.5′′ diameter transducer used had a nominal focal length of 9.0 cm 

(3.54 inches) in water. Its geometric focal length and actual focal length (peak amplitude from a 

small ball target in water) were measured to be 9.65 cm (3.80 inches) and 9.2 cm (3.62 inches) 

respectively [22]. For the inspections of the strip coupons, a 1.66′′ water path was used so that 

the actual focal depth in metal was about 0.5′′ deep, or one-quarter of the specimen thickness. 

The scan plan used for backscattered grain noise was similar to that used in section 3.1.2.1. The 

scan area was 10.4′′ x 2.4′′ and the step size was 0.04′′. The time gate extended from 2.9 μsec to 

6.9 μsec after the front-surface echo, or approximately from 0.33′′ to 0.79′′ deep in the metal. 

This bracketed the center of the focal zone where the absolute backscattered noise amplitudes 

tended to be the greatest. The resulting C-scan images of the Ni alloy strip coupons displayed 

the rectified gated-peak noise amplitude, i.e., the largest absolute noise amplitude seen anywhere 

within the time gate at a given transducer position. For a given strip coupon, the inspection gain 

was fixed; however, the gain varied from coupon to coupon. A #1 FBH located 0.5′′deep in a 

fine-grain IN100 step block served as a reference for absolute amplitude. The FBH was scanned 

at the same water path used for the noise scans; the scan area was 0.078′′ by 0.078′′ with a step 

size of 0.002′′. The peak amplitude seen in the reference scan differed slightly from day-to-day 

(by 1% to 3%) due to minor differences in water temperature and beam normalization. 

The four large (2′′ x 10′′) surfaces of each strip coupon were scanned to produce images of 

backscattered grain noise amplitudes. The results are shown in figures 3-19 through 3-22, 

respectively, for the four strip coupons, together with the associated images of the #1 FBH 

reference. It was found that grain noise, like attenuation, depended primarily on radial depth in 

the billet. At a given radial depth, noise measurements in the hoop and axial directions tended to 

yield similar results. 

3-18
Reference Scan:

C-scan of #1 FBH, 0.5” 

deep in fine grain IN100 

block at 10dB gain. 0.078” 

x 0.078” scan area with a 

step size of 0.002”.

Insp. thru Side 2

at 44 dB gain

1

3 6

4

1

2 5

4

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

0% 100%

Insp. thru Side 5

at 44 dB gain

Insp. thru Side 3

at 44 dB gain

Insp. thru Side 6

at 44 dB gain

Billet coupons scans :

Scan step size 0.04”

Scan area 2.4” x 10.4”

for all four sides.

Reference Scan:

C-scan of #1 FBH, 0.5” 

deep in fine grain IN100 

block at 10dB gain. 0.078” 

x 0.078” scan area with a 

step size of 0.002”.

Insp. thru Side 2

at 44 dB gain

1

3 6

4

1

2 5

4

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

0% 100%

Insp. thru Side 5

at 44 dB gain

Insp. thru Side 3

at 44 dB gain

Insp. thru Side 6

at 44 dB gain

Billet coupons scans :

Scan step size 0.04”

Scan area 2.4” x 10.4”

for all four sides.

FIGURE 3-19. BACKSCATTERED NOISE C-SCAN IMAGES FOR THE 

WASPALOY STRIP COUPON 

0% 100%

Reference Scan:

C-scan of #1 FBH, 0.5” 

deep in fine grain IN100 

block at 10dB gain. 0.078” 

x 0.078” scan area with a 

step size of 0.002”.

Billet coupons scans :

Scan step size 0.04”

Scan area 2.4” x 10.4”

for all four sides.

1

3 6

4

1

2 5

4

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 2

at 46 dB gain

Insp. thru Side 5

at 46 dB gain

Insp. thru Side 3

at 46 dB gain

Insp. thru Side 6

at 46 dB gain

0% 100%

Reference Scan:

C-scan of #1 FBH, 0.5” 

deep in fine grain IN100 

block at 10dB gain. 0.078” 

x 0.078” scan area with a 

step size of 0.002”.

Billet coupons scans :

Scan step size 0.04”

Scan area 2.4” x 10.4”

for all four sides.

1

3 6

4

1

2 5

4

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 2

at 46 dB gain

Insp. thru Side 5

at 46 dB gain

Insp. thru Side 3

at 46 dB gain

Insp. thru Side 6

at 46 dB gain

FIGURE 3-20. BACKSCATTERED NOISE C-SCAN IMAGES FOR THE 

GFM-A STRIP COUPON 

3-19
0% 100%

Reference Scan:

C-scan of #1 FBH, 0.5” 

deep in fine grain IN100 

block at 10dB gain. 0.078” 

x 0.078” scan area with a 

step size of 0.002”.

Billet coupons scans :

Scan step size 0.04”

Scan area 2.4” x 10.4”

for all four sides.

1

3 6

4

1

2 5

4

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 2

at 44 dB gain

Insp. thru Side 5

at 44 dB gain

Insp. thru Side 3

at 44 dB gain

Insp. thru Side 6

at 44 dB gain

0% 100%

Reference Scan:

C-scan of #1 FBH, 0.5” 

deep in fine grain IN100 

block at 10dB gain. 0.078” 

x 0.078” scan area with a 

step size of 0.002”.

Billet coupons scans :

Scan step size 0.04”

Scan area 2.4” x 10.4”

for all four sides.

1

3 6

4

1

2 5

4

1

6 3

4

1

5 2

4

Axial Axial Hoop Hoop

Insp. thru Side 2

at 44 dB gain

Insp. thru Side 5

at 44 dB gain

Insp. thru Side 3

at 44 dB gain

Insp. thru Side 6

at 44 dB gain

FIGURE 3-21. BACKSCATTERED NOISE C-SCAN IMAGES FOR THE 

V-DIE-A STRIP COUPON 

0% 100%

Reference Scan:

C-scan of #1 FBH, 0.5” 

deep in fine grain IN100 

block at 10dB gain. 0.078” 

x 0.078” scan area with a 

step size of 0.002”.

Billet coupons scans :

Scan step size 0.04”

Scan area 2.4” x 10.4”

for all four sides.

( ) p

2 5

1

3 6

4

1

2

4

1

6 3

4

1

5

4

Axial Axial Hoop Hoop

Insp. thru Side 2

at 48 dB gain

Insp. thru Side 5

at 48 dB gain

Insp. thru Side 3

at 48 dB gain

Insp. thru Side 6

at 48 dB gain

0% 100%

Reference Scan:

C-scan of #1 FBH, 0.5” 

deep in fine grain IN100 

block at 10dB gain. 0.078” 

x 0.078” scan area with a 

step size of 0.002”.

Billet coupons scans :

Scan step size 0.04”

Scan area 2.4” x 10.4”

for all four sides.

( ) p

2 5

1

3 6

4

1

2

4

1

6 3

4

1

5

4

Axial Axial Hoop Hoop

Insp. thru Side 2

at 48 dB gain

Insp. thru Side 5

at 48 dB gain

Insp. thru Side 3

at 48 dB gain

Insp. thru Side 6

at 48 dB gain

FIGURE 3-22. BACKSCATTERED NOISE C-SCAN IMAGES FOR THE 

V-DIE-B STRIP COUPON 

3-20
To better display the dependence of noise on radial depth, radial profiles of averaged noise 

amplitudes were constructed. The procedure used is illustrated in figure 3-23 using noise data 

from the Waspaloy coupon. Within each C-scan image, a rectangular area of dimension 0.52′′

(axial or hoop) by 10.4′′ (radial) was studied. This area, outlined by the dotted rectangle in 

figure 3-23(a), was centered at the center of the strip coupon. At each radial position, the noise 

amplitudes within the rectangle were averaged over the lateral coordinate (axial or hoop) to 

produce a noise-amplitude-versus-radial-position profile. Because of the random nature of 

backscattered noise, the radial profiles thus constructed have a somewhat jagged appearance. As 

shown in figure 3-23(b), a 9-point running average was performed to obtain a smoother curve 

that better displays the general trend of the data. 

Waspaloy @ 44dB

100

110

120

130

140

150

160

170

180

190

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 6

Running Avg ( 9 pts)

+5”

-5”

0”

Rectangular 

region of 13 x 260 

pixels used for 

radial profile 

construction. radial position

Waspaloy @ 44dB

100

110

120

130

140

150

160

170

180

190

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 6

Running Avg ( 9 pts)

+5”

-5”

0”

Rectangular 

region of 13 x 260 

pixels used for 

radial profile 

construction. radial position

(a) (b)

FIGURE 3-23. CONSTRUCTION OF RADIAL PROFILES OF BACKSCATTERED GRAIN 

NOISE (a) NOISE ANALYSIS REGION FOR THE INSPECTION THROUGH SIDE 6 OF 

THE WASPALOY STRIP COUPON AND (b) RESULTING CALCULATED AND 

SMOOTHED RADIAL NOISE PROFILES 

The smoothed versions of the radial noise profile are compared in figure 3-24, with the results 

shown for four sides of each strip coupon. Recall that the sonic beam propagates in the axial 

direction for inspections through sides 2 and 5 and in the hoop direction for inspections through 

sides 3 and 6. Note in figure 3-24 that a uniform amplitude scale has been adopted in which the 

peak amplitude of the FBH reference has a value of 100. Thus, all the grain noise amplitudes 

shown are less than 2% of that #1 FBH amplitude. The observed grain noise amplitudes depend 

on both the inherent noisiness of the microstructure (i.e., the FOM value) and on the UT 

attenuation, which varies greatly among the suite of specimens. Because of the effects of 

attenuation, one must not assume that relative FOM values are in the same proportion as the 

corresponding measured gated-peak noise amplitudes. For example, in figure 3-24, the 

displayed noise amplitude near the center of the Waspaloy coupon is only about 30% higher than 

that near the center of the GFM-A coupon. However, as discussed in section 3.1.4.2, the FOM 

values for these cases differ by about a factor of 4. The UT attenuation within the Waspaloy 

coupon is much larger than that within the GFM-A coupon; this attenuation difference acts to 

bring the measured noise amplitudes in figure 3-24 closer together. 

3-21
Waspaloy @ 44dB (Running Avgs.)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 2

Side 5

Side 3

Side 6

GFM-A @ 44dB (Running Avgs.)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 2

Side 5

Side 3

Side 6

VDIE-A @ 46dB (Running Avgs.)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 2

Side 5

Side 3

Side 6

VDIE-B @ 48dB (Running Avgs.)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 2

Side 5

Side 3

Side 6

Waspaloy @ 44dB (Running Avgs.)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 2

Side 5

Side 3

Side 6

GFM-A @ 44dB (Running Avgs.)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 2

Side 5

Side 3

Side 6

VDIE-A @ 46dB (Running Avgs.)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 2

Side 5

Side 3

Side 6

VDIE-B @ 48dB (Running Avgs.)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4 5

Distance from Billet center (inches)

Noise Amplitude (0 - 255) .

Side 2

Side 5

Side 3

Side 6

FIGURE 3-24. DEPENDENCE OF BACKSCATTERED NOISE ON RADIAL POSITION 

FOR EACH OF THE FOUR STRIP COUPONS, RELATIVE TO A #1 FBH AT THE 

SAME DEPTH (0.5′′) IN THE IN100 REFERENCE BLOCK 

In each panel of figure 3-24, four curves are shown corresponding to measurements through the 

central regions of the four major surfaces. Because of the choices of gate depth and lateral 

width, the insonified volumes for these four measurements do not overlap. However, the four 

curves for a given coupon tend to be quite similar in overall amplitude and shape. This indicates 

that backscattered noise levels at a given radial depth are (1) similar for axial and hoop sound 

propagation and (2) relatively unchanged by translations of an inch or so in either the axial or 

hoop direction. Again, this finding is consistent with having, within a given strip coupon, 

equiaxed microstructures in which the mean grain size principally varies with radial position. 

For example, under this interpretation, grain sizes would tend to decrease as one moved radially 

away from the center of the Waspaloy, GFM-A, or V-Die-B strip coupon. Although the 

dependence of backscattered noise on radial position is clear in figures 3-20 through 3-23, there 

are also dependences of noise (and hence microstructure) on axial and hoop position within a 

given billet. This fact is clear from the noise-banding patterns observed in the C-scans of the 

original billets (e.g., see figures 3-4, 3-7, and 3-8. It is also evident from the differences between 

the V-Die-A and V-Die-B curves in figure 3-24, since both strips were cut from different 

axial/hoop locations in the same billet. Recall that V-Die-A was cut from a high-noise band, and 

V-Die-B from a low-noise band. Thus, as expected, the measured noise amplitudes for V-Die-A, 

seen in figure 3-24, are generally larger than those for V-Die-B. However, the grain noise levels 

in the two V-Die coupons are much more similar to one another near the billet center than at 

other radial depths. This mirrors the behavior of back-wall echo amplitudes that were shown in 

section 3.1.2.1 to be most similar near the centers of these two coupons. 

3-22
In summary, back-wall echo and backscattered noise C-scans were performed at normal 

incidence through the four large surfaces of each strip coupon. Results were consistent with the 

hypothesis that the Ni billet microstructures have approximately equiaxed, randomly oriented 

grains whose mean diameter varies systematically with position (principally radial depth). Note 

that this hypothesis implies that local values of attenuation and backscattered noise should be 

well correlated with one another, with both attenuation and noise FOM increasing with mean 

grain diameter. A review of figures 3-18 and 3-24 does indeed reveal that for a given strip 

coupon, large (small) values of estimated attenuation are seen at sites having large (small) values 

of backscattered noise amplitude. This correspondence is also made clear by figure 3-25, which 

directly compares the back-wall and noise C-scan images for axial and hoop inspections of the 

GFM-A strip coupon. One sees that the sites having low back-wall amplitude (high attenuation) 

also have high backscattered noise. Similar visual comparisons for the other three strip coupons 

can be made by the interested reader by comparing figures 3-13 through 3-16 with figures 3-19 

through 3-22. 

FIGURE 3-25. COMPARISON OF BACKSCATTERED NOISE C-SCANS 

(Results ctions.) 

.1.2.3 

Back-Wall Echo Amplitude Grain Noise Amplitude

1

3 6

4

1

5 2

4

1

5 2

4

1

3 6

4

Axial Hoop Axial Hoop

High Back-wall 

Amplitude (low 

attenuation)

Low Back-wall 

Amplitude 

(high atten.)

High 

Grain 

Noise

Low 

Grain 

Noise

Thru side 2 Thru side 3 Thru side 2 Thru side 3

Back-Wall Echo Amplitude Grain Noise Amplitude

1

3 6

4

1

5 2

4

1

5 2

4

1

3 6

4

Axial Hoop Axial Hoop

High Back-wall 

Amplitude (low 

attenuation)

Low Back-wall 

Amplitude 

(high atten.)

High 

Grain 

Noise

Low 

Grain 

Noise

Thru side 2 Thru side 3 Thru side 2 Thru side 3

(RIGHT TWO IMAGES) AND BACK-WALL AMPLITUDE C-SCANS 

(LEFT TWO IMAGES) FOR THE GFM-A STRIP COUPON 

are shown for sound propagation in both the axial and hoop dire

3 Site Selection. 

The preliminary back-wall and backscattered noise C-scans revealed that the UT properties of 

the strip coupons were primarily dependent on radial depth and were approximately symmetric 

about the billet center. It was decided that detailed property measurements would be made at 

sites along a single billet radius rather than along a full diameter. Since the property variations 

with radial depth tended to be systematic and relatively smooth, it was decided that detailed 

measurements at five sites on each strip coupon would be sufficient to obtain a good general 

3-23
understanding of the degree of variability. The five sites selected are shown in figure 3-26 and 

were located 0′′, 1′′, 2′′, 3′′, and 4′′, respectively, from the billet center. For sonic beam 

propagation in the axial and hoop directions, UT measurements at these sites were made on the 

intact strip coupons. For measurements in the radial direction, the strip coupons were later cut 

into approximately 1′′ thick slices centered at these five sites (see figure 3-27). 

0”

Side 1

Side 4

3”4” 2” 1”

FIGURE 3-26. THE FIVE SITES ON A GIVEN STRIP COUPON WHERE DETAILED UT 

FIGURE 3-27. CUTTING AND LABELING PROTOCOLS FOR THE Ni ALLOY STRIP 

OD face for

preliminary

C-scans

Site 1Site 2Site 3Site 4Site 5

distance of 

site from 

billet center

0”

Side 1

Side 4

3”4” 2” 1”

OD face for

preliminary

C-scans

Site 1Site 2Site 3Site 4Site 5

distance of 

site from 

billet center

PROPERTY MEASUREMENTS WERE MADE 

COUPONS, ILLUSTRATED FOR THE GFM-A BILLET STRIP 

G1

The end piece

has a curved 

outer surface

with an engraved

identification

number on “side 1”.

P

Side 2

Side 3

G5 G4 G3 G2 G1 G0 G -1

These engraved markings

identify the slices. Markings shown above

are for GFMA. VdieA markings begin with 

“A”. VdieB markings begin with “B”.

Waspaloy markings begin with “W”.

Center of billet strip 

will be in the center 

of the first slice 

(slice G0)

Saw cut

End

piece

1.5”

0.5”

2.5”

3.5”

4.5”

0.5”

Exact

Center of

Strip Coupon

G5 G4 G3 G2 G1 G0 G - 1

P Side 2

Final saw cut removes

all or nearly all of the

curvature.

G1

The end piece

has a curved 

outer surface

with an engraved

identification

number on “side 1”.

P

Side 2

Side 3

G5 G4 G3 G2 G1 G0 G -1

These engraved markings

identify the slices. Markings shown above

are for GFMA. VdieA markings begin with 

“A”. VdieB markings begin with “B”.

Waspaloy markings begin with “W”.

Center of billet strip 

will be in the center 

of the first slice 

(slice G0)

Saw cut

End

piece

G1

The end piece

has a curved 

outer surface

with an engraved

identification

number on “side 1”.

P

Side 2

Side 3

G5 G4 G3 G2 G1 G0 G -1

These engraved markings

identify the slices. Markings shown above

are for GFMA. VdieA markings begin with 

“A”. VdieB markings begin with “B”.

Waspaloy markings begin with “W”.

Center of billet strip 

will be in the center 

of the first slice 

(slice G0)

Saw cut

End

piece

1.5”

0.5”

2.5”

3.5”

4.5”

0.5”

Exact

Center of

Strip Coupon

G5 G4 G3 G2 G1 G0 G - 1

P Side 2

Final saw cut removes

all or nearly all of the

curvature.

1.5”

0.5”

2.5”

3.5”

4.5”

0.5”

Exact

Center of

Strip Coupon

G5 G4 G3 G2 G1 G0 G - 1

P Side 2

Final saw cut removes

all or nearly all of the

curvature.

3-24
3.1.3 Methods Used for Detailed UT Property Measurements. 

This section will discuss the procedures used for detailed UT property measurements and show 

representative examples of the results. For a given site and beam propagation direction, 

longitudinal-wave measurements were performed in the following order: sound velocity, 

attenuation, and backscattered noise. This order was necessary because (1) sound velocity is a 

required input for the attenuation measurement procedure that was used and (2) both velocity and 

attenuation are inputs for the grain noise FOM extraction procedure. All the measurements were 

performed with the specimens immersed in distilled water using the same two transducers that 

were used for the preliminary C-scans. However, a different UT measurement system was used 

consisting of a Panametrics 5052 pulser/receiver, a Lecroy 9400A digitizing oscilloscope, a 

Testek water tank with a motorized XY scanning bridge, a manual gimble-gimble goniometer for 

adjusting the transducer orientation, and a personal computer to control scanning and data 

acquisition. The A-scans (RF waveforms) required for attenuation and FOM determination were 

digitized at a 100-MHz sampling rate. 

3.1.3.1 Velocity Measurements. 

The velocity at each measurement site was determined from the average time delay between 

successive back-wall echoes using the setup shown in figure 3-28. The first three back-wall (or 

back-surface) echoes were generally used, denoted BS1, BS2, and BS3, respectively, in 

figure 3-28. The two transducer orientation angles were adjusted so that the incident beam was 

as normal as possible to the front and back surfaces of the specimen; this was usually done by 

maximizing the amplitude of the second back-wall echo, BS2. The time delay between two 

echoes was specifically defined as the time difference between the corresponding zero crossing 

points just preceding the dominant positive peaks in the waveforms. In practice, time delays 

were measured with the LeCroy oscilloscope alone by (1) amplifying the second echo until its 

peak-to-peak amplitude approximately matched that of the first echo and (2) shifting the second 

echo to the left by the amount necessary to overlap the designated zero crossing points. This 

procedure is illustrated in figure 3-29 for two typical cases. Note in the figure that the successive 

back-wall echoes decrease more quickly in amplitude for Waspaloy than for GFM-A, indicating 

a higher attenuation in the Waspaloy specimen. Because of this higher attenuation, there are 

more pronounced changes in the shapes of the successive back-wall echoes. Early measurements 

for selected cases using both 5- and 10-MHz planar transducers found very little difference 

between the two measured velocities, i.e., typically less than one-tenth of 1 percent. 

Consequently, the dependence of velocity on frequency was deemed to have little impact on 

practical inspections, and subsequent velocity measurements were made using only the 10-MHz 

transducer. 

3-25
BS1 BS2 BS3

Ni

5.0 cm

water path

0.25”-diameter

10 MHz, planar

transducer

BS1 BS2 BS3

Ni

5.0 cm

water path

0.25”-diameter

10 MHz, planar

transducer

BS1 BS2 BS3

Ni

5.0 cm

water path

0.25”-diameter

10 MHz, planar

transducer

FIGURE 3-28. SETUP FOR UT VELOCITY MEASUREMENTS USING THE SUCCESSIVE 

BACK-SURFACE REVERBERATIONS OBSERVED AT A FIXED MEASUREMENT SITE 

GFM-A : "Slice" Coupon from Site 0"

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.0 0.1 0.2 0.3 0.4 0.5

Time (microseconds)

Amplitude (Volts)

BS1

BS2 (Shifted and Amplified)

Zero Crossing point

GFM-A : "Slice" Coupon from Site 0"

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30

Time (Microseconds)

Amplitude (Volts)

Saturated Front Surface echo

BS1 BS2 BS3

Waspaloy : "Slice" Coupon from Site 0"

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30

Time (microseconds)

Amplitude (Volts)

Saturated Front Surface echo

BS1 BS2 BS3

Waspaloy : "Slice" Coupon from Site 0"

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.0 0.1 0.2 0.3 0.4 0.5

Time (microseconds)

Amplitude (Volts)

BS1

BS2 (Shifted and Amplified)

Zero Crossing point

(a)

(c) (d)

(b)

GFM-A : "Slice" Coupon from Site 0"

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.0 0.1 0.2 0.3 0.4 0.5

Time (microseconds)

Amplitude (Volts)

BS1

BS2 (Shifted and Amplified)

Zero Crossing point

GFM-A : "Slice" Coupon from Site 0"

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30

Time (Microseconds)

Amplitude (Volts)

Saturated Front Surface echo

BS1 BS2 BS3

Waspaloy : "Slice" Coupon from Site 0"

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30

Time (microseconds)

Amplitude (Volts)

Saturated Front Surface echo

BS1 BS2 BS3

Waspaloy : "Slice" Coupon from Site 0"

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.0 0.1 0.2 0.3 0.4 0.5

Time (microseconds)

Amplitude (Volts)

BS1

BS2 (Shifted and Amplified)

Zero Crossing point

(a)

(c) (d)

(b)

FIGURE 3-29. (a)-(b) A-SCANS SHOWING THE FRONT-WALL (TRUNCATED) AND 

FIRST THREE BACK-WALL ECHOES FOR THE SLICE COUPONS FROM THE CENTERS 

OF THE WASPALOY AND GFM-A BILLETS AND (c)-(d) OVERLAP OF THE FIRST 

TWO BACK-WALL ECHOES AFTER THE SECOND HAS BEEN INCREASED IN 

AMPLITUDE AND TIME SHIFTED 

3-26
Examples of measured sound speeds within the Waspaloy coupon strip are shown in figure 3-30. 

The measured velocities occupied a narrow range of 0.6012 ±0.0003 cm/μsec (or ±0.05%). 

Absolute velocity measurements are typically believed to be accurate to about 1 part in 1000 

(0.10%), usually limited by the accuracy of the thickness measurement (via calipers). For 

example, a thickness measurement error of 1 mil out of 2′′ would shift a given plotted value, in 

figure 3-30, up or down by about 0.0003 cm/μsec. Figure 3-30 shows that measured velocities in 

the Waspaloy strip coupon tend to be slightly lower in the center of the billet than near the OD. 

The results for the full set of sound speed measurements on the Ni alloy coupons will be 

presented and discussed in section 3.1.4.1. 

0.6006

0.6007

0.6008

0.6009

0.6010

0.6011

0.6012

0.6013

0.6014

0.6015

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

Wasp-axial

Wasp-hoop

FIGURE 3-30. RESULTS OF SOUND VELOCITY MEASUREMENTS PERFORMED ON 

THE WASPALOY STRIP COUPON BEFORE SLICING 

3.1.3.2 Attenuation Measurements. 

In the preliminary study, two competing immersion methods for attenuation measurement were 

used, as illustrated in figure 3-31. Method 1 is based on comparing the first back-wall echo in 

the metal specimen to that in an FQ reference block of similar thickness [22]. The attenuation of 

the reference block is assumed to be negligible. Method 2 is based on comparing the first three 

metal back-wall echoes with one another [25]. Method 1 was originally developed for scanning 

applications where the effective attenuation varies from point to point and an average attenuation 

value within a scan region is desired. However, it can also be used for single-point 

measurements, as was the case here. Method 2 does not require a reference block; in addition, it 

is preferred for low-attenuation materials because longer sonic path lengths allow the effects of 

attenuation to accumulate, resulting in greater (and more readily measured) reductions to backwall amplitudes. In these attenuation measurements, a 1/4′′ diameter, 10-MHz, broadband planar 

transducer was used. The water path was typically 5 cm, and the incident sonic beam was 

perpendicular to the entry surface of the FQ, or metal block, with beam normalization 

accomplished in the same manner as discussed above for sound velocity measurements. 

3-27
BS1 BS2 BS3 ....

Ni

Method 1 Method 2

(software: MGMDec99.exe) (software: B123AttenA.exe)

Compare first backwall echo 

with that seen in a fused-quartz 

reference block of similar 

thickness. 

Compare spectra of BS1&BS2, 

BS1&BS3, BS1&BS4, etc.

to deduce one atten-vs-freq

curve for each pair of echoes.

FQ Ni

BS1 BS1

scan

Compatible with automatic

scanning.

More accurate in low-attenuation 

materials since travel path is longer.

BS1 BS2 BS3 ....

Ni

Method 1 Method 2

(software: MGMDec99.exe) (software: B123AttenA.exe)

Compare first backwall echo 

with that seen in a fused-quartz 

reference block of similar 

thickness. 

Compare spectra of BS1&BS2, 

BS1&BS3, BS1&BS4, etc.

to deduce one atten-vs-freq

curve for each pair of echoes.

FQ Ni

BS1 BS1

scan

Compatible with automatic

scanning.

More accurate in low-attenuation 

materials since travel path is longer.

FIGURE 3-31. THE TWO METHODS USED FOR L-WAVE ATTENUATION VERSUS 

FREQUENCY MEASUREMENTS 

In either method, the metal attenuation coefficient at a given frequency (f) is deduced from 

measured spectral components of two back-surface echoes. For any given back-surface echo, the 

spectral component at frequency (f), denoted by Γ(f), is written as a product of factors, which 

account for the effects of the measurement system, beam diffraction, interface losses, and 

attenuation, respectively. For example, in Method 2, the spectral amplitudes of the first two 

back-surface echoes are written respectively as 

( ) ( ) ( ) [ ( ) ( ) ] 1 101101 01100 TRTfDff 2-exp 2 ZfZf BS =Γ β α − α (3-1) 

( ) ()() ( ) [ ( ) ( ) ] 10 01100

3

2 1101 TRTfDff 2-exp 4 ZfZf BS =Γ β α − α (3-2) 

Here, β is related to the efficiency with which electric energy is converted to sound by the 

measurement system, and D describes the effect of beam spread (diffraction) on the back-wall 

echo. T01 and T10 are displacement field transmission coefficients for water-into-solid and solidinto-water, respectively, and R11 is the similar solid-into-solid reflection coefficient at the 

water/solid interface. α0 and α1 are attenuation coefficients for water and solid respectively, and 

Z0 and Z1 are the respective thickness of the water layer and the solid specimen. When 

evaluating the reflection and transmission coefficients, plane-wave values were used [22], which 

are independent of frequency and depend solely on the densities and sound speeds of the water 

and solid media. The diffraction correct D is calculated using the well-known Lommel formula 

[22] for diffraction from an ideal planar piston transducer. D depends on the sonic frequency, 

the sound speeds in the fluid and solid, the path lengths in each media, and the diameter of the 

3-28
transducer. The nominal diameter of the transducer (i.e., 0.25 in. = 0.635 cm) was assumed 

when diffraction corrections were computed. By measuring the spectral amplitudes Γ and using 

models, as described, to calculate the R, T, and D factors, equations 3-1 and 3-2 may be regarded 

as a system of two equations with two unknowns: the system efficiency factor β and the metal 

attenuation coefficient α1. β can be eliminated by dividing the two equations, leaving a single 

equation that expresses α1 in terms of the ratio of the spectral components of the two backsurface echoes. Since Fast Fourier Transform operations are applied to the discretized back-wall 

A-scan signals, the spectral components (and hence the deduced attenuation values) are obtained 

at a number of discrete frequencies within the transducer’s bandwidth. 

Two systems of units are commonly used for quoting attenuation values. The attenuation 

coefficient α1, as it appears in equation 3-1 or 3-2, is usually specified in Nepers per centimeter 

(N/cm), with the Neper being a dimensionless unit. In industrial settings, it is common to 

describe the loss in signal amplitude (in dB units) due to attenuation while propagating over a 

given distance (say, in inches). The attenuation value in dB/inch units can be obtained by 

multiplying the value in N/cm by 22.06. For example, 0.1 N/cm would be equivalent to 2.206 

dB/inch. 

It has been described in detail how the metal attenuation coefficient α1 is determined using 

Method 2 and the first two back-surface echoes. A similar logic can be followed for other cases 

in which α1 is deduced from the spectral components of a pair of echoes (e.g., Method 1, or 

Method 2 with other reflections). Additional discussion of the attenuation measurement 

techniques can be found in references 1, 17, 19, 26, and 29. Note that when Method 2 is used 

together with the first three back-surface echoes, there are three different ways of choosing a pair 

of echoes (BS1+BS2, BS1+BS3, BS2+BS3) and each such pair leads to an attenuation estimate. 

However, one can show that only two of the three estimates are independent results, and that the 

third estimate can be expressed in terms of the other two. BS1+BS2 and BS1+BS3 were 

regarded as the independent pairs. 

When making the diffraction corrections, it is tacitly assumed that the sound beam does not 

appreciably interact with the lateral sides of the specimen. The multi-Gaussian beam model [26

and 27] was used to calculate the lateral size of the propagating beam within the metal coupons, 

with examples shown in figure 3-32. Theory predicts that the strength of the back-wall response 

should be proportional to the square of the incident pressure field integrated over the back wall 

[15]. Thus, the square of the pressure amplitude for various frequencies was plotted, as shown in 

figure 3-32. A 0.25′′ diameter planar transducer operating at a 5-cm water path was assumed and 

a metal sound speed appropriate for Waspaloy was (0.601 cm/μsec). A distance-amplitude 

correction has been applied to keep the peak pressure-squared value independent of depth. This 

makes it easier to discern, at each metal depth, the relative decrease in field strength as one 

moves laterally away from the beam center. The vertical lines indicate the effective locations of 

the reflecting surface for the BS1, BS2, and BS3 echoes when measurements are made on the 2′′

thick strip specimens. For the 1′′ thick slice specimens, the three reflecting surfaces would be 

half as deep. For either the strip or slice coupons, the lateral width (perpendicular to the beam 

propagation direction) is 2′′, which is indicated on the figure. In figure 3-32, it is of interest to 

identify those cases in which the beam appreciably intersects the specimen’s sidewalls while on 

its way to the back wall. Note, for example, that the 3-MHz component of the beam for the BS3 

3-29
echo clearly interacts with the sidewalls of the 2′′ wide coupon. Based on such calculations, it 

was estimated that Method 2 diffraction corrections could likely be trusted above about 3.5 MHz 

when BS1 and BS2 echoes were used and above about 5.0 MHz when BS1 and BS3 echoes were 

used. 

6”

2”

BS1 BS2 BS3

10 MHz

7 MHz

2”

6”

2”

BS1 BS2 BS3

5 MHz

3 MHz

2”

0% 20% 40% 60% 80% 100%

Pressure Squared:

6”

2”

BS1 BS2 BS3

10 MHz

7 MHz

2”

6”

2”

BS1 BS2 BS3

10 MHz

7 MHz

2”

6”

2”

BS1 BS2 BS3

5 MHz

3 MHz

2”

6”

2”

BS1 BS2 BS3

5 MHz

3 MHz

2”

0% 20% 40% 60% 80% 100%

Pressure Squared:

FIGURE 3-32. COMPUTED RADIATION PATTERNS FOR THE SONIC BEAM FROM A 

0.25′′ DIAMETER TRANSDUCER AT FOUR FREQUENCIES OF INTEREST 

When deciding upon the valid frequency range for an attenuation measurement at a given site, 

one must also consider the bandwidths of the two echoes from which the attenuation 

determination is to be made, as illustrated in figure 3-33. For one site on the GFM-A strip 

coupon, the spectral components are shown for the various echoes used in the attenuation 

determination and the attenuation curves deduced from the spectra. Note that metal attenuation 

causes the peak frequency in the spectrum to shift downward from its nominal value of 10 MHz 

and, hence, to influence the bandwidths of the various back-surface echoes. For each analysis 

(using a pair of echoes), the resulting attenuation versus frequency curve possesses a general 

systematic trend (monotonically increasing faster than linear) with some scatter about that trend. 

The scatter tends to be larger at the lowest and highest frequencies in the plotted range where the 

spectral amplitudes of the echoes are weakest. Accurate attenuation measurements are most 

difficult for low-attenuation materials, since small errors in setup (probe alignment and position) 

can induce small back-wall echo changes on the same order as those resulting from material 

attenuation. For this reason, the scatter typically appears worse at low frequencies where the 

attenuation is smallest, other factors being equal. 

3-30
 (a) 

GFM-A Side2 (Axial) at Billet Center (0")

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0 2 4 6 8 10 12 14 16 18 20

Frequency (MHz)

Spect

 (b) 

FIGURE 3-33. ATTENUATION MEASUREMENT FOR AXIAL SOUND PROPAGATION 

AT THE CENTER OF THE IN718 GFM-A BILLET STRIP COUPON (a) SPECTRA OF 

BACK-WALL ECHOES ON WHICH THE MEASUREMENTS ARE BASED AND 

(b) DEDUCED ATTENUATION VS FREQUENCY CURVES 

Another example of measured back-wall spectra and the resulting attenuation values is shown in 

figure 3-34. For this specific Waspaloy case, the largest attenuation seen anywhere within the 

suite of specimens was observed. This large attenuation results in a pronounced downshifting of 

the center frequency with increasing travel path. Note that in both figures 3-33(b) and 3-34(b) 

good agreement between the Method 1 and Method 2 attenuation results is generally seen in the 

frequency range for which the relevant spectral components are all appreciable. This frequency 

range is significantly wider for the GFM-A example. ral Amplitude

GFM-A.BS1 (Gain = 6dB)

GFM-A.BS2 (Gain = 6dB)

GFM-A.BS3 (Gain = 6dB)

FQ (gain = 20dB)

GFM-A Side2 (Axial) at Billet Center

Comparison of Attenuation Measurement Methods

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

3 4 5 6 7 8 9 10 11 12 13 14 15 16

Frequency (MHz)

Attenu (dB/in) . ation

Method2 (Ave)

Method2 (BS1&BS2)

Method2 (BS1&BS2)

Method1 (FQ Ref)

In this figure only, “Gain” refers 

to the attenuator setting on the 

pulser/receiver. Thus the signal 

amplification level was 14 dB 

higher for the metal specimen 

than for FQ.

GFM-A Side2 (Axial) at Billet Center (0")

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0 2 4 6 8 10 12 14 16 18 20

Frequency (MHz)

Spect mplitude ral A

GFM-A.BS1 (Gain = 6dB)

GFM-A.BS2 (Gain = 6dB)

GFM-A.BS3 (Gain = 6dB)

FQ (gain = 20dB)

GFM-A Side2 (Axial) at Billet Center

Comparison of Attenuation Measurement Methods

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

3 4 5 6 7 8 9 10 11 12 13 14 15 16

Frequency (MHz)

Attenu (dB/in) . ation

Method2 (Ave)

Method2 (BS1&BS2)

Method2 (BS1&BS2)

Method1 (FQ Ref)

In this figure only, “Gain” refers 

to the attenuator setting on the 

pulser/receiver. Thus the signal 

amplification level was 14 dB 

higher for the metal specimen 

than for FQ.

3-31
 (a) 

0.0

1.0

2.0

3.0

4.0

5.0

0 2 4 6 8 10 12 14 16 18 20

Frequency (MHz)

Spectral

FQ (Gain = 0 dB)

 (b) 

FIGURE 3-34. ATTENUATION MEASUREMENT FOR AXIAL SOUND PROPAGATION 

AT THE CENTER OF THE WASPALOY BILLET STRIP COUPON (a) SPECTRA OF 

BACK-WALL ECHOES ON WHICH THE MEASUREMENTS ARE BASED AND 

(b) DEDUCED ATTENUATION VS FREQUENCY CURVES 

In summary, it was found that good agreement between the two methods was generally obtained 

if one 

• takes care to carefully reposition the transducer so that comparative measurements are 

made at the same physical location in the specimen. 

• discounts results where the sonic beam interacts appreciably with the lateral edges of the 

coupon. 

• examines the spectra of the back-wall echoes being used to determine the valid frequency 

range for the measurement. Amplitude .

Wasp.BS1 (Gain = 22 dB)

Wasp.BS2 (Gain = 22 dB)

Wasp.BS3 (Gain = 22 dB)

0.0

1.0

2.0

0

4.0

5.0

6.0

345678

Frequency (MHz)

Attenuat dB/inch)

3.

ion (

Method 1 (FQ Ref)

Method 2 (BS1 & BS2)

Method 2 (BS1 & BS3)

Method 2 (Ave)

0.0

1.0

2.0

3.0

4.0

5.0

0 2 4 6 8 10 12 14 16 18 20

Frequency (MHz)

Spectral plitude .

FQ (Gain = 0 dB)

Wasp.BS1 (Gain = 22 dB)

Wasp.BS2 (Gain = 22 dB)

Wasp.BS3 (Gain = 22 dB)

 Am

0.0

1.0

2.0

0

4.0

5.0

6.0

345678

Frequency (MHz)

Attenuat dB/inch)

Method 1 (FQ Ref)

Method 2 (BS1 & BS2)

Method 2 (BS1 & BS3)

Method 2 (Ave)

3.

ion (

3-32
• uses the same equipment gain setting for all echoes that contribute to the measured 

attenuation at a given site. The measured attenuation was quite low for many of the 

specimen/measurement site combinations considered. 

In such cases, the small effect of attenuation on the spectrum of an echo could be similar to that 

caused by small-equipment nonlinearities introduced when gain settings were changed. For a 

given measurements system, a gain change of 10 dB may not alter every frequency component of 

an echo by exactly 10 dB. To limit such nonlinearities, the gain setting of the pulser/receiver 

was fixed at each measurement site, and only the volts/division setting of the digitizing 

oscilloscope was altered. 

Because attenuation values for the suite of specimens as a whole tended to be rather small, a 

decision was made to adopt Method 2 as the preferred measurement technique. Two attenuation 

estimates were then made for each measurement case: one from the BS1 and BS2 echoes and 

one from BS1 and BS3. For each estimate, the valid frequency range was then estimated from 

beam spread and bandwidth considerations. At frequencies where both the BS1+BS2 and 

BS1+BS3 estimates were deemed valid, the two estimates were averaged. Otherwise, the quoted 

result is that for the BS1+BS2 analysis alone. In some cases, to check repeatability, multiple 

measurement trials were made. In those cases, the reported values are an average over trials. 

The decision was to only trust results for which the spectral amplitudes of both echoes were 

deemed to be appreciable. Appreciable was usually interpreted as meaning “having a spectral 

amplitude of at least 10%-15% of the maximum amplitude for that echo.” In practice, the lower 

limit on frequency was usually determined by the beam spread considerations (see figure 3-32), 

while the upper limit was determined by the 15% spectral amplitude rule. As the attenuation of 

the specimen increased, the upper limit on the trustworthy frequency range decreased. Table 3-3 

relates the upper frequency limit for trustworthy measurements to the deduced attenuation at one 

frequency of interest, namely, 8 MHz. This table was used in most instances to rapidly estimate 

frequency limits for quoted attenuation values. 

TABLE 3-3. APPROXIMATE UPPER FREQUENCY LIMIT FOR TRUSTWORTHY 

ATTENUATION MEASUREMENTS AS A FUNCTION OF THE MATERIAL 

ATTENUATION NEAR 8 MHz 

Atten. at 8 MHz 

(N/cm) 

Attn. at 8 MHz 

(dB/inch) 

Max freq. for 

BS1+BS2 

Max freq. for 

BS1+BS2 

0.18 4.0 7.7 6.6 

0.08 1.8 9.8 8.9 

0.02 0.4 13.2 12.4 

0.005 0.1 15.1 14.2 

<0.005 <0.1 15.7 ~15.7 

Note: Measurements used a 0.25′′ diameter, 10-MHz broadband planar probe. 

In C-scan studies (see section 3.1.2.1) of back-wall echo amplitude, it was noted that the 

amplitude varied significantly with radial position in all four of the strip coupons. As expected, 

3-33
the same trends were seen for measured UT attenuation values. Figure 3-35 shows one example 

of how measured attenuation-versus-frequency curves vary with radial position. For the case 

shown, namely, hoop propagation at sites in the IN718 GFM billet, the measured attenuation at a 

given frequency is highest near the billet center and decreases as the billet OD is approached. A 

similar behavior of decreasing attenuation with increasing distance from the billet center was 

also seen for the Waspaloy and V-Die-B coupons (but not for V-Die-A). Measured attenuation 

values, like those shown in figure 3-35, are typically believed to be reproducible to within about 

±0.01 N/cm or ±0.2 dB/inch, meaning that trained operators performing measurements at the 

same site on different days would generally obtain results differing by 0.01 N/cm or less. This 

error estimate is based on duplicate measurements made on selected specimens. When sufficient 

spectral strength is present to easily perform an attenuation measurement, the final accuracy of 

the measurement is usually dependent on the operator’s ability to adjust the two probe 

orientation angles so that the incident beam is normal to the entry surface. For low-attenuation 

materials, a small angulation error of 0.1 or 0.2 degrees can introduce an appreciable error into 

the measured attenuation. 

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0 5 10 15 20

Frequency (MHz)

L-wave Atten. (N/cm)

4" from center

3" from center

2" from center

1" from center

0" from center

3.08

2.65

2.21

1.76

1.32

0.88

0.44

0.00

Attenuation (dB/inch)

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0 5 10 15 20

Frequency (MHz)

L-wave Atten. (N/cm)

4" from center

3" from center

2" from center

1" from center

0" from center

3.08

2.65

2.21

1.76

1.32

0.88

0.44

0.00

Attenuation (dB/inch)

FIGURE 3-35. MEASURED ATTENUATION VALUES IN THE 5-15 MHz RANGE AT 

THE FIVE SITES IN THE GFM-A BILLET STRIP 

(Beam propagation is in the hoop direction, i.e., perpendicular to side 3 of the strip coupon.) 

In C-scan studies of the strip coupons (section 3.1.2.1), it was also noted that at a given radial 

site there was generally little difference in the back-wall amplitudes, as measured in the axial and 

hoop directions. As expected, this tendency was also seen in the measured attenuation values. 

Examples are shown figure 3-36, where one sees both the variation in measured attenuation with 

radial position and the similarity in attenuation values for propagation in the axial and hoop 

directions. In fact, as discussed in section 3.1.4.2, measured attenuation values at a given site in 

a given specimen tended to be fairly isotropic, i.e., similar for all three orthogonal propagation 

directions. 

3-34
 (a) 

Measured L-Wave Attenuation for the Waspaloy Strip Coupon

-0.02

0.00

0.02

0.04

 (b) 

FIGURE 3-36. DEDUCED ATTENUATION VALUES AT VARIOUS SITES FOR 

AXIAL AND HOOP PROPAGATION IN THE (a) WASPALOY AND 

(b) IN718 GFM-A STRIP COUPONS 

ISU models for simulating inspections often assume that the attenuation can be approximated as 

a power law in frequency: 

Attenuation in N/cm units = C (frequency in MHz) p (3-3) 

For this reason, each measured attenuation-versus-frequency curve was fit to such a power law. 

The fitting process consisted of adjusting the values of the multiplicative constant (C) and 

exponent (p) to minimize the sum of the squares of the differences between the measured 

attenuation values and those obtained by evaluating the power law at the discrete frequencies in 

question. Examples of four such fits are shown in figures 3-37 and 3-38. 

0.

wave Atten

06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

3 4 5 6 7 8 9 10 11 12 13 14

Frequency (MHz)

L- uation (Np/cm) .

Axial - 0"

Axial - 1"

Axial - 2"

Axial - 3"

Axial - 4"

Hoop - 0"

Hoop - 1"

Hoop - 2"

Hoop - 3"

Hoop - 4"

3.97

3.53

3.09

2.65

2.21

1.76

1.32

0.88

0.44

0.00

dB/in

Measured L-Wave Attenuation for the GFM-A Strip Coupon

-0.01

0.00

0.01

0.02

0.03

0.04

05

06

0.07

0.08

0.09

0.10

0.11

0.12

3 4 5 6 7 8 9 10 11 12 13 14 15 16

Frequency (MHz)

L-wave Atten n (Np/cm) .

0.

0.

uatio

Axial - 0"

Axial - 1"

Axial - 2"

Axial - 3"

Axial - 4"

Hoop - 0"

Hoop - 1"

Hoop - 2"

Hoop - 3"

Hoop - 4"

2.65

2.21

1.99

1.76

1.32

1.10

0.88

0.66

0.44

0.00

dB/in

1.54

0.22

3-35
-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

3 4 5 6 7 8 9 10 11 12 13 14

Frequency (MHz)

L-wave Attenuation (Np/cm) .

Hoop - 1"

Hoop - 4"

Hoop - 1" - fit

Hoop - 4" - fit

 C=0.0000064917

 P=3.7806

 C=0.0000013957

 P=3.8268

FIGURE 3-37. POWER-LAW FITS TO MEASURED ATTENUATION VALUES AT TWO 

LOCATIONS IN THE IN718 GFM-A BILLET SPECIMEN 

(Measurements are for hoop propagation at sites 1′′ and 4′′, respectively, from the billet center.) 

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

3 4 5 6 7 8 9 10 11 12 13 14

Frequency (MHz)

L-wave Attenuation (Np/cm) .

Hoop - 1"

Hoop - 4"

Hoop - 1" - fit

Hoop - 4" - fit

 C=0.000077678

 P=3.8129

 C=0.000011981

 P=3.6381

FIGURE 3-38. POWER-LAW FITS TO MEASURED ATTENUATION VALUES AT TWO 

LOCATIONS IN THE WASPALOY BILLET SPECIMEN 

(Measurements are for hoop propagation at sites 1′′ and 4′′, respectively, from the billet center.) 

3-36
Generally speaking, it was found that each of the power-law fits provided a good overall 

summary of the trend of the measured data. One should be aware, however, that there are often 

other {C, p} pairs for a given case that provide a fit of similar overall quality to that of the 

quoted pair. Also, if one alters the frequency range of the measured data contributing to the fit, a 

different {C’, p’} pair will generally result. For two fits using different ranges of the same data, 

it is not too unusual to find differences in exponents of |p-p’| > 0.2 or more. For the Ni billet 

specimens in the frequency range of interest (5-15 MHz), power-law fits, in virtually all cases, 

yielded powers in the 3 < p < 4 range. 

In all, attenuation-versus-frequency measurements were carried out for 60 cases: four billet 

specimens, five sites per specimen and three propagation directions per site. The results, 

including fitted power-law parameters, are summarized and discussed in section 3.1.4.2. 

3.1.3.3 Grain Noise FOM Measurement. 

Detailed backscattered noise measurements were made at the same sites where attenuation values 

were measured. The goal was to determine the frequency-dependent FOM parameter that is a 

measure of the noise-generating capacity of the microstructure. The basic measurement setup is 

shown in figure 3-39. A focused transducer, oriented at normal incidence, is scanned above the 

metal specimen. No distance-amplitude compensation is applied. For such a constant-gain 

measurement, the backscattered noise level tends to be highest for scattering from the focal zone 

[20, 21, and 22]. Digitized grain noise A-scans are acquired at a few hundred transducer 

positions and stored for later analysis. A reference echo is also acquired and subsequently used 

to deduce the measurement system efficiency factor. For FOM determination from measured 

noise data, it is important to accurately know the focal properties of the transducer being used. 

The same 1/2′′ diameter, 15-MHz broadband transducer used earlier for the preliminary noise 

C-scans was used for the FOM measurements. This transducer had previously been 

characterized using beam-mapping methods to determine its true focal characteristics. The basic 

characterization procedure is described in reference 22 and involves comparing measured 

responses from a small reflecting target scanned along the beam axis to model predictions for an 

ideal focused piston transducer (IFPP). The transducer in question was found to have an 

effective diameter of 1.21 cm (0.48′′), a geometric focal length in water of 9.65 cm (3.80′′), and 

an actual focal length of 9.2 cm (3.6′′) in water. 

Ni-alloy 

Coupon

Scan 

Fused Quartz 

Reference

Grains

Ni-alloy 

Coupon

Scan 

Fused Quartz 

Reference

Grains

FIGURE 3-39. IMMERSION SETUP FOR GRAIN NOISE FOM MEASUREMENT 

3-37
For the noise measurements on the Ni billet strip coupons, the water path was adjusted to place 

the actual beam focus 0.6′′ deep in the metal. Because of metal sound speed differences, the 

water path varied slightly from specimen to specimen but was typically near 1.3′′. For the slice 

coupons, the beam focus was approximately 0.5′′ deep in the metal, and the water path was 

typically 1.7′′. At each measurement site, the transducer was scanned over a 0.54′′ by 0.54′′ area 

using a step size of 0.03′′ in each direction. Backscattered noise A-scans were digitized at a 

100-MHz sampling rate and stored. The depth (time) region for which data was stored was 

approximately 1.8′′ (16 μsec) for the strip coupons and 0.9′′ (8 μsec) for the thinner slice 

coupons. At each transducer position, 500 A-scans were typically acquired, and these were 

averaged to reduce the contribution of measurement system (electronic) noise. In all cases, the 

reference signal was a back-wall echo from a 1/2′′ thick FQ block with the water path adjusted to 

position the geometric focus at the back wall. 

In deducing the value of the grain noise FOM at a given frequency, a time gate is first selected 

for the analysis, typically one bracketing the focal zone where the average noise level tends to be 

highest. Using that gate, the spectrum of each stored noise A-scan is computed, and the spectra 

are averaged. For technical reasons, a root mean-squared (rms) average is computed at each 

frequency rather than a simple average. The rms noise spectrum is then compared to the 

spectrum of the reference echo, and the formalism of the so-called independent scatterer noise 

model is used to deduce an FOM-versus-frequency curve. 

The independent scatterer noise model, the associated FOM extraction procedure, and examples 

of deduced FOM-versus-frequency curves for metals are discussed in detail in references 20-22 

and references 28-34. In addition, appendix B of reference 1 describes the historical 

development of the evolving grain noise models and summarizes their practical uses. As 

illustrated in figure 3-40(a), the independent scatterer model makes the assumption that the 

observed noise is dominated by single-scattering events, i.e., events in which sound energy is 

directly reflected back to the transducer by a metal grain. In particular, the model assumes that 

the received noise is an incoherent superposition of the direct echoes from all the insonified 

grains. As summarized in figure 3-40(b), the model formalism expresses the rms average 

spectral amplitude of the received noise at a given frequency in terms of a product of factors. 

The factors depend on both the details of the measurement system (e.g., transducer focal 

properties, time gate, system efficiency) and on the properties of the metal specimen (sound 

speed, attenuation, and noise capacity). One of the factors, known as the FOM, is a direct 

measure of the noise generation capacity of the microstructure. By using the measured reference 

signal spectrum to deduce the measurement system efficiency factor (β), the FOM can be 

deduced from the measured noise data. The standard unit for the FOM is cm –1/2. 

The determination of the grain noise FOM requires analysis of backscattered noise RF 

waveforms within some designated time gate. A time gate is generally used that brackets the 

focal zone of the transducer being used because the grain noise level tends to be highest there; 

this reduces the impact of any residual electronic noise that remains after signal averaging. In 

particular, for the results quoted in section 3.1.4.2, a time gate of 5.11 μsec duration (512 

discrete time points with 0.01 μsec spacing) centered at the focal depth in the metal was used 

with the focal depth computed from the actual focal depth in water and the measured sound 

speeds in the water and metal. The locations of this principal time gate and several other time 

3-38
gates used in preliminary analyses are illustrated in figure 3-41(a). Times are measured relative 

to the center of the front-surface echo (t = 0) so that noise arrival time is directly proportional to 

sound penetration depth. The smaller time gates are designated G1, G2, ..., G6 from left to right 

in the figure, and the larger (principal) gate is designated G7. 

Neglect multiple 

scattering events.

Assume phases are random,

and hence powers add.

Yes

No

1/ 2

4 4 | ( )| | | Z

noise RMS FOM P C e dx dy dz α ω β − ⎡ ⎤ <Γ > ∝ ⎢ ⎥ ⎣ ⎦ ∫∫∫  

Microstructure

factor

Fraction of 

backscattered 

power that that 

falls within the 

time gate

Transducer

efficiency

factor

Integral over 

volume which can 

contribute 

backscattered 

signals within the 

time gate

Incident UT field 

amplitude in the 

metal

(probe radiation 

pattern)

Ultrasonic 

attenuation of 

the metal

(a)

(b)

Neglect multiple 

scattering events.

Assume phases are random,

and hence powers add.

Yes

No

Neglect multiple 

scattering events.

Assume phases are random,

and hence powers add.

Yes

No

1/ 2

4 4 | ( )| | | Z

noise RMS FOM P C e dx dy dz α ω β − ⎡ ⎤ <Γ > ∝ ⎢ ⎥ ⎣ ⎦ ∫∫∫  

Microstructure

factor

Fraction of 

backscattered 

power that that 

falls within the 

time gate

Transducer

efficiency

factor

Integral over 

volume which can 

contribute 

backscattered 

signals within the 

time gate

Incident UT field 

amplitude in the 

metal

(probe radiation 

pattern)

Ultrasonic 

attenuation of 

the metal

1/ 2

4 4 | ( )| | | Z

noise RMS FOM P C e dx dy dz α ω β − ⎡ ⎤ <Γ > ∝ ⎢ ⎥ ⎣ ⎦ ∫∫∫  

Microstructure

factor

Fraction of 

backscattered 

power that that 

falls within the 

time gate

Transducer

efficiency

factor

Integral over 

volume which can 

contribute 

backscattered 

signals within the 

time gate

Incident UT field 

amplitude in the 

metal

(probe radiation 

pattern)

Ultrasonic 

attenuation of 

the metal

1/ 2

4 4 | ( )| | | Z

noise RMS FOM P C e dx dy dz α ω β − ⎡ ⎤ <Γ > ∝ ⎢ ⎥ ⎣ ⎦ ∫∫∫  

Microstructure

factor

Fraction of 

backscattered 

power that that 

falls within the 

time gate

Transducer

efficiency

factor

Integral over 

volume which can 

contribute 

backscattered 

signals within the 

time gate

Integral over 

volume which can 

contribute 

backscattered 

signals within the 

time gate

Incident UT field 

amplitude in the 

metal

(probe radiation 

pattern)

Ultrasonic 

attenuation of 

the metal

(a)

(b)

FIGURE 3-40. (a) THE INDEPENDENT SCATTERER NOISE MODEL NEGLECTS 

GRAIN NOISE CONTRIBUTIONS FROM MULTIPLE-SCATTERING EVENTS AND 

(b) RELATIONSHIP BETWEEN THE AVERAGE MEASURED NOISE SPECTRUM 

AND VARIOUS MICROSTRUCTURE AND MEASUREMENT SYSTEM 

FACTORS IN THE NOISE MODEL 

Figure 3-41(a) also shows the depth dependence of the grain noise level at each of the five 

measurement sites. The plotted quantity is the so-called rms noise level [22, 28, and 29]. This is 

computed directly from the stored A-scan voltages. Recall that at a given measurement site (i.e., 

the billet center) several hundred A-scans are acquired by scanning the transducer over a small 

area centered at the site. At a fixed sound arrival time, i.e., 3.0 μsec after the front surface echo, 

the noise voltages seen at that time in the various A-scans are squared. These squares are then 

averaged and the square root of the average is taken. This process is repeated at each time 

instant. The resulting rms noise level curve generally has a slowly varying underlying trend with 

statistical jitter superimposed on it. The jitter decreases as the number of A-scans used in the 

average increases. In figure 3-41(a), a prominent focal maximum is seen in the rms noise level 

near t = 5 μsec at each of the five measurement sites. In addition, the absolute rms noise level is 

seen to systematically decrease as one moves from the center of the billet toward the OD. The 

3-39
lower panel in figure 3-41(a) displays deduced FOM-versus-frequency curves for four of the 

measurement trials conducted on the GFM-A billet strip. The results are shown for two 

inspection directions (axial and hoop) at two sites (0′′ and 4′′ from the billet center). In each 

case, the principal time gate (G7 in figure 3-41(a)) was used in the analysis. One notices that the 

FOM values tend to rise with frequency at a rate faster than linear, and that the results are 

relatively independent of inspection direction at each site. The deduced FOM values are 

considerably higher near the billet center than near the OD for this specimen. This is as expected 

from the preliminary noise C-scans shown in figure 3-20. 

RMS Noise Levels and Analysis Gates for FOM

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Time (usec)

RMS Noise Level (Volts) .

GFM-A: side 2 site 0"

GFM-A: side 2 site 1"

GFM-A: side 2 site 2"

GFM-A: side 2 site 3"

GFM-A: side 2 site 4"

Smaller time gates have 2.55 usec duration 

and centers (listed below) are spaced about 

2.3 usec apart. Large gate has 5.11 usec 

duration and is centered at 5.58 usec.

3.28 (G1) 5.58 (G2) 7.88 (G3)

10.18 (G4) 12.41 (G5) 14.71 G(6)

5.58 (G7)

(a)

RMS Noise Levels and Analysis Gates for FOM

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Time (usec)

RMS Noise Level (Volts) .

GFM-A: side 2 site 0"

GFM-A: side 2 site 1"

GFM-A: side 2 site 2"

GFM-A: side 2 site 3"

GFM-A: side 2 site 4"

Smaller time gates have 2.55 usec duration 

and centers (listed below) are spaced about 

2.3 usec apart. Large gate has 5.11 usec 

duration and is centered at 5.58 usec.

3.28 (G1) 5.58 (G2) 7.88 (G3)

10.18 (G4) 12.41 (G5) 14.71 G(6)

5.58 (G7)

RMS Noise Levels and Analysis Gates for FOM

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Time (usec)

RMS Noise Level (Volts) .

GFM-A: side 2 site 0"

GFM-A: side 2 site 1"

GFM-A: side 2 site 2"

GFM-A: side 2 site 3"

GFM-A: side 2 site 4"

Smaller time gates have 2.55 usec duration 

and centers (listed below) are spaced about 

2.3 usec apart. Large gate has 5.11 usec 

duration and is centered at 5.58 usec.

3.28 (G1) 5.58 (G2) 7.88 (G3)

10.18 (G4) 12.41 (G5) 14.71 G(6)

5.58 (G7)

(a)

0.00

0.01

0.02

0.03

0.04

0.07

5 6 7 8 9 10 11 12 13 14 15

Frequency (MHz)

Noise FOM (cm^

FIGURE 3-

S USED 

41. (a) ROOT MEAN SQUARE NOISE LEVELS MEASURED AT FIVE SITES 

IN THE GFM-A IN718 BILLET (AXIAL PROPAGATION), AND TIME GATE

FOR FOM DETERMINATION AND (b) DEDUCED FOM VALUES FOR 

SELECTED GFM-A CASES USING THE PRINCIPAL TIME GATE (G7) 

0.05

0.06

-0.5)

Axial-0"

Hoop-0"

Axial-4"

Hoop-4"

GFMA Strip Coupon

(b)

0.00

0.01

0.02

0.03

0.04

0.07

5 6 7 8 9 10 11 12 13 14 15

Frequency (MHz)

Noise FOM (cm^

0.05

0.06

-0.5)

Axial-0"

Hoop-0"

Axial-4"

Hoop-4"

GFMA Strip Coupon

(b)

3-40
Figure 3-42 shows examples of rms noise levels and deduced FOM values for the Waspaloy 

billet. As the billet center is approached, the attenuation becomes substantial (approximately 11 

dB/inch at 10 MHz). This large attenuation causes grain noise from sites deep within a billet 

specimens to be diminished relative to noise from shallower sites. When the attenuation is large 

enough, the focal maximum in the rms noise curve can be suppressed, as is the case in 

figure 3-42(a). It was noted that the UT attenuation, as measured earlier, is used in the FOM 

extraction procedure. As illustrated in figure 3-42(b), the noise analysis leads to FOM values 

near the center of the Waspaloy billet, which are significantly larger than those near the OD. 

Again, this is as expected from the preliminary noise C-scans shown in figure 3-19. As in the 

GFM-A example, the deduced FOM values for Waspaloy at a given measurement site are 

approximately independent of inspection direction, although some differences in the axial and 

hoop FOM values near the billet center are seen at the higher frequencies. This measured 

difference is believed to largely be a manifestation of the uncertainty in attenuation values at the 

higher frequencies. At the 0′′ site in Waspaloy, measured attenuation values are trustworthy up 

to about 8 MHz. For FOM extraction at higher frequencies, the attenuation was estimated by 

extrapolating the fitted power law to higher frequencies. As was shown in figure 36(a), 

attenuation values below 8 MHz are very similar for axial and hoop propagation at the 0′′ site in 

Waspaloy. However, at higher frequencies, the extrapolated attenuation values for axial 

propagation are somewhat larger than those for hoop propagation. This boost in assumed 

attenuation leads to a similar boost in the deduced FOM value and accounts for most of the 

difference between the axial and hoop results at the 0′′ site that are seen at the higher frequencies 

in figure 3-42(b). Larger assumed attenuation values will always lead to larger deduced FOM 

values. This is because a larger attenuation value implies that grain noise echoes will be 

suppressed more by attenuation, thus requiring a larger FOM value (larger backscatter capacity) 

to produce the grain noise that is observed. 

In principle, if the assumptions of the independent scatterer noise model hold, if the radiation 

pattern of the transducer is well modeled, and if the microstructure does not vary with inspection 

depth, then the FOM values deduced from the noise data should be independent of the time gate 

used in the analysis. Several analyses were carried out to examine the dependence of the 

deduced FOM values on the choice of time gate. In one set of calculations, the backscattered 

noise data for the GFM-A IN718 specimen was reanalyzed using the seven time gates shown in 

figure 3-41(a). The resulting FOM-versus-frequency curves are shown in figure 3-43. The 

extraction of FOM values from measured noise echoes requires knowledge of the attenuation of 

the specimen. In particular, the attenuation value used in the analysis should be that which 

describes the rate of loss of sound energy with increasing propagation distance [22, 28, and 31]. 

Two attenuation choices were made in the analyses leading to figure 3-43: (1) using the 

attenuation-versus-frequency curve deduced from earlier back-wall echo measurements and 

(2) assuming the energy loss attenuation to be negligible. The attenuation value appropriate for 

backscattered noise should be between these two limits. This is because attenuation values 

deduced from back-wall reverberations generally contain contributions from both energy loss 

and beam distortion during propagation [15]. Only the energy loss contribution is of importance 

in the backscattered noise analysis. Figure 3-43 shows that there is a systematic dependence of 

the deduced FOM on the time gate choice, especially at higher frequencies, with deeper gates 

generally leading to higher FOM values. This trend is less prominent, but still present, when 

negligible attenuation is assumed. One indication that beam distortion effects are a significant 

3-41
contributor to back-wall signal attenuation is the observation of back-wall echo fluctuations 

when the probe is scanned over a region of uniform average attenuation [15]. Such fluctuations, 

which are often very prominent in Ti alloy billet specimens, were not seen in these Ni billet 

specimens at the inspection frequencies. This suggests that the energy loss attenuation is quite 

close to the measured back-wall attenuation. The measured back-wall attenuation values were 

consequently used as input energy loss values for the final FOM determinations; these results are 

summarized in section 3.1.4.2. 

RMS Noise Levels and Analysis Gates for FOM

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Time (usec)

RMS Noise Level (Volts)

Waspaloy: side 2 site 0"

Waspaloy: side 2 site 1"

Waspaloy: side 2 site 2"

Waspaloy: side 2 site 3"

Waspaloy: side 2 site 4"

Smaller time gates have 2.55 usec 

duration and centers (listed below) 

are spaced about 2.3 usec apart. 

Large gate has 5.11 usec duration 

and is centered at 5.58 usec.

3.28 (G1) 5.58 (G2) 7.88 (G3)

10.18 (G4) 12.41 (G5) 14.71 G(6)

5.58 (G7)

(a)

RMS Noise Levels and Analysis Gates for FOM

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Time (usec)

RMS Noise Level (Volts)

Waspaloy: side 2 site 0"

Waspaloy: side 2 site 1"

Waspaloy: side 2 site 2"

Waspaloy: side 2 site 3"

Waspaloy: side 2 site 4"

Smaller time gates have 2.55 usec 

duration and centers (listed below) 

are spaced about 2.3 usec apart. 

Large gate has 5.11 usec duration 

and is centered at 5.58 usec.

3.28 (G1) 5.58 (G2) 7.88 (G3)

10.18 (G4) 12.41 (G5) 14.71 G(6)

5.58 (G7)

RMS Noise Levels and Analysis Gates for FOM

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Time (usec)

RMS Noise Level (Volts)

Waspaloy: side 2 site 0"

Waspaloy: side 2 site 1"

Waspaloy: side 2 site 2"

Waspaloy: side 2 site 3"

Waspaloy: side 2 site 4"

Smaller time gates have 2.55 usec 

duration and centers (listed below) 

are spaced about 2.3 usec apart. 

Large gate has 5.11 usec duration 

and is centered at 5.58 usec.

3.28 (G1) 5.58 (G2) 7.88 (G3)

10.18 (G4) 12.41 (G5) 14.71 G(6)

5.58 (G7)

(a)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

4 5 6 7 8 9 10 11 12

Frequency (MHz)

Noise FOM (cm^-0.5) .

Axial Gate 7 - 0"

Hoop Gate 7 - 0"

Axial Gate 7 - 4"

Hoop Gate 7 - 4"

Waspaloy Strip Coupon

(b)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

4 5 6 7 8 9 10 11 12

Frequency (MHz)

Noise FOM (cm^-0.5) .

Axial Gate 7 - 0"

Hoop Gate 7 - 0"

Axial Gate 7 - 4"

Hoop Gate 7 - 4"

Waspaloy Strip Coupon

(b)

FIGURE 3-42. (a) ROOT MEAN SQUARE NOISE LEVELS MEASURED AT FIVE SITES 

IN THE WASPALOY BILLET (AXIAL PROPAGATION) AND TIME GATES USED FOR 

FOM DETERMINATION AND (b) DEDUCED FOM VALUES FOR SELECTED 

WASPALOY CASES USING THE PRINCIPAL TIME GATE (G7) 

3-42
GFM-A ( Axial ) Site 0"

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

4 5 6 7 8 9 10 11 12

Frequency ( MHz )

Noise FOM ( cm^-0.5 ) .

Side 2 Gate 1 - 0"

Side 2 Gate 2 - 0"

Side 2 Gate 3 - 0"

Side 2 Gate 4 - 0"

Side 2 Gate 5 - 0"

Side 2 Gate 6N - 0"

Side 2 Gate 7N - 0"

GFM-A ( Axial ) Site 0"

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

4 5 6 7 8 9 10 11 12

Frequency ( MHz )

Noise FOM ( cm^-0.5 ) .

Side 2 Gate 1 - 0" (no att)

Side 2 Gate 2 - 0" (no att)

Side 2 Gate 3 - 0" (no att)

Side 2 Gate 4 - 0" (no att)

Side 2 Gate 5 - 0" (no att)

Side 2 Gate 6N - 0" (no att)

Side 2 Gate 7N - 0" (no att)

(a) (b) 

Using back-wall attenuation Assuming zero attenuation

GFM-A ( Axial ) Site 0"

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

4 5 6 7 8 9 10 11 12

Frequency ( MHz )

Noise FOM ( cm^-0.5 ) .

Side 2 Gate 1 - 0"

Side 2 Gate 2 - 0"

Side 2 Gate 3 - 0"

Side 2 Gate 4 - 0"

Side 2 Gate 5 - 0"

Side 2 Gate 6N - 0"

Side 2 Gate 7N - 0"

GFM-A ( Axial ) Site 0"

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

4 5 6 7 8 9 10 11 12

Frequency ( MHz )

Noise FOM ( cm^-0.5 ) .

Side 2 Gate 1 - 0" (no att)

Side 2 Gate 2 - 0" (no att)

Side 2 Gate 3 - 0" (no att)

Side 2 Gate 4 - 0" (no att)

Side 2 Gate 5 - 0" (no att)

Side 2 Gate 6N - 0" (no att)

Side 2 Gate 7N - 0" (no att)

(a) (b) 

Using back-wall attenuation Assuming zero attenuation

FIGURE 3-43. GRAIN NOISE FOM VALUES FOR AXIAL PROPAGATION AT THE 

CENTER OF THE GFM-A IN718 BILLET DEDUCED USING THE SEVEN TIME GATES 

INDICATED IN FIGURE 40(a); (a) USING THE MEASURED BACK-WALL 

ATTENUATION FOR THE ENERGY LOSS ATTENUATION AND (b) ASSUMING 

THAT THE ENERGY LOSS ATTENUATION IS NEGLIGIBLE 

The fundamental reasons behind the apparent dependence of FOM on gate choice are not fully 

understood. The gate dependence may arise from the neglect of multiple-scattering events by the 

underlying noise model. Near the centers of the Ni alloy billets, the UT properties are relatively 

uniform, suggesting that the microstructures are relatively uniform as well. This affords an 

opportunity to test some predictions of the independent scatterer noise model. For example, the 

model can be used to predict the depth dependence of the rms noise level from an input FOMversus-frequency curve. Such model predictions, which assume a homogeneous (depth 

independent) microstructure, are compared with the experiment in figure 3-44 for axial sound 

propagation near the center of the GFM-A billet. Two model results are shown; these use the 

measured attenuation and zero, respectively, for the energy loss attenuation in the FOM 

extraction analysis and the ensuing rms noise level prediction. The model was shown to do a 

good job of predicting the overall shape and location of the focal maximum. However, the 

measured noise levels tend to be above the predicted result for times that are either well before or 

well after the focal maximum. Departures from the model predictions at early times are not 

unexpected, since ring-down of the front-wall echo tends to boost the measured rms noise level 

over that due to grain noise alone. Departures at later times are of more interest. One possible 

explanation is that multiple-scattering events are contributing significantly to the observed noise, 

particularly in the insonified regions away from the focal zone. These multiple-scattering 

contributions, which are not treated by the model, tend to boost the grain noise level above that 

expected based on single scattering alone. Consequently, when noise data is analyzed using the 

single-scattering model, the enhanced noise level is translated into an overstated FOM value. 

3-43
Measured and Predicted RMS Noise Level 

GFMA Side 2 (axial) Site 0" (billet center)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1

Time (usec)

RMS Noise Level (Volts) .

6

GFM-A: side 2 site 0"

model no atten

model with atten

3.28 5.58

7.88

10.18 12.41

14.7

512 points centered at 5.58usec

Model calculations use 

FOM values deduced by 

analyzing noise within 512-

time point gate

FIGURE 3-44. MEASURED AND PREDICTED RMS GRAIN NOISE LEVELS FOR AXIAL 

BEAM PROPAGATION AT THE CENTER OF THE IN718 GFM-A BILLET 

The observed dependence of FOM on the time gate choice may indeed be due to multiplescattering events and the manner in which they contribute to the observed noise in and away 

from the focal zone. Alternatively, the gate dependence may be due to the accuracy of the ideal 

piston probe assumption: the transducer’s actual radiation pattern may be similar to that of a 

piston probe near the focal zone but depart from it systematically as one moves away from the 

focal zone. Since transducer characterization procedures tend to emphasize data acquired near 

the focal zone, noise data analyses and model predictions based on such transducer 

characterizations are likely more trustworthy in the focal zone. 

Additional experiments were conducted to shed further light on the dependence of deduced FOM 

values on the choice of time gate. The experimental setups and results for two complementary 

FOM measurements in the GFM-A billet are contrasted in figure 3-45. In the first experiment, 

the inspection water path was fixed and grain noise data was gathered. The resulting rms noiseversus-time curve is shown in figure 3-45(b), together with three of the time gates that were 

subsequently used for FOM determination. The FOM-versus-frequency curves, deduced by 

analyzing the spectral components of the backscattered noise within each time gate, are shown in 

figure 3-45(c). One sees that backscattered noise data beyond the focal zone generally led to a 

larger deduced FOM value than noise data from within the focal zone. A second round of 

measurements was then made using the same transducer. As depicted in figure 3-45(d), the 

water path was varied to alter the depth of the beam focus in metal. Shifting the beam focus 

downward caused the peak in the rms grain noise level to shift to later arrival times, as expected. 

For each of the three water paths, the FOM-versus-frequency curve was deduced using a time 

gate enclosing the focal maximum, as shown in figure 3-45(e). The resulting FOM curves 

(figure 3-45(f)) were very similar to one another and in good agreement with the earlier result 

that used noise data from the focal zone. Thus, measured FOM does not appear to depend on 

metal depth per se but rather on the location of the analysis region relative to the focal zone. 

3-44
RMS Noise Level and Analysis Gates 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

2 4 6 8 10 12 14

Time (usec)

RMS Noise Level (Volts) .

GFM-A: side 2 site 0"

A

B

C D

RMS Noise Level and Analysis Gates 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

2 4 6 8 10 12 14

Time (usec)

RMS Noise Level (Volts) .

GFM-A: side 2 site 0"

A

B

C D

Mid Focus

Deep Focus

Shallow Focus

water

metal

Analysis

gates

RMS Noise Levels and Analysis Gates

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

234567

 Time (usec )

RMS Noise Level (Volts) 

8

.

Shallow Focus Deep Focus

Mid Focus

Specimen GFM-A ( Radial ) Site 0"

0.00

0.02

0.04

0.06

0.08

0.10

4 6 8 10 12 14 16

Frequency ( MHz )

Noise FOM ( cm^-0.5 )

Deep-Focus

Mid-Focus

Shallow-Focus

Specimen GFM-A ( Axial ) Site 0"

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

4 5 6 7 8 9 10 11 12

Frequency ( MHz )

Noise FOM ( cm^-0.5 ) 

Gate A

Gate B

Gate C

Gate D

(a) (d)

(b) (e)

(c) (f)

RMS Noise Level and Analysis Gates 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

2 4 6 8 10 12 14

Time (usec)

RMS Noise Level (Volts) .

GFM-A: side 2 site 0"

A

B

C D

RMS Noise Level and Analysis Gates 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

2 4 6 8 10 12 14

Time (usec)

RMS Noise Level (Volts) .

GFM-A: side 2 site 0"

A

B

C D

Mid Focus

Deep Focus

Shallow Focus

water

metal

Analysis

gates

RMS Noise Levels and Analysis Gates

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

234567

 Time (usec )

RMS Noise Level (Volts) 

8

.

Shallow Focus Deep Focus

Mid Focus

Specimen GFM-A ( Radial ) Site 0"

0.00

0.02

0.04

0.06

0.08

0.10

4 6 8 10 12 14 16

Frequency ( MHz )

Noise FOM ( cm^-0.5 )

Deep-Focus

Mid-Focus

Shallow-Focus

Specimen GFM-A ( Axial ) Site 0"

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

4 5 6 7 8 9 10 11 12

Frequency ( MHz )

Noise FOM ( cm^-0.5 ) 

Gate A

Gate B

Gate C

Gate D

(a) (d)

(b) (e)

(c) (f)

FIGURE 3-45. ADDITIONAL FOM MEASUREMENTS AND ANALYSES AT THE 

CENTER OF THE IN718 GFM-A BILLET SPECIMEN (a AND d) MEASUREMENT 

SETUPS (b AND e) RMS GRAIN NOISE PROFILES WITH ANALYSIS GATES 

INDICATED, AND (c AND f) RESULTING FOM VALUES 

Note that the trends evident in figure 3-45 are consistent with preliminary computations of a twodimensional noise model recently developed by R. Roberts and A. Li [35]. In those 

computations, ensembles of model grains are generated with each grain having a randomly 

selected sound velocity in the beam propagation direction (simulating random grain 

orientations). Backscattered rms grain noise levels were then computed, assuming an incidentfocused sound beam. Two types of computations were made: one using the full-model 

formalism that includes both single- and multiple-scattering effects and another in which only 

single-scattering effects were accounted for. The difference in rms noise levels between the fullscattering computation (including multiple-scattering effects) and the single-scattering 

computation was found to increase as one moved away from the focal zone in either direction 

[35]. This led to deduced FOM values that increased away from the focal zone when the singlescatterer model was used in the extraction of FOM from measured noise data. 

3-45
Alternatively, one can craft an explanation for the trends in figure 3-45 that does not rely on the 

presence of a significant multiple-scattering contribution to the observed grain noise. One could 

posit that the transducer in question has a radiation pattern that differs from that of an idea IFPP. 

If, as in figure 3-45(d), one performs a series of measurements that all use the same portion of 

the radiation field (e.g., the focal zone), the error made in assuming that the radiation field is the 

same as that of an IFPP would be similar for each measurement. Thus, the deduced FOM values, 

although in error, would be the same for each measurement, as is the case in figure 3-45(f). 

However, if different experiments use different regions of the radiation field in their analyses (as 

is the case for figure 3-45(a)), the error made by assuming an IFPP radiation field would be 

different for each experiment and hence lead to different deduced FOM values, as is the case in 

figure 3-45(c). Thus, although the results of figure 3-45 are quite interesting, they are open to 

different interpretations. The reasons underlying the dependence of deduced FOM on gate 

choice are expected to be the subject of future research that is beyond the scope of the current 

study. 

Section 3.1.4.2 summarizes the grain noise measurements by tabulating the best estimates of 

FOM for several inspection frequencies of interest, namely, 5, 7.5, 10, and 12.5 MHz. For a 

given coupon site and inspection direction, this involves analyzing backscattered noise data 

within a time gate, fitting a curve through the data in a localized frequency region, and 

evaluating the fitted curve at the frequency of interest. Curve fitting was deemed necessary 

because the FOM-versus-frequency results deduced from a limited number of RF noise 

waveforms display a significant scatter about the general trend, as evidenced by the wiggles seen 

in the FOM curves shown in figures 3-41 through 3-43. The curve-fitting operation is 

demonstrated in figure 3-46 for a typical case. Since the fitting interval is relatively small in 

frequency (about 1.5 MHz in length), it was found that a simple linear fit suffices. 

0.008

0.009

0.010

0.011

0.012

0.013

0.014

6789

Frequency ( MHz)

Noise FOM ( cm^-0.5 ) .

Side 2 Gate 1 - 0"

Side 2 Gate 1 - 0"-FIT

FIGURE 3-46. EXAMPLES OF LOCAL LINEAR FITS USED TO DETERMINE THE BEST 

ESTIMATE OF THE GRAIN NOISE FOM AT 7.5 MHz (a) USING A 256-POINT GATE 

CENTERED 2.38 μsec AFTER THE FRONT-WALL ECHO AND (b) USING A 512-POINT 

GATE CENTERED 5.58 μsec AFTER THE FRONT-WALL ECHO 

(Both fits are for axial propagation near the center of the GFM-A billet.) 

y = 0.003154x - 0.012486

0.008

0.009

0.010

0.011

0.012

0.013

0.014

6789

Frequency ( MHz )

Noise FOM ( cm^-0.5 ) .

Side 2 Gate 7N - 0"

Side 2 Gate 7N - 0"-FIT

y = 0.002940x - 0.011396

(a) (b)

GFMA GFMA

0.008

0.009

0.010

0.011

0.012

0.013

0.014

6789

Frequency ( MHz)

Noise FOM ( cm^-0.5 ) .

Side 2 Gate 1 - 0"

Side 2 Gate 1 - 0"-FIT

0.008

0.009

0.010

0.011

0.012

0.013

0.014

6789

Frequency ( MHz )

Noise FOM ( cm^-0.5 ) .

Side 2 Gate 7N - 0"

Side 2 Gate 7N - 0"-FIT

y = 0.003154x - 0.012486 y = 0.002940x - 0.011396

(a) (b)

GFMA GFMA

3-46
As a practical matter, the dependence of FOM on gate choice and assumed energy loss 

attenuation leads to an uncertainty in measured FOM values. For a given billet specimen, 

measurement site, and inspection direction, uncertainty can be quantified by repeatedly 

analyzing the stored noise data using different time gates and different attenuation choices and 

examining the spread in the resulting FOM values. Such studies have been performed, and 

typical results are shown in figures 3-47 and 3-48 for IN718 and Waspaloy examples, 

respectively. In each figure, the deduced FOM values at 7.5 MHz are plotted versus the 

measured back-wall attenuation, with results shown for each of the seven time gate choices 

indicated in figures 3-41(a) and 3-42(a). In the leftmost panel of figures 3-47 and 3-48, the 

energy loss attenuation assumed in the FOM analysis was equal to the measured back-wall 

attenuation. In the rightmost panel, the energy loss attenuation was assumed to be zero. In both 

figures 3-47 and 3-48, the dependence of deduced FOM on time gate choice is substantial, with 

time gates in or near the focal zone tending to yield the smaller FOM values, as discussed earlier 

in this section. It was believed that the most accurate FOM estimates are obtained when gate 7 is 

used because (1) this gate encloses the bulk of the focal zone where noise levels are high and, 

hence, easily measured and (2) when characterizing the radiation pattern of the transducer in 

question (to obtain the diameter and focal length subsequently used in FOM analyses), data from 

the focal zone were principally used. Thus, plotted and tabulated FOM values, discussed in 

section 3.1.4.2, are always those for gate 7, i.e., a wide gate centered in the focal zone. 

FOM Vs Attenuation for GFM-A (Axial; 7.5 MHz)

(Atten. accounted for in FOM calculations)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.00 0.10 0.20 0.30

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^-0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” 0”

2”

3”

4”

FOM Vs Attenuation for GFM-A (Axial; 7.5 MHz)

(Atten. neglected in FOM calculations)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.00 0.10 0.20 0.30

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^-0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” 0”

2”

3”

4”

FOM Vs Attenuation for GFM-A (Axial; 7.5 MHz)

(Atten. accounted for in FOM calculations)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.00 0.10 0.20 0.30

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^-0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” 0”

2”

3”

4”

FOM Vs Attenuation for GFM-A (Axial; 7.5 MHz)

(Atten. accounted for in FOM calculations)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.00 0.10 0.20 0.30

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^-0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” 0”

2”

3”

4”

FOM Vs Attenuation for GFM-A (Axial; 7.5 MHz)

(Atten. neglected in FOM calculations)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.00 0.10 0.20 0.30

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^-0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” 0”

2”

3”

4”

FOM Vs Attenuation for GFM-A (Axial; 7.5 MHz)

(Atten. neglected in FOM calculations)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.00 0.10 0.20 0.30

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^-0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” 0”

2”

3”

4”

FIGURE 3-47. COMPARISON OF MEASURED FOM AND ATTENUATION VALUES AT 

7.5 MHz FOR THE GFM IN718 BILLET 

(Results are shown for axial beam propagation at five measurement sites located 0′′, 1′′, 

2′′, 3′′, and 4′′ from the billet center, respectively. Deduced FOM values depend on 

the time gate used in the analysis and the treatment of attenuation.) 

For the IN718 specimens, the measured attenuations tend to be rather low and the influence of 

attenuation on the deduced FOM values is rather small, as can be seen by comparing the two 

panels of figure 3-48. For the Waspaloy billet, however, the attenuation is quite large, 

particularly near the billet center, and uncertainty in the knowledge of the energy loss attenuation 

can lead to a much larger uncertainty in the FOM value, as can be seen by comparing the two 

panels in figure 3-48. Because of the smooth variation of back-wall amplitudes seen during 

scanning, it was believed that the back-wall attenuation is a good estimate of the energy loss 

3-47
attenuation in these Ni billet specimens. Consequently, unless explicitly stated otherwise, the 

back-wall attenuation was used in the FOM analysis for all the results quoted in the remainder of 

this report. 

FOM Vs Attenuation for Waspaloy (Axial; 7.5 MHz)

(Atten. accounted for in FOM calculations)

0.000

0.025

0.050

0.075

0.100

0.125

0.150

0.175

0.200

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^- 0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” , 0”

2”

3”

4”

FOM Vs Attenuation for Waspaloy (Axial; 7.5 MHz)

(Atten. neglected for in FOM calculations)

0.000

0.010

0.020

0.030

0.040

0.050

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^- 0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7 2”

3”

4”

1” , 0”

FOM Vs Attenuation for Waspaloy (Axial; 7.5 MHz)

(Atten. accounted for in FOM calculations)

0.000

0.025

0.050

0.075

0.100

0.125

0.150

0.175

0.200

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^- 0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” , 0”

2”

3”

4”

FOM Vs Attenuation for Waspaloy (Axial; 7.5 MHz)

(Atten. accounted for in FOM calculations)

0.000

0.025

0.050

0.075

0.100

0.125

0.150

0.175

0.200

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^- 0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7

1” , 0”

2”

3”

4”

FOM Vs Attenuation for Waspaloy (Axial; 7.5 MHz)

(Atten. neglected for in FOM calculations)

0.000

0.010

0.020

0.030

0.040

0.050

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^- 0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7 2”

3”

4”

1” , 0”

FOM Vs Attenuation for Waspaloy (Axial; 7.5 MHz)

(Atten. neglected for in FOM calculations)

0.000

0.010

0.020

0.030

0.040

0.050

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Attenuation at 7.5 MHz (dB/in)

FOM at 7.5 MHz (cm^- 0.5) . 

Gate 1 Gate 2

Gate 3 Gate 4

Gate 5 Gate 6

Gate 7 2”

3”

4”

1” , 0”

FIGURE 3-48. COMPARISON OF MEASURED FOM AND ATTENUATION VALUES AT 

7.5 MHz FOR THE WASPALOY BILLET, AFTER THE FASHION OF FIGURE 3-47 

Finally, it was noted that the spectra of the reference echo and the backscattered noise A-scans 

must be considered when assessing the reliability of the deduced FOM values. The situation is 

similar to that discussed at length for the attenuation measurements: one desires that spectral 

strength at a frequency of interest be appreciable, e.g., greater that 15% of the strength at the 

peak frequency. This policy has been followed as well for the FOM data. In addition, because 

FOM analysis requires knowledge of the attenuation, FOM values were not quoted for 

frequencies where the attenuation values were deemed to be unreliable, even if there was 

appreciable spectral strength in the reference and (averaged) noise echoes. For example, near the 

center of the Waspaloy billet, FOM values for 10 and 12.5 MHz are not quoted due solely to the 

unreliability of the attenuation data. 

3.1.4 Results of UT Property Measurements. 

This section summarizes the results of the longitudinal-wave velocity, attenuation, and 

backscattered grain noise FOM measurements. For each of these three basic UT properties, 60 

measurement trials were conducted, corresponding to four billet strip specimens, five 

measurement sites per strip, and three inspection directions per site. 

The focus will be on the manner in which UT properties vary with position and inspection 

direction and the correlation between attenuation and FOM values. As demonstrated earlier, 

attenuation and FOM are strong functions of frequency for these Ni alloys, as they are for metals 

in general. However, to maintain focus and to keep this section reasonably short, the main points 

will be graphically illustrated using data for just one frequency of interest, namely, 7.5 MHz. 

3-48
Some of the results shown in this report have been published in other venues [36-38], and the 

latter reference in particular contains additional graphs and tables that do not appear here. These 

may be of interest to readers desiring a more detailed summary of certain aspects of the 

measured results, such as the manner in which deduced FOM values depend on the time gate 

used in the analysis. 

3.1.4.1 Velocity Summary. 

As noted in section 3.1.2.1, the TOF C-scan images for the strip coupons (like that shown in 

figure 3-12) were consistent with having velocity variations of less than about 0.2% throughout 

each coupon. This finding was confirmed by the subsequent detailed velocity measurements, 

which are summarized in table 3-4. For the set of 60 measurements, longitudinal velocities were 

found to range over 0.5805 ±0.0009 cm/μsec (or ±0.15%) for the IN718 specimens and over 

0.6008 ±0.0006 cm/μsec (or ±0.10%) for the Waspaloy specimens. The measured velocities are 

also displayed graphically in figure 3-49. A horizontal line in each panel indicates the average 

for all measurements for a given alloy, i.e., 0.58075 cm/μsec for IN718 (average over GFM-A, 

V-Die-A, V-Die-B) and 0.60098 cm/μsec for Waspaloy. No obvious trends in the velocity 

results are seen in figure 3-49. The minor velocity variations within each of the four billet strips 

may simply reflect the inherent errors in the measurement process. As noted in section 3.1.3.1, 

the absolute velocity measurements are believed to typically be accurate to about 1 part in 1000 

(0.10%), usually limited by the accuracy of the thickness measurement. For the axial and hoop 

directions, a thickness measurement error of 1 mil out of 2′′ would shift a given plotted value in 

figure 3-49 up or down by about 0.0003 cm/μsec. For the radial direction, where the coupons 

are only about 1′′ thick, a 1 mil error would result in a 0.0006 cm/μsec shift. 

In practice, the small velocity variations seen in these Ni alloy billets, in and of themselves, are 

expected to have no significant impact on inspectability. The sonic velocity influences the 

radiation pattern within the billet that is produced by the insonifying transducer. Sizeable 

velocity inhomogeneities can shift the focal depth and alter the degree of focus at the focal zone. 

For the specimens studied, the velocity variations are very small and are unlikely to cause 

noticeable changes in the radiation pattern. For example, in typical multizone billet inspections, 

the focal depth and spot size are expected to be very similar for, say, the GFM billet studied here, 

and an ideal billet having a perfectly uniform sound velocity of 0.58075 cm/μsec, if other factors 

(e.g., attenuation) are the same for both. 

Although this study was focused on gathering UT data to support longitudinal-wave inspections, 

a limited number of shear-wave velocity measurements were also made. These were carried out 

to obtain representative velocity values for testing models that relate sonic velocity to the singlecrystal elastic constants of the metal. The measurements used a 1/2′′ diameter, 5-MHz, shearwave contact transducer with a thin layer of honey serving as a couplant. Time delays between 

successive back-wall echoes were measured (in essentially the same manner as described earlier 

for the longitudinal-wave case) and used to deduce velocities. Results are summarized in 

table 3-5. 

3-49
TABLE 3-4. RESULTS OF LONGITUDINAL-WAVE SOUND VELOCITY 

MEASUREMENTS 

Inspection Site 

Coupon 

Inspection 

Direction 0′′ (center) 1′′ 2′′ 3′′ 4′′

Waspaloy-A axial thickness (in.): 1.9975 1.9975 1.9975 1.9975 1.9975 

Dt (μsec): 16.8853 16.8843 16.8855 16.8795 16.8715 

v (cm/μsec): 0.6010 0.6010 0.6009 0.6012 0.6014 

Waspaloy-A hoop thickness (in.): 2.0000 2.0000 2.0000 2.0000 2.0000 

Dt (μsec): 16.9035 16.9060 16.9040 16.9020 16.8950 

v (cm/μsec): 0.6011 0.6010 0.6010 0.6011 0.6014 

Waspaloy-A radial thickness (in.): 0.9682 0.9722 0.9700 0.9700 0.9700 

Dt (μsec): 8.2065 8.2075 8.2115 8.19475 8.19425 

v (cm/μsec): 0.6006 0.6003 0.6002 0.6013 0.6014 

GFM-A axial thickness (in.): 1.9645 1.9645 1.9645 1.9645 1.9645 

Dt (μsec): 17.1695 17.1663 17.1665 17.1758 17.1833 

v (cm/μsec): 0.5812 0.5814 0.5813 0.5810 0.5808 

GFM-A hoop thickness (in.): 1.9905 1.9905 1.9905 1.9905 1.9905 

Dt (μsec): 17.4030 17.4038 17.4033 17.4023 17.4140 

v (cm/μsec): 0.5810 0.5810 0.5810 0.5811 0.5807 

GFM-A radial thickness (in.): 0.9755 0.9755 0.9755 0.9755 0.9755 

Dt (μsec): 8.5340 8.5375 8.5425 8.5420 8.5412 

v (cm/μsec): 0.5806 0.5806 0.5801 0.5801 0.5802 

V-Die-A axial thickness (in.): 2.0050 2.0050 2.0050 2.0050 2.0050 

Dt (μsec): 17.5183 17.5170 17.5220 17.5320 17.5440 

v (cm/μsec): 0.5814 0.5815 0.5813 0.5810 0.5806 

V-Die-A hoop thickness (in.): 1.9945 1.9945 1.9945 1.9945 1.9945 

Dt (μsec): 17.4548 17.4550 17.4458 17.4358 17.4668 

v (cm/μsec): 0.5805 0.5805 0.5808 0.5811 0.5801 

V-Die-A radial thickness (in.): 0.9755 0.9755 0.9755 0.9755 0.9755 

Dt (μsec): 8.54225 8.52625 8.543 8.5305 8.53775 

v (cm/μsec): 0.5801 0.5811 0.5804 0.5807 0.5805 

V-Die-B axial thickness (in.): 2.0080 2.0080 2.0080 2.0080 2.0080 

Dt (μsec): 17.5480 17.5490 17.5503 17.5593 17.5648 

v (cm/μsec): 0.5813 0.5813 0.5812 0.5809 0.5807 

V-Die-B hoop thickness (in.): 2.0070 2.0070 2.0070 2.0070 2.0070 

Dt (μsec): 17.5390 17.5378 17.5420 17.5553 17.5783 

v (cm/μsec): 0.5813 0.5813 0.5812 0.5808 0.5800 

V-Die-B radial thickness (in.): 0.9820 0.9820 0.9820 0.9820 0.9820 

Dt (μsec): 8.5915 8.60175 8.5955 8.6065 8.6045 

v (cm/μsec): 0.5807 0.5800 0.5804 0.5796 0.5797 

Note: Dt is the average time delay between two successive back-wall echoes. 

3-50
V-Die-A Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

VdieA-Axial

VdieA-Hoop

VdieA-Radial

Ave. over all IN718 

measurements

V-Die-B Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

VdieB-Axial

VdieB-Hoop

VdieB-Radial

Ave. over all IN718 

measurements

GFM-A Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

GFMA-Axial

GFMA-Hoop

GFMA-Radial

Ave. over all IN718 

measurements

Waspaloy Billet Coupons

0.5990

0.5995

0.6000

0.6005

0.6010

0.6015

0.6020

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

Wasp-Axial

Wasp-Hoop

Wasp-Radial

Ave. over all Wasp. 

measurements

V-Die-A Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

VdieA-Axial

VdieA-Hoop

VdieA-Radial

Ave. over all IN718 

measurements

V-Die-A Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

VdieA-Axial

VdieA-Hoop

VdieA-Radial

Ave. over all IN718 

measurements

V-Die-B Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

VdieB-Axial

VdieB-Hoop

VdieB-Radial

Ave. over all IN718 

measurements

V-Die-B Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

VdieB-Axial

VdieB-Hoop

VdieB-Radial

Ave. over all IN718 

measurements

GFM-A Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

GFMA-Axial

GFMA-Hoop

GFMA-Radial

Ave. over all IN718 

measurements

GFM-A Billet Coupons

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

GFMA-Axial

GFMA-Hoop

GFMA-Radial

Ave. over all IN718 

measurements

Waspaloy Billet Coupons

0.5990

0.5995

0.6000

0.6005

0.6010

0.6015

0.6020

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

Wasp-Axial

Wasp-Hoop

Wasp-Radial

Ave. over all Wasp. 

measurements

Waspaloy Billet Coupons

0.5990

0.5995

0.6000

0.6005

0.6010

0.6015

0.6020

0" 1" 2" 3" 4"

Measurement Site (0" = billet center)

Velocity (cm/usec)

Wasp-Axial

Wasp-Hoop

Wasp-Radial

Ave. over all Wasp. 

measurements

FIGURE 3-49. MEASURED LONGITUDINAL-WAVE SOUND VELOCITIES AS 

FUNCTIONS OF RADIAL DEPTH IN THE BILLET 

(Results are shown for three sound propagation directions.) 

3.1.4.2 Attenuation and Backscattered Noise FOM Summary. 

In contrast to the velocity behavior, attenuation and FOM values varied significantly with both 

frequency and position. As illustrated in figures 3-37 and 3-38, a power law was fit to the 

measured attenuation-versus-frequency data for each case studied. Generally speaking, it was 

found that each of the power-law fits provided a good overall summary of the trend of the 

measured data. The power-law fits are summarized in table 3-6. For each measurement case, 

the following are listed: the frequency range for the data used in the fitting range, the 

multiplicative (C) and power (p) parameters resulting from the fitting, and a tabulation of 

attenuation values at selected frequencies obtained by evaluating the power law expressions. 

Tabulated attenuation values listed in blue indicate cases where an evaluation was made by 

extrapolating the power law slightly beyond the range of the trustworthy attenuation data used in 

the fit. Except for a few cases with very low measured attenuation (where measurement 

uncertainties will be a larger percentage of the attenuation value), fits to the Ni alloy data yielded 

power law exponents in the 3 < p < 4 range. 

3-51
TABLE 3-5. RESULTS OF SHEAR-WAVE SOUND VELOCITY MEASUREMENTS 

Coupon 

Inspection 

Direction 

Measured 

Quantities Results 

Waspaloy Radial thickness (in.): 0.9682 

0′′ (center) Dt (μsec): 15.2925 

v (cm/μsec): 0.3216 

GFM-A Radial thickness (in.): 0.9755 

0′′ (center) Dt (μsec): 16.0725 

v (cm/μsec): 0.3083 

V-Die-A Radial thickness (in.): 0.9755 

0′′ (center) Dt (μsec): 16.035 

v (cm/μsec): 0.3090 

V-Die-B Radial thickness (in.): 0.9820 

0′′ (center) Dt (μsec): 16.1975 

v (cm/μsec): 0.3080 

Note: Dt is the average time delay between two successive back-wall echoes. 

Note that table 3-6 is complete for 5 MHz (the center frequency for existing Ni billet inspections) 

and for 7.5 MHz (the likely center frequency for a next-generation inspection with improved 

sensitivity). However, for some high-attenuation cases, e.g., the 0′′ site in Waspaloy at 10 MHz, 

no tabulated value is listed because the measured attenuation data at that frequency were not 

deemed to be sufficiently trustworthy. For such cases, the attenuation values are likely too high 

to permit practical billet inspections at those frequencies; lower-frequency inspections with 

enhanced focusing would probably present a better alternative. Since the billet specimens will 

be preserved, property measurements at the higher frequencies could be carried out in the future 

using other transducers if necessary. 

The best estimates of the backscattered noise FOM values at the same five frequencies of interest 

are listed in table 3-7. The best estimate at a given frequency was obtained from a local linear fit 

to the measured FOM-versus-frequency curve that resulted from an analysis of the backscattered 

noise using the large time gate centered near the focal depth (gate 7 in figures 3-41 and 3-42). 

Deduced FOM values are listed for two separate analyses: one in which the measured 

attenuation power law of table 3-6 was used as an input, and one in which the input attenuation 

was assumed to be negligible. The first analysis (using measured attenuation) is believed to lead 

to the more accurate FOM value. By comparing the two results for a given case, the effect of 

attenuation on the FOM determination can be discerned. 

3-52
TABLE 3-6. SUMMARY OF MEASURED ATTENUATION VALUES FOR THE Ni ALLOY BILLET SPECIMENS 

Fitting Range Power Law Parameters

(N/cm Units) 

Coupon Side - Site C P 

f_min 

(MHz) 

f_max 

(MHz) 

Atten. at 

5 MHz 

(dB/inch) 

Atten. at 

7.5 MHz 

(dB/inch) 

Atten. at 

10 MHz 

(dB/inch) 

Atten. at 

12.5 MHz 

(dB/inch) 

Atten. at 

15 MHz 

(dB/inch) 

Waspaloy Axial - 0′′ 1.378E-04 3.526 3.91 7.81 0.885 3.696 

Axial - 1′′ 7.279E-05 3.828 3.91 7.81 0.761 3.596 Axial - 2′′ 8.757E-05 3.644 3.91 7.81 0.681 2.986 Axial - 3′′ 5.070E-05 3.585 3.91 9.77 0.358 1.533 4.298 Axial - 4′′ 7.778E-06 3.849 3.91 13.3 0.084 0.400 1.211 2.858 

Waspaloy Hoop - 0′′ 2.227E-04 3.242 3.91 7.81 0.907 3.376 

Hoop - 1′′ 7.768E-05 3.813 3.91 7.42 0.793 3.719 Hoop - 2′′ 9.284E-05 3.582 3.91 7.81 0.653 2.791 Hoop - 3′′ 3.818E-05 3.667 3.91 9.77 0.308 1.362 3.912 Hoop - 4′′ 1.198E-05 3.638 3.91 13.3 0.092 0.403 1.149 2.587 

Waspaloy Radial - 0′′ 1.280E-04 3.551 4.3 8.2 0.857 3.615 

Radial - 1′′ 9.822E-05 3.685 4.3 8.2 0.815 3.632 Radial - 2′′ 6.200E-05 3.803 4.3 8.2 0.623 2.911 Radial - 3′′ 8.065E-05 3.394 4.3 8.2 0.420 1.661 Radial - 4′′ 1.794E-05 3.563 4.3 8.2 0.122 0.518 

GFM-A Axial - 0′′ 6.558E-06 3.765 3.91 13.3 0.062 0.285 0.841 1.949 

Axial - 1′′ 9.736E-06 3.593 3.91 13.3 0.070 0.299 0.840 1.873 Axial - 2′′ 9.686E-06 3.495 3.91 13.3 0.059 0.244 0.668 1.456 Axial - 3′′ 3.015E-06 3.713 3.91 15.2 0.026 0.118 0.343 0.786 1.546 Axial - 4′′ 7.931E-07 4.029 3.91 15.6 0.011 0.059 0.187 0.459 0.958 

GFM-A Hoop - 0′′ 1.075E-05 3.585 3.91 13.3 0.076 0.325 0.912 2.030 

Hoop - 1′′ 6.492E-06 3.781 3.91 13.3 0.063 0.291 0.864 2.009 Hoop - 2′′ 3.968E-06 3.801 3.91 13.3 0.040 0.185 0.553 1.292 Hoop - 3′′ 3.174E-06 3.703 3.91 15.2 0.027 0.122 0.354 0.808 1.587 Hoop - 4′′ 1.396E-06 3.827 3.91 15.6 0.015 0.069 0.207 0.485 0.975 

GFM-A Radial - 0′′ 1.132E-05 3.567 3.91 16 0.078 0.330 0.920 2.039 3.907 

Radial - 1′′ 7.136E-06 3.724 3.91 16 0.063 0.286 0.835 1.916 3.778 Radial - 2′′ 9.800E-06 3.478 3.91 16 0.058 0.239 0.650 1.412 2.663 Radial - 3′′ 3.033E-06 3.735 3.91 16 0.027 0.124 0.364 0.837 1.654 Radial - 4′′ 1.601E-05 2.896 3.91 16 0.037 0.121 0.278 0.530 0.898 

3-53

Values in blue require extrapolations of the power-law curve beyond the frequency range of the measured data. 
TABLE 3-6. SUMMARY OF MEASURED ATTENUATION VALUES FOR THE Ni ALLOY BILLET SPECIMENS (Continued) 

Fitting Range Power Law Parameters

(N/cm Units) 

Coupon Side - Site C P 

f_min 

(MHz) 

f_max 

(MHz) 

Atten. at 

5 MHz 

(dB/inch) 

Atten. at 

7.5 MHz 

(dB/inch) 

Atten. at 

10 MHz 

(dB/inch) 

Atten. at 

12.5 MHz 

(dB/inch) 

Atten. at 

15 MHz 

(dB/inch) 

V-Die-A Axial - 0′′ 2.985E-06 3.734 3.91 15.2 0.027 0.122 0.357 0.821 1.621 

Axial - 1′′ 2.995E-06 3.697 3.91 15.2 0.025 0.114 0.329 0.750 1.472 Axial - 2′′ 3.021E-06 3.691 3.91 15.2 0.025 0.113 0.327 0.745 1.460 Axial - 3′′ 5.663E-06 3.563 3.91 14.5 0.039 0.164 0.456 1.010 1.935Axial - 4′′ 1.795E-05 3.199 3.91 13.3 0.068 0.249 0.626 1.278 

V-Die-A Hoop - 0′′ 1.899E-06 3.889 3.91 15.2 0.022 0.106 0.324 0.773 1.570 

Hoop - 1′′ 2.914E-06 3.715 3.91 15.2 0.025 0.114 0.333 0.763 1.502 Hoop - 2′′ 2.776E-06 3.736 3.91 15.2 0.025 0.114 0.334 0.768 1.518 Hoop - 3′′ 1.189E-05 3.331 3.91 14.5 0.056 0.216 0.562 1.182 2.170Hoop - 4′′ 1.330E-05 3.292 3.91 13.3 0.059 0.223 0.574 1.197 

V-Die-A Radial - 0′′ 1.003E-05 3.249 6.25 14.1 0.041 0.154 0.392 0.810 

Radial - 1′′ 7.161E-06 3.363 6.25 14.1 0.035 0.138 0.364 0.771 Radial - 2′′ 4.178E-06 3.603 6.25 14.1 0.030 0.131 0.369 0.825 Radial - 3′′ 1.371E-05 3.348 6.25 14.1 0.066 0.257 0.674 1.422 Radial - 4′′ 1.137E-06 3.454 6.25 14.1 0.065 0.264 0.713 1.542 

V-Die-B Axial - 0′′ 1.699E-06 3.898 3.91 15.2 0.020 0.097 0.296 0.707 1.439 

Axial - 1′′ 3.952E-06 3.558 3.91 15.2 0.027 0.113 0.315 0.697 1.334 Axial - 2′′ 4.298E-06 3.440 3.91 15.2 0.024 0.097 0.261 0.563 1.054 Axial - 3′′ 2.634E-06 3.322 3.91 15.6 0.012 0.047 0.122 0.256 0.469 Axial - 4′′ 1.998E-09 5.474 10 15.6 0.000 0.003 0.013 0.045 0.121 

V-Die-B Hoop - 0′′ 1.707E-06 3.889 3.91 15.6 0.020 0.095 0.292 0.695 1.412 

Hoop - 1′′ 2.809E-06 3.668 3.91 15.6 0.023 0.100 0.289 0.654 1.277 Hoop - 2′′ 8.309E-06 3.243 3.91 15.2 0.034 0.126 0.321 0.662 1.195 Hoop - 3′′ 6.766E-06 2.995 3.91 15.6 0.018 0.062 0.147 0.288 0.496 Hoop - 4′′ 9.009E-08 4.361 10 13.3 0.002 0.013 0.046 0.121 

V-Die-B Radial - 0′′ 3.791E-06 3.606 7.43 14.1 0.028 0.120 0.338 0.755 

Radial - 1′′ 3.004E-06 3.681 7.43 14.1 0.025 0.110 0.318 0.722 Radial - 2′′ 7.937E-06 3.249 7.43 14.1 0.033 0.122 0.311 0.642 Radial - 3′′ 2.750E-06 3.342 7.43 14.1 0.013 0.051 0.133 0.281 Radial - 4′′ 6.444E-06 2.881 7.43 14.1 0.015 0.047 0.108 0.206 

3-54

Values in blue require extrapolations of the power-law curve beyond the frequency range of the measured data. 
TABLE 3-7. SUMMARY OF MEASURED GRAIN NOISE FOM VALUES FOR THE Ni ALLOY BILLET SPECIMENS 

Coupon Side - Site 

Noise FOM at 

5 MHz (cm-0.5) 

Measured 

Atten. Used 

Noise FOM at 

7.5 MHz (cm-0.5) 

Measured 

Atten. Used 

Noise FOM at 

10 MHz (cm-0.5) 

Measured 

Atten. Used 

Noise FOM at 

12.5 MHz (cm-0.5) 

Measured 

Atten. Used 

Noise FOM at 

5 MHz (cm-0.5) 

Atten. = 0 

Assumed 

Noise FOM at

7.5 MHz (cm-0.5) 

Atten. = 0 

Assumed 

Noise FOM at 

10 MHz (cm-0.5) 

Atten. = 0 

Assumed 

Noise FOM at 

12.5 MHz (cm-0.5) 

Atten. = 0 

Assumed 

Waspaloy Axial - 0′′ 0.0142 0.0425 0.0124 0.0251 

Axial - 1′′ 0.0135 0.0427 0.0120 0.0254 Axial - 2′′ 0.0122 0.0386 0.0110 0.0250 Axial - 3′′ 0.0086 0.0238 0.0081 0.0190 Axial - 4′′ 0.0049 0.0119 0.0048 0.0112 

Waspaloy Hoop - 0′′ 0.0128 0.0413 0.0112 0.0256 

Hoop - 1′′ 0.0130 0.0407 0.0115 0.0239 Hoop - 2′′ 0.0114 0.0360 0.0103 0.0241 Hoop - 3′′ 0.0088 0.0250 0.0084 0.0205 Hoop - 4′′ 0.0049 0.0122 0.0048 0.0115 

Waspaloy Radial - 0′′ 0.0156 0.0473 0.0136 0.0291 

Radial - 1′′ 0.0153 0.0454 0.0134 0.0278 Radial - 2′′ 0.0134 0.0401 0.0121 0.0266 Radial - 3′′ 0.0099 0.0275 0.0093 0.0219 Radial - 4′′ 0.0062 0.0144 0.0060 0.0134 

GFM-A Axial - 0′′ 0.0045 0.0107 0.0201 0.0360 0.0045 0.0102 0.0178 0.0271 

Axial - 1′′ 0.0049 0.0110 0.0201 0.0351 0.0048 0.0105 0.0178 0.0267 Axial - 2′′ 0.0040 0.0092 0.0171 0.0291 0.0040 0.0088 0.0155 0.0235 Axial - 3′′ 0.0030 0.0065 0.0122 0.0214 0.0030 0.0064 0.0116 0.0191 Axial - 4′′ 0.0026 0.0051 0.0093 0.0163 0.0026 0.0050 0.0091 0.0152 

GFM-A Hoop - 0′′ 0.0047 0.0109 0.0201 0.0360 0.0046 0.0104 0.0178 0.0271 

Hoop - 1′′ 0.0048 0.0112 0.0201 0.0351 0.0047 0.0108 0.0178 0.0267 Hoop - 2′′ 0.0040 0.0093 0.0171 0.0291 0.0040 0.0091 0.0155 0.0235 Hoop - 3′′ 0.0030 0.0070 0.0122 0.0214 0.0030 0.0069 0.0116 0.0191 Hoop - 4′′ 0.0026 0.0054 0.0093 0.0163 0.0026 0.0054 0.0091 0.0152 

GFM-A Radial - 0′′ 0.0050 0.0118 0.0214 0.0361 0.0049 0.0113 0.0190 0.0279 

Radial - 1′′ 0.0050 0.0110 0.0204 0.0334 0.0049 0.0106 0.0183 0.0262 Radial - 2′′ 0.0048 0.0102 0.0188 0.0306 0.0047 0.0098 0.0173 0.0256 Radial - 3′′ 0.0041 0.0076 0.0128 0.0211 0.0041 0.0740 0.0122 0.0190 Radial - 4′′ 0.0032 0.0066 0.0118 0.0196 0.0032 0.0065 0.0113 0.0184 

3-55
TABLE 3-7. SUMMARY OF MEASURED GRAIN NOISE FOM VALUES FOR THE Ni ALLOY 

BILLET SPECIMENS (Continued) 

Coupon Side - Site 

Noise FOM at 

5 MHz (cm-0.5) 

Measured 

Atten. Used 

Noise FOM at 

7.5 MHz (cm-0.5) 

Measured 

Atten. Used 

Noise FOM at 

10 MHz (cm-0.5) 

Measured 

Atten. Used 

Noise FOM at 

12.5 MHz (cm-0.5) 

Measured 

Atten. Used 

Noise FOM at 

5 MHz (cm-0.5) 

Atten. = 0 

Assumed 

Noise FOM at

7.5 MHz (cm-0.5) 

Atten. = 0 

Assumed 

Noise FOM at 

10 MHz (cm-0.5) 

Atten. = 0 

Assumed 

Noise FOM at 

12.5 MHz (cm-0.5) 

Atten. = 0 

Assumed 

V-Die-A Axial - 0′′ 0.0034 0.0064 0.0121 0.0204 0.0034 0.0063 0.0115 0.0181 

Axial - 1′′ 0.0032 0.0061 0.0119 0.0199 0.0031 0.0060 0.0114 0.0179 Axial - 2′′ 0.0030 0.0063 0.0117 0.0197 0.0030 0.0062 0.0111 0.0177 Axial - 3′′ 0.0036 0.0074 0.0129 0.0218 0.0036 0.0072 0.0120 0.0188 Axial - 4′′ 0.0040 0.0089 0.0174 0.0304 0.0040 0.0086 0.0159 0.0252 

V-Die-A Hoop - 0′′ 0.0036 0.0080 0.0137 0.0223 0.0036 0.0079 0.0130 0.0199 

Hoop - 1′′ 0.0035 0.0073 0.0126 0.0201 0.0035 0.0072 0.0120 0.0180 Hoop - 2′′ 0.0034 0.0072 0.0122 0.0197 0.0034 0.0071 0.0116 0.0176 Hoop - 3′′ 0.0036 0.0080 0.0145 0.0242 0.0036 0.0078 0.0133 0.0203 Hoop - 4′′ 0.0037 0.0082 0.0149 0.0251 0.0037 0.0079 0.0137 0.0211 

V-Die-A Radial - 0′′ 0.0038 0.0080 0.0214 0.0361 0.0038 0.0078 0.0190 0.0279 

Radial - 1′′ 0.0034 0.0075 0.0204 0.0334 0.0034 0.0073 0.0183 0.0262 Radial - 2′′ 0.0035 0.0076 0.0188 0.0306 0.0035 0.0074 0.0173 0.0256 Radial - 3′′ 0.0048 0.0109 0.0128 0.0211 0.0048 0.0107 0.0122 0.0190 Radial - 4′′ 0.0045 0.0102 0.0118 0.0196 0.0045 0.0099 0.0113 0.0184 

V-Die-B Axial - 0′′ 0.0030 0.0061 0.0111 0.0193 0.0030 0.0060 0.0106 0.0174 

Axial - 1′′ 0.0030 0.0059 0.0105 0.0185 0.0030 0.0058 0.0100 0.0167 Axial - 2′′ 0.0026 0.0052 0.0094 0.0167 0.0026 0.0051 0.0090 0.0153 Axial - 3′′ 0.0020 0.0033 0.0058 0.0101 0.0020 0.0033 0.0057 0.0098 Axial - 4′′ 0.0018 0.0022 0.0037 0.0065 0.0018 0.0022 0.0037 0.0065 

V-Die-B Hoop - 0′′ 0.0033 0.0071 0.0116 0.0193 0.0033 0.0070 0.0111 0.0174 

Hoop - 1′′ 0.0037. 0.0078 0.0122 0.0196 0.0037 0.0077 0.0117 0.0178 Hoop - 2′′ 0.0030 0.0064 0.0112 0.0179 0.0030 0.0062 0.0107 0.0162 Hoop - 3′′ 0.0025 0.0045 0.0076 0.0124 0.0025 0.0044 0.0074 0.0119 Hoop - 4′′ 0.0017 0.0029 0.0048 0.0076 0.0017 0.0029 0.0048 0.0075 

V-Die-B Radial - 0′′ 0.0038 0.0078 0.0137 0.0230 0.0038 0.0077 0.0131 0.0209 

Radial - 1′′ 0.0037 0.0074 0.0137 0.0233 0.0036 0.0073 0.0131 0.0213 Radial - 2′′ 0.0030 0.0059 0.0111 0.0191 0.0030 0.0058 0.0106 0.0176 Radial - 3′′ 0.0021 0.0040 0.0072 0.0125 0.0021 0.0039 0.0071 0.0121 Radial - 4′′ 0.0019 0.0028 0.0046 0.0077 0.0019 0.0028 0.0045 0.0075 

3-56
The attenuation values of the various coupons are displayed graphically in figure 3-50 for one 

frequency of interest, namely, 7.5 MHz. The plotted values shown were obtained from table 3-6, 

i.e., by evaluating the power-law fits at that frequency. Figure 3-51 displays, in a similar format, 

the measured FOM values at 7.5 MHz. The plotted values have been taken from table 3-7 for the 

analysis that used the measured attenuation as an input. 

The same trends seen earlier in the preliminary C-scans of back-wall echo strength and 

backscattered noise are also evident in figures 3-50 and 3-51. 

1. The attenuation or FOM value varies significantly with radial position, being largest near 

the billet center (0′′) for Waspaloy, GFM-A, and V-Die-B, and largest near the OD for 

V-Die-A. 

2. On average, at a given measurement site, there is little difference between the results for 

the axial, radial, and hoop directions. 

3. Averaged over radial position, the attenuation or FOM value increases for the sequence: 

V-Die-B, V-Die-A, GFM-A, and Waspaloy. 

4. Attenuation and FOM values tend to be correlated: sites with higher (lower) attenuation 

also have higher (lower) FOM. 

Waspalloy Attenuation at 7.5 MHz

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0" 1" 2" 3" 4"

Measurement Site

Attenuation (dB/in) .

Axial

Hoop

Radial

(center)

GFM-A Attenuation at 7.5 MHz

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0" 1" 2" 3" 4"

Measurement Site

Attenuation (dB/in) .

Axial

Hoop

Radial

(center)

V-Die-A Attenuation at 7.5 MHz

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0" 1" 2" 3" 4"

Measurement Site

Attenuation (dB/in) .

Axial

Hoop

Radial

(center)

V-Die-B Attenuation at 7.5 MHz

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0" 1" 2" 3" 4"

Measurement Site

Attenuation (dB/in) .

Axial

Hoop

Radial

(center)

Waspalloy Attenuation at 7.5 MHz

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0" 1" 2" 3" 4"

Measurement Site

Attenuation (dB/in) .

Axial

Hoop

Radial

(center)

GFM-A Attenuation at 7.5 MHz

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0" 1" 2" 3" 4"

Measurement Site

Attenuation (dB/in) .

Axial

Hoop

Radial

(center)

V-Die-A Attenuation at 7.5 MHz

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0" 1" 2" 3" 4"

Measurement Site

Attenuation (dB/in) .

Axial

Hoop

Radial

(center)

V-Die-B Attenuation at 7.5 MHz

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0" 1" 2" 3" 4"

Measurement Site

Attenuation (dB/in) .

Axial

Hoop

Radial

(center)

FIGURE 3-50. MEASURED ATTENUATION VALUES AT 7.5 MHz FOR 

THE Ni ALLOY BILLET SPECIMENS 

(Results are shown for three orthogonal inspection directions. The distance 

from each measurement site to the billet center is indicated.) 

3-57
Waspaloy Noise FOM at 7.5 MHz

0.000

0.010

0.020

0.030

0.040

0.050

01234

Measurement Site

Noise FOM (cm^-0.5) .

Axial

Hoop

Radial

GFM-A Noise FOM at 7.5 MHz

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0123

Measurement Site

Noise FOM (cm^-0.5) .

4

Axial

Hoop

Radial

V-DIE-A Noise FOM at 7.5 MHz 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

01234

Measurement Site

Noise FOM (cm^-0.5) 

.

Axial

Hoop

Radial

V-DIE-B Noise FOM at 7.5 MHz 

0.000

0.002

0.004

0.006

0.008

0.010

0123 4

Measurement Site

Noise FOM (cm^-0.5) .

Axial

Hoop

Radial

Waspaloy Noise FOM at 7.5 MHz

0.000

0.010

0.020

0.030

0.040

0.050

01234

Measurement Site

Noise FOM (cm^-0.5) .

Axial

Hoop

Radial

GFM-A Noise FOM at 7.5 MHz

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0123

Measurement Site

Noise FOM (cm^-0.5) .

4

Axial

Hoop

Radial

V-DIE-A Noise FOM at 7.5 MHz 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

01234

Measurement Site

Noise FOM (cm^-0.5) 

.

Axial

Hoop

Radial

V-DIE-B Noise FOM at 7.5 MHz 

0.000

0.002

0.004

0.006

0.008

0.010

0123 4

Measurement Site

Noise FOM (cm^-0.5) .

Axial

Hoop

Radial

FIGURE 3-51. MEASURED GRAIN NOISE FOM VALUES AT 7.5 MHz FOR THE 

Ni ALLOY BILLET SPECIMENS 

(Results are shown for three orthogonal inspection directions. The distance from 

each measurement site to the billet center is indicated.) 

The attenuation and FOM values vary widely over the suite of specimens. Values for both 

properties tend to be much larger for Waspaloy than for IN718. Averaged over all the coupons 

and all inspection directions, the Waspaloy attenuation at 5 MHz (0.56 dB/inch) is 15 times 

larger than the IN718 attenuation (0.036 dB/inch). At 7.5 MHz, the average Waspaloy 

attenuation is 16 times larger (2.41 dB/inch versus 0.15 dB/inch). The average FOM values for 

Waspaloy at 5 and 7.5 MHz are larger than the corresponding averages for IN718 by factors of 

3.1 and 4.5, respectively. Within a given billet strip coupon, attenuation values at these 

frequencies can vary by as much as an order of magnitude with position, and FOM values can 

vary by as much as a factor of 3. Table 3-8 lists the range of measured attenuation and FOM 

values at 5 and 7.5 MHz for each strip coupon. 

TABLE 3-8. RANGES OF ATTENUATION AND FOM VALUES MEASURED WITHIN 

EACH OF THE Ni ALLOY BILLET STRIP COUPONS 

Range of Attenuation Values Range of FOM Values 

Coupon 

(dB/inch) 

at 5 MHz 

(dB/inch) 

at 7.5 MHz 

(cm-0.5) 

at 5 MHz 

(cm-0.5) 

at 7.5 MHz 

Waspaloy 0.084-0.907 0.400-3.719 0.0049-0.0156 0.0119-0.0473 

GFM-A 0.011-0.078 0.059-0.330 0.0026-0.0050 0.0051-0.0118 

V-Die-A 0.022-0.068 0.106-0.264 0.0030-0.0048 0.0061-0.0109 

V-Die-B 0.000-0.034 0.003-0.126 0.0017-0.0038 0.0022-0.0078 

3-58
It was noted that for the Ni alloy billet specimens, high-attenuation regions generally produce 

high backscattered noise levels. This correlation can now be demonstrated more explicitly. In 

figure 3-52(a)-(b), the measured noise FOM was plotted against the measured attenuation for a 

fixed frequency of 7.5 MHz. Each point on the plot represents data from one site on one coupon, 

inspected from one direction. Thus, for each of the four strip coupons, there are 15 plotted points 

in figure 3-52(a) (i.e., five sites with three directions per site). It was observed that the Ni alloys 

exhibit a clear general trend, with the noise FOM tending to increase with increasing attenuation. 

This general trend is expected for specimens with equiaxed grains: as the mean grain size 

increases, the attenuation and noise FOM both increase. However, this trend is not always seen 

in jet-engine materials. For example, for the Ti alloy billet specimens studied under ETC 

Phase I, the direction of maximum back-wall attenuation was usually the direction of minimum 

grain noise [15-17]. This led to an inverse relationship between attenuation and noise, as shown 

in figure 3-52(c) [17]. These two different trends were attributed to the role of recrystallization 

during the billet formation process. In Ni alloys, there is a significant amount of recrystallization 

that occurs due to mechanical working of the billet during its formation. This evidently leads to 

nearly equiaxed grains in the Ni billet specimens under study. In contrast, very little or no 

recrystallization occurs during working of titanium alloy billets. Hence, the large-scale 

macrograins present in the Ti ingot become elongated in the axial direction. These elongated 

grains are believed to be responsible for the inverse correlation between noise FOM and 

attenuation that is seen in figure 3-52(c) [17]. 

FIGURE 3-52. CORRELATION BETWEEN BACKSCATTERED NOISE FOM AND 

ATTENUATION (a)-(b) COUPONS FROM 10′′ DIAMETER Ni BILLETS AT 7.5 MHz WITH 

PANEL (b) BEING A BLOWUP OF THE LOW-ATTENUATION PORTION OF PANEL (a), 

AND (c) COUPONS FROM A 6′′ DIAMETER Ti-6-4 BILLET AT 15 MHz 

Relationship of Noise and Attenuation

at 15 MHz in Six Ti 6-4 (CBS) Billet Specimens

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 2 4 6 8 10

Attenuation (dB/inch))

Noise FOM (cm ^ - 0.5) .

Axial

Radial

Hoop

Trendline

Correlation Betw een Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

01234

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

Trendline

Correlation Betw een Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.0 0.1 0.2 0.3 0.4

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

Trendline

Ti 6-4 results at left are from measurements 

performed during ETC Phase-I on six 

rectangular coupons from a 6”-diameter billet 

(the so-called “contaminated billet”) . Three 

coupons each were cut from “high noise” and 

“low noise” bands. Results are at 15-MHz for 

sound propagation in the axial, radial, and 

hoop directions of each coupon.

Ni-alloy results are at 7.5 MHz for coupons cut 

from 10”-diameter billets.

FOM values are in standard units of

cm ^ -1/2.

(a) (b)

(c)

Relationship of Noise and Attenuation

at 15 MHz in Six Ti 6-4 (CBS) Billet Specimens

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 2 4 6 8 10

Attenuation (dB/inch))

Noise FOM (cm ^ - 0.5) .

Axial

Radial

Hoop

Trendline

Relationship of Noise and Attenuation

at 15 MHz in Six Ti 6-4 (CBS) Billet Specimens

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 2 4 6 8 10

Attenuation (dB/inch))

Noise FOM (cm ^ - 0.5) .

Axial

Radial

Hoop

Trendline

Correlation Betw een Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

01234

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

Trendline

Correlation Betw een Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.0 0.1 0.2 0.3 0.

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

Trendline

4

Ti 6-4 results at left are from measurements 

performed during ETC Phase-I on six 

rectangular coupons from a 6”-diameter billet 

(the so-called “contaminated billet”) . Three 

coupons each were cut from “high noise” and 

“low noise” bands. Results are at 15-MHz for 

sound propagation in the axial, radial, and 

hoop directions of each coupon.

Ni-alloy results are at 7.5 MHz for coupons cut 

from 10”-diameter billets.

FOM values are in standard units of

cm ^ -1/2.

(a) (b)

(c)

3-59
The systematic variation of attenuation and FOM with position will make some locations within 

billets more difficult to inspect than others. For a given defect, SNR will vary with defect 

position, generally being smaller at locations having larger FOM values. The UT properties 

reported in this section can be used as inputs to model calculations that simulate billet 

inspections and estimate SNR values. Examples of such calculations performed during ETC 

Phase I were reported in reference 39. 

3.1.4.3 The Relationship Between Attenuation and FOM in Equiaxed Pure Ni Microstructures. 

As noted in section 3.1.4.2, for the Ni alloy specimens, the UT properties at a given 

measurement site are approximately independent of inspection direction, and attenuation and 

FOM values tend to rise or fall together as the measurement site is moved. This behavior is 

consistent with an equiaxed, untextured billet microstructure in which the average grain diameter 

varies systematically with position. It is informative to ask how attenuation and FOM are 

expected to depend upon grain size and frequency for such simple microstructures. Models were 

used to calculate attenuation and FOM values for pure Ni microstructures containing equiaxed, 

randomly oriented grains of constant density. Such calculations require, as inputs, the elastic 

stiffness constants of a single crystal. These constants are known for pure Ni but not for 

Waspaloy or IN718. Thus, the calculations were only performed for the pure Ni case. 

Attenuation values, as functions of frequency and average grain diameter, were obtained using 

an internet-available calculator developed by Joseph Turner [40]. It is based on the so-called 

first-order smoothing approximation to a stochastic elastic wave equation [41 and 42] and is 

equivalent to the formalism developed by Stanke and Kino [43] to describe attenuation resulting 

from grain scattering during sound propagation through a polycrystalline solid. FOM values 

were predicted using a model expression developed by James Rose [22 and 44]. Both the 

attenuation and FOM model calculations assume that each microstructure considered contains a 

distribution of grain sizes with a well-defined average diameter. Grain size information is input 

through a function P(L) that describes the probability (P) that a line segment of length (L), 

randomly drawn through the microstructure, has both ends in the same grain. Here, P(L) is 

assumed to be an exponential function, P(L) = exp(-L/b), where parameter b is equal to one-half 

of the average grain diameter. The P(L) function will be discussed further in section 3.1.5.2. 

Selected results of the model calculations are shown in figures 3-53 through 3-55. Figure 3-53 

displays predicted attenuation-versus-frequency curves for three pure Ni microstructures having 

average grain diameters of 10, 20, and 30 microns, respectively. As the average grain diameter 

increases, the attenuation is shown to increase rapidly. For each of the three diameters, the 

attenuation is a rapidly rising function of frequency, which, over a limited frequency range, can 

be well approximated by a power law [α = C f p

]. Such power-law fits to the attenuation-versusfrequency curves in the 5-15 MHz range are also shown in figure 3-53. As shown in figure 3-54, 

the power-law exponent for pure Ni microstructures is near 4 for small grain diameters, and 

systematically decreases as the average grain diameter rises, being about 3.0 for 50-micron 

grains and 2.3 for 100-micron grains. Recall that power-law fits to the Ni alloy attenuation data 

generally yielded exponents in the 3 to 4 range. Figure 3-55 shows predicted FOM-versusfrequency curves for eight pure Ni microstructures with different average grain diameters. The 

curves all rise with frequency within the horizontal range shown, but in each case, the rate of rise 

is significantly lower than that of the associated attenuation-versus-frequency curve. 

3-60
0.00

0.05

0.10

0.15

0.20

0.25

0 5 10 15 20

Frequency (MHz)

Attenuation (N / cm) .

10 micron (theory)

10 micron (fit)

20 micron (theory)

20 micron (fit)

30 micron (theory)

30 micron (fit)

30 microns

p = 3.50

20 microns

p = 3.74

10 microns

p = 3.93

Power law fits are over

5 MHz < f < 15 MHz

0.00

0.05

0.10

0.15

0.20

0.25

0 5 10 15 20

Frequency (MHz)

Attenuation (N / cm) .

10 micron (theory)

10 micron (fit)

20 micron (theory)

20 micron (fit)

30 micron (theory)

30 micron (fit)

30 microns

p = 3.50

20 microns

p = 3.74

10 microns

p = 3.93

Power law fits are over

5 MHz < f < 15 MHz

FIGURE 3-53. PREDICTED ATTENUATION-VERSUS-FREQUENCY CURVES AND 

ASSOCIATED POWER-LAW FITS FOR PURE Ni MICROSTRUCTURES HAVING 

AVERAGE GRAIN DIAMETERS OF 10, 20, AND 30 microns 

FIGURE 3-54. PREDICTED RELATIONSHIP BETWEEN THE AVERAGE GRAIN 

DIAMETER, THE ATTENUATION AT 7.5 MHz, AND THE EXPONENT IN THE 

POWER-LAW FIT TO THE ATTENUATION-VERSUS-FREQUENCY CURVE FOR 

PURE Ni MICROSTRUCTURES 

Model Prediction for Equiaxed Pure Nickel

2.0

2.5

3.0

3.5

4.0

012345678

Attenuation at 7.5 MHz (dB/inch)

Power Law Exponent, p .

Ave. Grain Attenuation Power Law

Diameter at 7.5 MHz Exponent

(microns) (dB/in)

5 0.002 3.981

10 0.019 3.927

15 0.062 3.844

20 0.143 3.739

30 0.452 3.499

40 0.984 3.254

50 1.738 3.029

60 2.683 2.828

70 3.776 2.651

80 4.971 2.495

90 6.225 2.356

100 7.502 2.232

Power law fits are over

5 MHz < frequency < 15 MHz

100

microns

5

microns

50

microns

Model Prediction for Equiaxed Pure Nickel

2.0

2.5

3.0

3.5

4.0

012345678

Attenuation at 7.5 MHz (dB/inch)

Power Law Exponent, p .

Ave. Grain Attenuation Power Law

Diameter at 7.5 MHz Exponent

(microns) (dB/in)

5 0.002 3.981

10 0.019 3.927

15 0.062 3.844

20 0.143 3.739

30 0.452 3.499

40 0.984 3.254

50 1.738 3.029

60 2.683 2.828

70 3.776 2.651

80 4.971 2.495

90 6.225 2.356

100 7.502 2.232

Power law fits are over

5 MHz < frequency < 15 MHz

100

microns

5

microns

50

microns

3-61
0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0 10 20

Frequency (MHz)

FOM (cm**-0.5)

100 microns

80 microns

60 microns

50 microns

40 microns

30 microns

20 microns

10 microns

Assumes P(L)=exp(-L/b)

with b= half of mean dia. 2b = D

FIGURE 3-55. PREDICTED FOM-VERSUS-FREQUENCY CURVES 

FOR PURE Ni MICROSTRUCTURES HAVING AVERAGE GRAIN 

DIAMETERS RANGING FROM 10 TO 100 microns 

The relationship between attenuation and FOM for the pure Ni microstructures is illustrated in 

figure 3-56, assuming a fixed frequency of 7.5 MHz. As the average grain size increases, the 

FOM value rises less rapidly than the attenuation, leading to a FOM-versus-attenuation curve 

whose slope decreases as one moves further to the right in the figure (i.e., as the average grain 

size increases). For the model curve, the FOM value is approximately proportional to the square 

root of the attenuation value. Also shown for comparison are the measured (attenuation, FOM) 

pairs at 7.5 MHz for the Ni alloy billet specimens (figure 3-52). The general trend of the Ni 

alloy data is similar to that predicted for pure Ni microstructures; however, the Ni alloy results 

tend to lie above the pure Ni curve. Attenuation and FOM values for polycrystals depend, in 

part, on the single-crystal elastic constants of the metal. The differences seen in figure 3-56 may 

be a result of associated differences in the elastic constants of pure Ni and the two alloys under 

study. 

Correlation Between Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.0 0.1 0.2 0.3 0.4

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

MODEL: Pure Ni

Correlation Between Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.010

0.020

0.030

0.040

0.050

0.060

01234

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

MODEL: Pure Ni

Smaller grains

Larger grains

Correlation Between Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.0 0.1 0.2 0.3 0.4

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

MODEL: Pure Ni

Correlation Between Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.010

0.020

0.030

0.040

0.050

0.060

01234

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

MODEL: Pure Ni

Smaller grains

Larger grains

Correlation Between Measured Attenuation and 

Measured Noise FOM for Nickel Superalloys

0.000

0.010

0.020

0.030

0.040

0.050

0.060

01234

Attenuation (dB/in)

Noise FOM (cm^-0.5)

WASPALOY

GFM-A

V-DIE-A

V-DIE-B

MODEL: Pure Ni

Smaller grains

Larger grains

(Solid curve: model prediction for pure Ni. Plotted points: measured data for Ni alloy billet specimens. Right panel shows 

a blowup of the low attenuation region of the left panel.) 

FIGURE 3-56. RELATIONSHIP BETWEEN FOM AND ATTENUATION AT 7.5 MHz 

3-62
Be te 

FIGURE 3-57. PREDICTED BEHAVIOR OF THE ABSOLUTE GRAIN NOISE 

ote that figure 3-54 contains, for pure Ni microstructures, a short table relating the attenuation 

cause attenuation and FOM increase at different rates as the grain size grows, the absolu

grain noise level seen in an inspection does not necessarily increase with grain size. The grain 

noise level (e.g., the rms noise voltage) seen at a particular frequency, f and depth z, in the metal 

is expected to be proportional to the product [exp(-2a(f)z)][FOM(f)], with the first factor 

describing the effect of attenuation on the backscattered noise. In addition, a third depthdependent factor would describe the effect of beam focusing and diffraction. Consider the case 

of a nondiffracting sonic beam that maintains a constant lateral area during propagation through 

the metal. For such a beam, figure 3-57 shows the predicted manner in which the noise level 

depends on the average grain diameter. Results are shown for three depths in pure Ni 

microstructures. In each case, the noise level peaks at a particular average grain size; beyond 

that size, signal loss due to attenuation outpaces the increase in signal resulting from the stronger 

scattering by the larger grains. Note that as the depth in the metal increases, the grain size that 

produces the largest absolute noise level decreases. It is this interplay between attenuation and 

FOM that is responsible for the fact that observed absolute noise levels for the Waspaloy and 

IN718 specimens (figure 3-24) are much more similar than would be expected based on their 

relative FOM values (figure 3-51). 

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 20 40 60 80 10

Average Grain Diameter (microns)

0.035

0

Absolute 

N

oise 

Level . .

Noise at 0.5"

Noise at 1.0"

Noise at 1.5"

Vertical coordinate is

[exp(-2αz)][FOM]

evaluated at 10 MHz.

LEVEL SEEN AT THREE DEPTHS IN PURE Ni AS A FUNCTION OF 

MEAN GRAIN DIAMETER 

N

at 7.5 MHz to the average grain diameter. One can compare the values in that table to the 

measured attenuations for the Ni alloy specimens listed in tables 3-6 or 3-8. If one assumes that 

the relationship between attenuation and grain diameter for the Ni alloys is similar to that for 

pure Ni, one concludes that average grain diameters within the billet specimens range from 

approximately 5 to 70 microns. A similar comparison of measured FOM values at 7.5 MHz and 

those predicted for pure Ni (figure 3-55) leads to estimated grain diameters of 10-100 microns 

for the billet specimens. The measurement of average grain diameter is the topic of the next 

section. 

3-63
3.1.5 Metallography Characterization and the Relationship Between UT Properties

and Grain Size. 

3.1.5.1 Metallography Coupons and Grain Structure Photography. 

One goal of the study was to correlate the measured UT properties with the local billet 

microstructure. To this end, small metallography coupons from selected billet sites were 

polished and etched to reveal the grain structure, and grain sizes were then measured from 

associated photographs. The metallography coupons were not cut directly from the rectangular 

specimens used for UT property measurements because it was desired to keep these specimens 

fully intact. Instead, as shown in figure 3-58, the metallography coupons were cut from a 1/2′′

thick rough-cut slab that was adjacent in the billet to a given strip coupon. For each of the four 

strip coupons studied, there were 12 metallography coupons: three each from sites located 0′′, 

2′′, 3′′, and 4′′ from the billet center. To economize, the 1′′ site was skipped because it typically 

showed very similar attenuation and noise values as the central (0′′) site. One face of each 

metallography coupon was polished, as shown in figure 3-58(b), so that the collection of 

12 coupons provided axial, radial, and hoop views of the microstructure at each of the four 

inspection sites. As shown in figure 3-58(b), the 12 coupons associated with a given UT 

property strip were denoted A-L. These can be divided into three groups based on their polished 

surfaces: 

• Axial view coupons: A, D, G, and J 

• Radial view coupons: B, E, H, and K 

• Hoop view coupons: C, F, I, and L 

The photographs that were used for grain-sizing work were taken at points located as close as 

possible to the center of the corresponding UT measurement site. Thus, in the axial-radial plane, 

the photographed regions of the polished metallography coupons were as close as possible to the 

blue dot shown in figure 3-58(c). That blue dot indicates a point precisely 0′′, 2′′, 3′′, or 4′′ from 

the billet center. 

Grain structure photographs were printed on 8.5′′ x 11′′ stock, with the magnification chosen 

such that 150-300 grains were typically seen. An example is shown in figure 3-59(a). 

Figure 3-59(b) depicts a smaller, representative area of the entire micrograph. Such subimages 

are used in this document for discussion purposes and to illustrate variations in average grain 

size with position. For example, figure 3-60 uses subimages to compare the microstructures seen 

in the 12 Waspaloy metallography coupons. At a given radial location in the Waspaloy billet, 

the grains are approximately equiaxed, i.e., the grains tend to have approximately circular crosssections rather than being systematically elongated, and the grain structure appears similar from 

the three viewing directions. In addition, figure 3-60 shows that the average grain size tends to 

decrease as one moves radially outward from the center of the billet. In section 3.1.4.2, the UT 

measurements on the Waspaloy coupons found both attenuation and grain noise to be 

approximately independent of inspection direction and largest near the billet center. Generally 

speaking, the grain structures were seen to be approximately equiaxed in all the Ni alloy billet 

specimens studied. A second example is shown in figure 3-61 for one of the IN718 cases. Note 

the different magnifications of the two images in figure 3-61; the average grain diameter for the 

3-64
left-hand image (billet center) is approximately twice that of the right-hand image (1′′ from 

billet OD). 

(a)

“strip coupon” 

for UT property 

measurements

“rough cut” slab for 

metallography

Metallography

sites

(c)

A

B

C

D

E

F

G

H

I

J

K

L

10”

Face polished for axial view.

Face polished for radial view.

Face polished for hoop view.

(polished surface shaded blue)

(b)

2”

Metallography coupon dimensions: 

(hoop) x (radial) x (axial):

1/4” x 3/8” x 1/2” ; polished: 1/4” x 3/8”

1/4” x 3/16” x 1/2”; polished: 1/4” x 1/2”

1/4” x 3/4” x 1/2”; polished: 3/4” x 1/2”

radial

x

axial

hoop

EDM

cuts

(a)

“strip coupon” 

for UT property 

measurements

“rough cut” slab for 

metallography

Metallography

sites

“strip coupon” 

for UT property 

measurements

“rough cut” slab for 

metallography

Metallography

sites

“strip coupon” 

for UT property 

measurements

“rough cut” slab for 

metallography

Metallography

sites

(c)

A

B

C

D

E

F

G

H

I

J

K

L

10”

Face polished for axial view.

Face polished for radial view.

Face polished for hoop view.

(polished surface shaded blue)

(b)

2”

A

B

C

D

E

F

G

H

I

J

K

L

A

B

C

D

E

F

G

H

I

J

K

L

10”

Face polished for axial view.

Face polished for radial view.

Face polished for hoop view.

(polished surface shaded blue)

(b)

2”

Metallography coupon dimensions: 

(hoop) x (radial) x (axial):

1/4” x 3/8” x 1/2” ; polished: 1/4” x 3/8”

1/4” x 3/16” x 1/2”; polished: 1/4” x 1/2”

1/4” x 3/4” x 1/2”; polished: 3/4” x 1/2”

radial

x

axial

hoop

EDM

cuts

Metallography coupon dimensions: 

(hoop) x (radial) x (axial):

1/4” x 3/8” x 1/2” ; polished: 1/4” x 3/8”

1/4” x 3/16” x 1/2”; polished: 1/4” x 1/2”

1/4” x 3/4” x 1/2”; polished: 3/4” x 1/2”

radial

x

axial

hoop

EDM

cuts

radial

x

axial

hoop

EDM

cuts

FIGURE 3-58. METALLOGRAPHY COUPONS (a) SLAB FROM WHICH COUPONS ARE 

CUT, (b) DESIGNATIONS OF THE 12 COUPONS FROM EACH SLAB, AND 

(c) COUPON DIMENSIONS 

250μm

Hoop

Radial

Coupon A

Waspaloy, 0” Site, Axial View

Subregion of 

photo at left:

(a) (b)

250μm

Hoop

Radial

Coupon A

250μm

Hoop

Radial

Coupon A

Waspaloy, 0” Site, Axial View

Subregion of 

photo at left:

(a) (b)

FIGURE 3-59. AXIAL VIEW OF MICROSTRUCTURE AT THE CENTER (0′′) OF THE 

WASPALOY BILLET (a) FULL REGION USED FOR GRAIN SIZE ANALYSIS 

AND (b) SMALLER SUBREGION 

3-65
250μm 250μm 100μm

Axial Views :

250μm

Hoop

Radial

Coupon A Coupon D Coupon G Coupon J

Coupon B

Radial Views :

Hoop Views :

Hoop

Axial

Radial

Axial

Coupon H Coupon K

Coupon I Coupon L

100μm 250μm 250μm

250μm 250μm

Coupon E

250μm

Coupon F

250μm

Center of the Billet 2” away from center 3” away from center 4” away from center

Center of the Billet 2” away from center 3” away from center 4” away from center

Center of the Billet 2” away from center 3” away from center 4” away from center

250μm

Coupon C

250μm 250μm 100μm

Axial Views :

250μm

Hoop

Radial

Coupon A

250μm

Hoop

Radial

Coupon A Coupon D Coupon G Coupon J

Coupon B

Radial Views :

Hoop Views :

Hoop

Axial

Radial

Axial

Coupon H Coupon K

Coupon I Coupon L

100μm 250μm 250μm

250μm 250μm

Coupon E

250μm

Coupon F

250μm

Center of the Billet 2” away from center 3” away from center 4” away from center

Center of the Billet 2” away from center 3” away from center 4” away from center

Center of the Billet 2” away from center 3” away from center 4” away from center

250μm

Coupon C

FIGURE 3-60. AXIAL, RADIAL, AND HOOP VIEWS OF THE WASPALOY 

MICROSTRUCTURE AT FOUR METALLOGRAPHY SITES 

Coupon B

Hoop

Axial

100μm

Coupon K

50μm

GFM-A Radial View

Center of Billet

GFM-A Radial View

4” from Center

Coupon B

Hoop

Axial

100μm

Coupon K

50μm

GFM-A Radial View

Center of Billet

GFM-A Radial View

4” from Center

FIGURE 3-61. RADIAL VIEWS OF THE IN718 GFM-A MICROSTRUCTURE 

AT TWO SITES 

3-66
3.1.5.2 Grain-Sizing Methods. 

Grain size information was deduced from a digitized image of the grain boundaries. The first 

phase of the analysis is illustrated in figure 3-62. A clear transparency is placed over the 

microphotograph and the presumed grain boundaries are traced by hand in black ink. The image 

on the transparency, which has excellent black and white contrast, is scanned and digitized. 

Commercial software is then used to skeletonize the image, i.e., to reduce the thickness of each 

grain boundary line to a single pixel. When the tracings are made, an attempt is made to identify 

the so-called twin boundaries and exclude them from the tracing. This is done because it is 

believed that these boundaries do not significantly contribute to attenuation and backscattered 

noise. Twin grain pairs are characterized by a shared boundary that consists of one or more very 

straight-line segments. Typical examples of presumed twin boundaries are shown in figure 3-63. 

Photograph Digitized image of

grain boundary tracing

Skeletonized image

(grain boundaries 1 pixel wide)

(a) (b) (c) 

FIGURE 3-62. EXAMPLE OF PROCESSING A GRAIN STRUCTURE 

(a) PHOTOGRAPH, (b) DIGITIZED IMAGE, AND (c) SKELETONIZED 

IMAGES OF GRAIN BOUNDARIES 

Microstructure photographed. Grain boundaries traced and digitized.

(Twin boundaries ignored)

TB

TB

TB

TB

(a) (b)

Microstructure photographed. Grain boundaries traced and digitized.

(Twin boundaries ignored)

TB

TB

TB

TB

(a) (b)

FIGURE 3-63. (a) MICROGRAPH SUBSECTION WITH SEVERAL PRESUMED 

TWIN BOUNDARIES IDENTIFIED AND (b) BOUNDARIES IGNORED IN THE FINAL 

DIGITIZED IMAGE 

3-67
Twin grains typically have atomic lattice directions that are nearly aligned with one another. 

Sound energy crossing the boundary between twin grains will, thus, encounter only a minor 

velocity shift, and the sonic reflection coefficient at the boundary consequently tends to be much 

smaller than at a typical grain boundary. Since relatively little energy is scattered at twin 

boundaries, they are not believed to play a major role in determining UT attenuation and 

backscattered noise levels. Since the goal is to correlate grain diameter estimates with these two 

UT properties, a conscious effort has been made to identify and exclude obvious twin boundaries 

from the grain size analysis. However, as will be discussed in section 3.1.5.3, some photographs 

were reanalyzed counting twin boundaries as bona fide boundaries. This allowed one to 

determine how the decision to exclude twin boundaries affected the deduced average grain 

diameter. 

For grain size determinations, it is important to accurately determine the scaling factor that 

translates from pixel coordinates in the skelontonized image to physical coordinates on the metal 

specimen. This was done by (1) selecting two widely separated points on opposite corners of the 

image (e.g., easily recognizable vertices where three grains meet), (2) determining the separation 

between the two points on the skeletonized image in pixel coordinates, (3) determining the 

separation between the two points on the original photograph in microns, and (4) equating the 

two separation measurements to determine the scaling factor in microns/pixel. The process is 

discussed in more detail in references 38 and 45. 

Two different methods were used to deduce average grain diameters from a given skeletonized 

image of the grain boundaries. The first, known as the mean chord length (MCL) method, is 

illustrated in figure 3-64. This method is equivalent to (1) drawing a set of parallel straight lines 

through the micrograph, (2) examining each line to determine the points where it crosses grain 

boundaries, (3) calculating the average length of the small line segments that connect the points 

(i.e., segments which lie within a single grain), and (4) multiplying this average chord length by 

1.5 to obtain a grain diameter estimate. A computer program operating on the digital image of 

the grain boundaries is used to draw all possible lines in either the horizontal (X) or vertical (Y) 

direction, and to calculate the chord lengths for each line [45]. The conversion factor of 1.5, 

which translates from two-dimensional images to three-dimensional diameters, is believed to be 

appropriate for equiaxed grains of the kind seen in these billets. This is because if a line is 

randomly passed through a sphere, the length of the line segment within the sphere can range 

from 0 (line tangent to the sphere) to the sphere diameter (line passing through center of the 

sphere). If one considers all possible lines (parallel to some given direction) that intersect the 

sphere, the average line segment within the sphere is 2/3 of the sphere diameter. Hence, the 

sphere diameter is 3/2 of the MCL. 

The second, known as the P(L) method, is an alternative approach to grain sizing and is 

summarized in figure 3-65. There, one begins by analyzing the skeletonized grain boundary 

image to determine: 

P(L) = the probability P that a line segment of length L, randomly placed on the 

micrograph, lies entirely within a single grain. 

3-68
X

Y

S1

S2

S3 S4

MCL Method:

Horizontal (X) or vertical (Y) 

lines are drawn through the 

tracing. 

The average of the chord 

lengths (S j ) is calculated.

The average chord length is 

multiplied by 1.5 to obtain an 

estimate of the average grain 

diameter.

S6

S5

X

Y

S1

S2

S3 S4

MCL Method:

Horizontal (X) or vertical (Y) 

lines are drawn through the 

tracing. 

The average of the chord 

lengths (S j ) is calculated.

The average chord length is 

multiplied by 1.5 to obtain an 

estimate of the average grain 

diameter.

S6

S5

FIGURE 3-64. MEAN CHORD LENGTH METHOD FOR DETERMINING 

AVERAGE GRAIN DIAMETER 

For a given length L, line segments of that 

length are randomly placed on the tracing. 

Some cross grain boundaries, some do 

not. Count each type to determine:

P(L) = probability that a segment of length

L lies entirely within one grain.

Repeat for a series of different lengths.

Fit an exponential function e –L / b to P(L).

Use 2b as an estimate of the average grain 

diameter.

L

L

L

L

L

X

Y

P(L) Method:

L

L

L

L

For a given length L, line segments of that 

length are randomly placed on the tracing. 

Some cross grain boundaries, some do 

not. Count each type to determine:

P(L) = probability that a segment of length

L lies entirely within one grain.

Repeat for a series of different lengths.

Fit an exponential function e –L / b to P(L).

Use 2b as an estimate of the average grain 

diameter.

L

L

L

L

L

X

Y

P(L) Method:

L

L

L

L

FIGURE 3-65. P(L) METHOD FOR DETERMINING AVERAGE GRAIN DIAMETER 

For a given line segment length (L), a computer program is used to generate the line segments, 

place them on the image, and decide whether a given segment crosses a grain boundary [45]. 

One begins by considering short line segments having a length of one pixel and progresses 

systematically to longer lengths until the line segment becomes so large that it always crosses a 

grain boundary regardless of where it is placed on the image [P(L)=0]. As with the MCL 

method, the P(L) method can be applied separately in the X and Y directions of the image. The 

X or Y analysis of a micrograph yields a probability-versus-length curve, P(L). This curve is 

then fit to an exponential function P(L) = exp(-L/b), and 2b is used as an estimate of the mean 

grain diameter [43]. Typical measured P(L) curves do not exactly follow an exponential 

function, leading to some uncertainty in the fitting parameter b and the associated grain diameter 

estimate. To document this uncertainty, separate fits were performed to different regions of the 

3-69
measured P(L) curve. This is illustrated in figure 3-66, where fits to the low L and high L halves 

of the data are shown for one case. In practice, separate fits were performed to ten overlapping 

regions of the measured P(L) curve, with each such region comprising 50% of the horizontal 

range of the P(L) data; the minimum, maximum, and mean values of b thus obtained are then 

tabulated. Note that the P(L) curve for a given microstructure, sometimes referred to as a twopoint correlation function, is of interest because it appears in a direct manner in certain models of 

UT beam propagation and scattering, playing a key role in determining the levels of UT 

attenuation and backscattered grain noise [29 and 43]. 

V-DIE-A coupon "D" (axial view)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50

L (microns)

P(L)

60

Measured P(L) - x&y combined

Low-L exponential fit (b = 11.50 microns)

High L exponential fit (b = 9.26 microns)

The blue curve is a fit to the lower 

half of the data (0 < L < 27 microns). 

The red curve is a fit to the upper 

half of the data (27 < L < 54 microns) 

V-DIE-A coupon "D" (axial view)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50

L (microns)

P(L)

60

Measured P(L) - x&y combined

Low-L exponential fit (b = 11.50 microns)

High L exponential fit (b = 9.26 microns)

The blue curve is a fit to the lower 

half of the data (0 < L < 27 microns). 

The red curve is a fit to the upper 

half of the data (27 < L < 54 microns) 

FIGURE 3-66. EXAMPLES OF EXPONENTIAL FITS TO MEASURED P(L) CURVES 

FOR A TYPICAL CASE 

A Fortran program named GrnSize3 was developed at ISU to perform both the MCL and P(L) 

analyses [45]. For a given skeletonized image that is input to the program, grain-sizing results 

are summarized in the manner illustrated in table 3-9. The P(L) and MCL analyses are 

summarized in the upper and lower portion of the table, respectively. Note that for each sizing 

method, separate analyses are carried out for X alone, for Y alone, and for XY. The latter 

describes an analysis in which the data for the X and Y directions is combined and then 

processed to determine a grain size estimate. For the P(L) method, the third analysis involves 

averaging the measured X and Y P(L) curves, and then fitting exponentials to the average curve. 

For the MCL method, the third analysis is equivalent to simply averaging the MCL estimates 

from the separate X and Y analyses to obtain a new MCL estimate. Notice the “Where” 

parameter in table 3-9, which describes the portion of the measured P(L) curve that is being fit to 

an exponential function. As Where varies from 0 to 100, the 50% of the data being fit varies 

from the extreme low-L portion to the extreme high-L portion in a systematic manner. For the 

case shown in table 3-9, the P(L) method yielded an average grain diameter estimate of 30.20 

microns for the analysis in which (1) the X and Y data were combined to determine the overall 

P(L) curve; (2) ten separate exponential fits were made to various portions of that P(L) curve; 

3-70
and (3) the ten grain size estimates resulting from the 10 separate fits were averaged. For the 

case shown in table 3-9, the MCL method yielded an average grain diameter of 33.07 microns 

when averaged over the X and Y data. Details of the inner workings and use of the Grnsize3 

software can be found in the User’s Guide [45]. 

TABLE 3-9. SUMMARY OF GRAIN-SIZING ANALYSES FOR ONE MICROGRAPH OF 

METALLOGRAPHY COUPON C OF THE GFM-A SPECIMEN (0′′ site; hoop view) 

Table of b values for P(L) in microns:

“Where” b for x b for y b for xy

AVE b = 14.55 15.81 15.10

AVE. GRAIN DIA. VIA P(L) (microns, for xy) = 2b = 30.20

Grain Diameter

Mean chord length along x (microns) : 21.4803

Mean chord length along y (microns) : 22.6075

Average MCL for x & y (microns) : 22.0439

AVE. GRAIN DIA. VIA MCL (microns, for xy) = 33.07

AVE. FROM THE TWO METHODS= 31.64 microns

MCL Average

0.00E+00 1.63E+01 1.77E+01 1.69E+01

1.00E+01 1.61E+01 1.75E+01 1.68E+01

2.00E+01 1.58E+01 1.73E+01 1.65E+01

3.00E+01 1.55E+01 1.69E+01 1.61E+01

4.00E+01 1.50E+01 1.64E+01 1.56E+01

5.00E+01 1.46E+01 1.59E+01 1.52E+01

6.00E+01 1.41E+01 1.54E+01 1.47E+01

7.00E+01 1.37E+01 1.49E+01 1.42E+01

8.00E+01 1.33E+01 1.44E+01 1.38E+01

9.00E+01 1.30E+01 1.40E+01 1.34E+01

1.00E+02 1.27E+01 1.35E+01 1.30E+01

Ave. Grain

Diameter

Highest b for x:

Lowest b for x:

Highest b for y:

Lowest b for y:

Highest b for xy:

Lowest b for xy:

b 

16.27 32.54

12.66 25.32

17.72 35.44

13.49 26.98

16.91 33.83

12.99 25.99

P(L)

32.22

33.91

33.07

Table of b values for P(L) in microns:

“Where” b for x b for y b for xy

AVE b = 14.55 15.81 15.10

AVE. GRAIN DIA. VIA P(L) (microns, for xy) = 2b = 30.20

Grain Diameter

Mean chord length along x (microns) : 21.4803

Mean chord length along y (microns) : 22.6075

Average MCL for x & y (microns) : 22.0439

AVE. GRAIN DIA. VIA MCL (microns, for xy) = 33.07

AVE. FROM THE TWO METHODS= 31.64 microns

MCL Average

0.00E+00 1.63E+01 1.77E+01 1.69E+01

1.00E+01 1.61E+01 1.75E+01 1.68E+01

2.00E+01 1.58E+01 1.73E+01 1.65E+01

3.00E+01 1.55E+01 1.69E+01 1.61E+01

4.00E+01 1.50E+01 1.64E+01 1.56E+01

5.00E+01 1.46E+01 1.59E+01 1.52E+01

6.00E+01 1.41E+01 1.54E+01 1.47E+01

7.00E+01 1.37E+01 1.49E+01 1.42E+01

8.00E+01 1.33E+01 1.44E+01 1.38E+01

9.00E+01 1.30E+01 1.40E+01 1.34E+01

1.00E+02 1.27E+01 1.35E+01 1.30E+01

0.00E+00 1.63E+01 1.77E+01 1.69E+01

1.00E+01 1.61E+01 1.75E+01 1.68E+01

2.00E+01 1.58E+01 1.73E+01 1.65E+01

3.00E+01 1.55E+01 1.69E+01 1.61E+01

4.00E+01 1.50E+01 1.64E+01 1.56E+01

5.00E+01 1.46E+01 1.59E+01 1.52E+01

6.00E+01 1.41E+01 1.54E+01 1.47E+01

7.00E+01 1.37E+01 1.49E+01 1.42E+01

8.00E+01 1.33E+01 1.44E+01 1.38E+01

9.00E+01 1.30E+01 1.40E+01 1.34E+01

1.00E+02 1.27E+01 1.35E+01 1.30E+01

Ave. Grain

Diameter

Highest b for x:

Lowest b for x:

Highest b for y:

Lowest b for y:

Highest b for xy:

Lowest b for xy:

b 

16.27 32.54

12.66 25.32

17.72 35.44

13.49 26.98

16.91 33.83

12.99 25.99

P(L)

Ave. Grain

Diameter

Highest b for x:

Lowest b for x:

Highest b for y:

Lowest b for y:

Highest b for xy:

Lowest b for xy:

b 

16.27 32.54

12.66 25.32

17.72 35.44

13.49 26.98

16.91 33.83

12.99 25.99

16.27 32.54

12.66 25.32

17.72 35.44

13.49 26.98

16.91 33.83

12.99 25.99

P(L)

32.22

33.91

33.07

3.1.5.3 Grain-Sizing Results. 

The results of the grain-sizing analyses performed on micrographs from the 48 metallographic 

coupons are listed in tables 3-10 through 3-13. The first two columns of each table list the 

coupon site (distance from billet center) and coupon designation as defined in figure 3-58(b). 

The third column lists grain diameter estimates inferred from the attenuation measurements; the 

tabulated value is the mean grain diameter of an equiaxed pure Ni microstructure having a 

predicted attenuation-versus-frequency curve that approximately matches the measured curve. 

Note that the third column results are for reference only; one does not expect these predicted 

grain diameters to match measured ones because the elastic properties of pure Ni are different 

than those of the alloys. However, the predicted grain diameters serve to remind us of the trend 

of the attenuation data, with larger predicted diameters associated with larger measured 

3-71
TABLE 3-10. AVERAGE GRAIN DIAMETERS DEDUCED FROM WASPALOY METALLOGRAPHY 

WASP 

Coupon 

From 

Atten. 

From XY 

ave. of P(L) 

and MCL 

Methods 

From P(L) 

Using 

Highest b 

for X 

From P(L) 

Using 

Lowest b 

for X 

From P(L) 

Using 

Highest b 

for Y 

From P(L) 

Using 

Lowest b 

for Y 

From P(L) 

Using 

Highest b 

for XY 

From P(L) 

Using 

Lowest b 

for XY 

From P(L) 

Using 

Average b

for X 

From P(L) 

Using 

Average b

for Y 

From P(L) 

Using 

Average b

for XY 

0′′0′′0′′0′′

A A* B C 

69 69 66 

115.88 

82.33 

117.16 113.08 

125.01 

89.18 

139.60 119.26 

92.21 73.74 

103.14 

87.93 

128.93 

92.41 

115.79 130.89 

101.78 

79.51 90.06 

106.31 

126.52 

90.54 

126.70 123.85 

95.90 75.88 96.02 95.98 

110.40 

82.63 

123.28 105.14 

117.01 

87.03 

103.75 119.32 

113.01 

84.37 

112.87 110.91 

2′′2′′2′′2′′

D D* E F 

62 62 60 

91.85 70.94 65.29 

105.07 

95.36 79.28 

102.24 112.99 

68.91 66.27 74.80 80.01 

104.18 

76.15 

102.85 114.23 

76.44 65.35 79.88 89.43 

98.82 77.20 

102.03 111.72 

70.33 62.15 76.45 79.88 

81.89 72.12 90.03 97.11 

92.05 71.38 92.49 

102.32 

85.55 70.04 90.59 96.86 

3′′3′′3′′3′′

G G* H I 

47 47 45 

74.83 63.37 87.91 72.76 

83.37 69.48 

112.99 

80.71 

61.36 53.76 80.01 60.24 

76.52 66.56 

114.23 

78.38 

55.87 51.16 89.43 56.83 

79.78 67.82 

111.72 

78.27 

58.73 51.75 79.88 55.04 

73.75 62.72 97.11 71.74 

67.40 59.93 

102.32 

69.17 

70.54 60.91 96.86 68.42 

4′′4′′4′′4′′

J J* K L 

28 28 28 

44.36 36.04 50.01 47.70 

51.10 42.84 54.22 50.24 

36.93 33.27 40.61 37.63 

50.14 38.99 58.48 50.38 

37.45 30.92 45.40 36.21 

50.56 40.67 55.80 50.20 

37.05 31.46 41.86 36.60 

44.90 38.78 48.35 44.53 

44.75 35.48 52.77 44.24 

44.74 36.77 49.80 44.14 

Notes:

Coupon 

From 

Atten. 

From MCL 

for X 

From MCL 

for Y 

From MCL 

for XY 

0′′0′′0′′0′′

A A* B C 

69 69 69 

116.70 

79.07 

131.26 111.05 

120.82 

81.49 

111.65 119.46 

118.76 

80.28 

121.46 115.25 

2′′2′′2′′2′′

D D* E F 

62 62 60 

97.33 72.19 

102.08 114.44 

98.96 71.49 97.88 

112.10 

98.14 71.84 99.98 

113.27 

3′′3′′3′′3′′

G G* H I 

47 47 45 

81.12 66.40 81.53 75.97 

77.13 65.25 76.37 78.22 

79.13 65.83 78.95 77.09 

4′′4′′4′′4′′

J J* K L 

28 28 28 

42.92 36.13 49.46 50.91 

45.02 34.49 51.00 51.58 

43.97 35.31 50.23 51.25 

All numerical values are average grain diameters in microns. A,B, C,...L identify the 12 metallography coupons:  A, D. G, J present axial views of the microstructure.  B, E, H, K present radial views of the microstructure.  C, F, I, L present hoop views of the microstructure. Results are given for analyses in the X and Y directions of each micrograph, and for a combined (XY) analysis. In most cases, apparent twin boundaries were not counted as bona fide grain boundaries when making tracings. However for entries denoted A*, D*, G*, and J*, twin boundaries were counted as bona fide grain boundaries (leading to a smaller average grain size.) The result labeled “From Atten.” is the average grain diameter of a pure Ni microstructure, which has the same attenuation near 7.5 MHz as measured for the Ni alloy specimen in the corresponding inspection direction. 

3-72
TABLE 3-11. AVERAGE GRAIN DIAMETERS DEDUCED FROM GFM-A METALLOGRAPHY 

GFM-A 

Coupon 

From 

Atten. 

From XY 

Ave. of 

P(L) and 

MCL 

Methods 

From P(L) 

Using 

Highest b 

for X 

From P(L) 

Using 

Lowest b 

for X 

From P(L) 

Using 

Highest b 

for Y 

From P(L) 

Using 

Lowest b 

for Y 

From P(L) 

Using 

Highest b 

 for XY 

From P(L) 

Using 

Lowest b 

 for XY 

From P(L) 

Using 

Average b 

for X 

From P(L) 

Using 

Average b

 for Y 

From P(L) 

Using 

Average b 

for XY 

0′′0′′0′′

A B C 

25 26 

36.28 32.63 31.64 

41.07 34.57 32.54 

39.68 26.39 25.32 

39.17 36.59 35.44 

37.57 29.13 26.98 

39.47 35.55 33.83 

38.20 27.82 25.99 

40.74 30.86 29.10 

38.14 33.55 31.61 

39.09 32.22 30.20 

2′′2′′2′′

D E F 

24 21 

30.40 26.58 25.06 

32.85 30.22 28.29 

27.25 25.16 22.04 

31.99 30.01 27.27 

23.49 22.68 20.48 

32.37 29.68 27.50 

25.40 22.84 20.64 

30.05 27.52 25.51 

28.27 26.79 24.24 

29.11 26.46 24.47 

3′′3′′3′′

G H I 

18 18 

24.69 23.07 19.84 

26.23 25.31 21.84 

21.82 17.55 15.60 

27.12 24.82 22.54 

26.23 19.09 15.43 

26.68 24.82 21.56 

23.04 17.86 13.93 

24.61 21.90 19.05 

26.92 22.20 19.58 

25.54 21.70 18.29 

4′′4′′4′′

J K L 

14 15 

16.75 17.27 14.60 

17.91 19.29 14.15 

12.87 15.41 

9.75 

17.70 17.62 16.55 

13.58 15.20 14.28 

17.69 18.22 15.15 

12.83 14.94 12.29 

15.69 17.48 12.20 

15.69 16.24 15.21 

15.47 16.44 13.50 

Notes:

Coupon 

From 

Atten. 

From MCL 

Along X 

From MCL 

Along Y 

From MCL 

Along XY 

0′′0′′0′′

A B C 

25 26 

34.77 31.89 32.22 

32.17 34.21 33.91 

33.47 33.05 33.07 

2′′2′′2′′

D E F 

24 21 

32.49 26.17 26.14 

30.92 27.23 25.17 

31.70 26.70 25.66 

3′′3′′3′′

G H I 

18 18 

23.59 24.66 20.11 

24.10 24.21 22.66 

23.85 24.44 21.39 

4′′4′′4′′

J K L 

14 15 

17.96 18.57 14.67 

18.10 17.61 16.72 

18.03 18.09 15.69 

All numerical values are average grain diameters in microns. A,B, C,...L identify the 12 metallography coupons:  A, D. G, J present axial views of the microstructure.  B, E, H, K present radial views of the microstructure.  C, F, I, L present hoop views of the microstructure. Results are given for analyses in the X and Y directions of each micrograph, and for a combined (XY) analysis. In most cases, apparent twin boundaries were not counted as bona fide grain boundaries when making tracings. The result labeled “From Atten.” is the average grain diameter of a pure Ni microstructure, which has the same attenuation near 7.5 MHz as measured for the Ni alloy specimen in the corresponding inspection direction. 

3-73
TABLE 3-12. AVERAGE GRAIN DIAMETERS DEDUCED FROM V-DIE-A METALLOGRAPHY 

V-Die-A 

Coupon 

From 

Atten. 

From XY 

Ave. of 

P(L) and 

MCL 

Methods 

From P(L) 

Using 

Highest b 

 for X 

From P(L) 

Using 

Lowest b 

for X 

From P(L) 

Using 

Highest b 

for Y 

From P(L) 

Using 

Lowest b 

 for Y 

From P(L) 

Using 

Highest b 

for XY 

From P(L) 

Using 

Lowest b 

 for XY 

From P(L) 

Using 

Average b

 for X 

From P(L) 

Using 

Average b

 for Y 

From P(L) 

Using 

Average b

 for XY 

0′′0′′0′′

A B C 

19 18 

24.84 25.53 21.15 

27.05 29.97 25.47 

20.74 21.25 22.96 

28.85 26.20 21.73 

24.89 18.89 16.68 

27.72 27.95 21.87 

21.27 20.03 19.63 

24.31 26.11 23.67 

27.06 22.93 19.48 

25.08 24.45 20.63 

2′′2′′2′′

D E F 

18 18 

21.63 20.48 23.89 

24.43 22.48 24.19 

19.24 18.83 18.82 

23.12 21.48 27.90 

17.80 18.91 25.19 

23.74 21.88 25.93 

18.50 18.41 22.08 

22.05 20.51 21.99 

20.42 20.22 26.78 

21.20 20.15 24.31 

3′′3′′3′′

G H I 

21 22 

17.89 15.58 14.14 

19.38 18.08 14.99 

16.16 16.70 13.29 

18.84 15.66 14.55 

15.62 11.87 11.48 

19.10 16.49 14.76 

15.89 14.08 12.43 

17.79 17.23 14.07 

17.31 13.97 12.97 

17.55 15.09 13.51 

4′′4′′4′′

J K L 

24 22 

20.60 23.78 15.62 

23.11 31.11 15.27 

18.14 29.28 11.45 

21.14 20.02 18.21 

17.06 16.66 15.77 

22.03 25.00 16.51 

17.43 23.69 13.51 

21.02 29.99 13.58 

19.28 18.37 17.02 

20.05 24.28 15.06 

Notes:

Coupon 

From 

Atten. 

From MCL 

Along X 

From MCL 

Along Y 

From MCL 

Along XY 

0′′0′′0′′

A B C 

19 18 

22.45 27.06 22.15 

26.78 26.21 21.21 

24.61 26.63 21.68 

2′′2′′2′′

D E F 

18 18 

21.92 21.12 22.33 

22.23 20.53 24.65 

22.08 20.83 23.49 

3′′3′′3′′

G H I 

21 22 

18.41 16.60 14.85 

18.08 15.55 14.71 

18.25 16.08 14.78 

4′′4′′4′′

J K L 

24 22 

21.65 26.99 15.47 

20.65 19.60 16.90 

21.15 23.30 16.18 

All numerical values are average grain diameters in microns. A,B, C,...L identify the 12 metallography coupons:  A, D. G, J present axial views of the microstructure.  B, E, H, K present radial views of the microstructure.  C, F, I, L present hoop views of the microstructure. Results are given for analyses in the X and Y directions of each micrograph, and for a combined (XY) analysis. In most cases, apparent twin boundaries were not counted as bona fide grain boundaries when making tracings. The result labeled “From Atten.” is the average grain diameter of a pure Ni microstructure, which has the same attenuation near 7.5 MHz as measured for the Ni alloy specimen in the corresponding inspection direction. 

3-74
TABLE 3-13. AVERAGE GRAIN DIAMETERS DEDUCED FROM V-DIE-B METALLOGR

V-Die-B 

Coupon 

From 

Atten. 

From XY 

Ave. of 

P(L) and 

MCL 

Methods 

From P(L) 

Using 

Highest b 

for X 

From P(L) 

Using 

Lowest b 

for X 

From P(L) 

Using 

Highest b 

for Y 

From P(L) 

Using 

Lowest b 

for Y 

From P(L) 

Using 

Highest b 

for XY 

From P(L) 

Using 

Lowest b 

for XY 

From P(L) 

Using 

Average b

 for X 

0′′0′′0′′

A B C 

17 17 

23.41 27.36 24.58 

26.24 31.04 25.89 

22.97 24.89 18.95 

24.80 31.14 29.11 

19.15 23.59 25.66 

25.49 30.77 26.29 

21.02 23.39 21.08 

24.87 28.59 22.77 

2′′2′′2′′

D E F 

17 19 

21.32 21.25 18.11 

22.37 23.42 18.57 

18.56 17.14 14.49 

23.95 22.89 20.21 

19.63 18.77 16.90 

23.00 23.06 19.15 

18.53 17.68 15.11 

20.67 20.60 16.66 

3′′3′′3′′

G H I 

13 14 

16.73 17.10 15.49 

18.36 19.29 15.99 

12.79 15.30 13.44 

18.15 17.11 17.07 

13.76 13.90 13.19 

17.84 18.13 16.39 

12.46 14.52 12.89 

15.86 17.49 14.65 

4′′4′′4′′

J K L 

<5 

7 

13.24 12.39 13.82 

14.57 13.95 14.30 

11.01 10.24 10.80 

13.02 12.03 14.52 

9.27 9.25 

10.04 

13.75 12.90 14.35 

10.11 

9.65 

10.30 

12.95 12.32 12.69 

Notes:

Coupon 

From 

Atten. 

From MCL 

Along X 

From MCL 

Along Y 

From MCL 

Along XY 

0′′0′′0′′

A B C 

17 17 

23.18 25.89 23.90 

23.28 28.06 27.11 

23.23 26.97 25.51 

2′′2′′2′′

D E F 

17 19 

20.48 22.77 18.22 

22.76 20.91 19.83 

21.62 21.84 19.02 

3′′3′′3′′

G H I 

13 14 

17.94 18.59 15.82 

18.03 16.83 16.57 

17.98 17.71 16.20 

4′′4′′4′′

J K L 

<5 

7 

14.96 14.06 14.97 

13.80 12.61 15.29 

14.38 13.34 15.13 

All numerical values are average grain diameters in microns. A,B, C,...L identify the 12 metallography coupons:  A, D. G, J present axial views of the microstructure.  B, E, H, K present radial views of the microstructure.  C, F, I, L present hoop views of the microstructure. Results are given for analyses in the X and Y directions of each micrograph, and for a combined (XY) analysis. In most cases, apparent twin boundaries were not counted as bona fide grain boundaries when making tracings. The result labeled “From Atten.” is the average grain diameter of a pure Ni microstructure, which has the same attenuation near 7.5 MHz as measured for the Ni alloy specimen in the corresponding inspection direction. 

3-75
attenuation values. The fourth column in the upper half of tables 3-10 through 3-13 lists an 

overall estimate of the mean grain diameter; this is obtained by averaging the results of the MCL 

and P(L) analyses, which use the combined data from the X and Y directions of the micrograph 

in question. The remainder of the columns in the upper half of each table list various results 

from the P(L) analyses. These include mean grain diameters deduced using the highest, lowest, 

and average b-parameters from the various exponential fits to the measured P(L) curve, with 

values listed separately for the X, Y, and combined (XY) micrograph directions. The lower half 

of tables 3-10 through 3-13 lists the mean grain diameters resulting from the MCL analyses of 

the X, Y, and combined (XY) data. 

It was endeavored to make grain diameter estimates near the sites where UT property 

measurements were conducted without cutting the property measurement coupons themselves. 

Absolute errors in grain diameter estimates at the UT sites are believed to mainly arise from 

three factors: (1) the process of identifying nontwin grain boundaries is somewhat subjective, 

(2) a given micrograph containing a few hundred grains represents only a very minute fraction of 

the specimen volume that is insonified in a typical UT measurement, and (3) because the 

metallography coupons were taken from the rough-cut slab shown in figure 3-58(a), there is 

approximately a 1′′ axial shift between the locations where metallography was done and the 

center of the associated UT property measurement region. 

It is difficult to estimate the absolute grain diameter error that results from these combined 

factors, but it was suspected that it could be 25% or more in isolated cases. 

The effect of eliminating the twin grain boundaries from the analysis was also studied. For four 

of the Waspaloy micrographs, two analyses were carried out: one in which apparent twin 

boundaries were identified and excluded from the tracing (the standard procedure) and another in 

which apparent twin boundaries were included in the tracing. The results, taken from column 4 

of table 3-10, are graphically summarized in figure 3-67. Retaining twin boundaries as bona fide 

grain boundaries will, of course, reduce the average grain diameter below that obtained from the 

standard analysis. In the Waspaloy study, retaining twin boundaries decreased the mean grain 

diameter to about 76% of the standard treatment. That ratio is likely similar for the IN718 cases 

as well, since twin boundaries were observed to be present in roughly similar proportions in the 

micrographs of both alloys. 

Each of the grain structure micrographs had its X and Y axes approximately aligned with the 

principal billet axes, i.e., with the axial, radial, or hoop directions. For example, in figure 3-58, 

the X and Y axes for micrographs of coupons A, D, G, and J are parallel to the radial and hoop 

directions, respectively. In all the cases examined, the grain cross-sections shown in the 

micrographs had roughly similar average dimensions in the X and Y directions. This indicated 

that there was no general tendency for grains to be preferentially longer, for instance, in the 

radial direction than in the hoop direction. It also reinforced the finding, from visual observation 

of the micrographs, that the grain structures were approximately equiaxed (i.e., spherical). This 

is demonstrated in figure 3-68 for the MCL method. For each of the 4 × 12 = 48 metallography 

coupons, the grain diameter estimate from the analysis in the Y direction is plotted versus the 

result for the X direction. The grain diameter estimates vary widely, from about 15 microns for 

some IN718 coupons to about 120 microns for some Waspaloy coupons. However, for each 

3-76
g

y = 0.761x

0

20

40

60

80

100

120

0 50 100 150

Mean Grain Diameter (Microns)

(Standard Treatment)

(Twin Boundaries Not Traced)

Mean Grain Diameter (Microns)

(Trace all Boundaries)

A

DG

J

Results for 4

Waspaloy Coupons

FIGURE 3-67. EFFECT OF TWIN GRAIN TREATMENT ON DEDUCED 

MEAN DIAMETER 

micrograph, the X and Y diameter estimates tend to be very similar to each other, and thus, the 

plotted points in figure 3-68 lie close to the diagonal line that would result from a perfectly 

equiaxed microstructure. The P(L) analysis similarly finds nearly equiaxed grain diameters. 

GRAIN DIAMETER ESTIMATES 

USING THE MCL METHOD

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140

Estimate along X-direction

Estimate along Y-direction .

GFMA

V-DIE-A

V-DIE-B

WASP

Equiaxed Grains

GRAIN DIAMETER ESTIMATES 

USING THE MCL METHOD

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40

Estimate along X-direction

Estimate along Y-direction .

GFMA

V-DIE-A

V-DIE-B

Equiaxed Grains

(in microns) (in microns)

GRAIN DIAMETER ESTIMATES 

USING THE MCL METHOD

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140

Estimate along X-direction

Estimate along Y-direction .

GFMA

V-DIE-A

V-DIE-B

WASP

Equiaxed Grains

GRAIN DIAMETER ESTIMATES 

USING THE MCL METHOD

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40

Estimate along X-direction

Estimate along Y-direction .

GFMA

V-DIE-A

V-DIE-B

Equiaxed Grains

(in microns) (in microns)

FIGURE 3-68. COMPARISONS OF AVERAGE GRAIN DIAMETERS IN THE X AND 

Y DIRECTIONS OF Ni ALLOY MICROGRAPHS USING THE MCL METHOD 

(The right-hand panel is a blowup of the small diameter region where the IN718 values lie.) 

When a given micrograph is analyzed, the MCL grain size estimate usually falls in between the 

minimum and maximum estimates from the P(L) method. This is demonstrated in figure 3-69 

for average grain sizes deduced from the 12 V-Die-B metallography coupons. In the figure, the 

coupons have been arranged in order of increasing MCL grain size estimate to give the plot a 

3-77
smoother appearance. When the P(L) method was applied to a given micrograph, exponentials 

were fitted separately to ten regions of the P(L) data as usual, and the resulting minimum, 

maximum, and mean estimates of average grain diameter are all shown in figure 3-69. One sees 

that in all but one case (coupon K), the MCL grain diameter estimate lies between the minimum 

and maximum estimates from the P(L) method. 

0

5

10

15

20

25

30

I HLGEJ CDKFAB

Metallography Coupon

Ave. Grain Diameter (microns) .

D from 1.5*MCL

D from 2*(ave b)

D from 2*(low b)

D from 2*(high b)

MCL = Mean Chord Length.

Fitting exponentials [exp(-L/b)] to 

different regions of the P(L) data

leads to three different estimates of b: 

highest b, low est b, and average b.

FIGURE 3-69. COMPARISONS OF AVERAGE GRAIN DIAMETERS IN THE 12 V-DIE-B 

MICROGRAPHS AS ESTIMATED USING THE MCL AND P(L) METHODS 

(In each case, results are averaged over the X and Y directions.) 

Overall, it was found that the average grain diameter estimate from the P(L) method tended to be 

about 5% below that from the MCL method. This is demonstrated in figure 3-70. For each of 

the 48 metallography coupons, the average grain diameter estimate from the P(L) method is 

plotted versus the estimate from the MCL method. For both methods, an average over the X and 

Y directions has been taken, and the P(L) results are averages over the ten exponential fits to 

various regions of the P(L) data. The plotted points in the figure lie nearly along a straight line, 

indicating a high degree of correlation between the two grain diameter estimates. A best-fit line 

through the plotted points is also shown and shows a slope of 0.946. This indicates that, on 

average, the P(L) grain diameter estimate is 94.6% of the MCL estimate. 

Since the grains generally appear to be nearly equiaxed and the two analysis methods yield 

similar diameter estimates, the data were averaged over the two analysis directions for [X and Y] 

and over the two methods [MCL and average P(L)] to arrive at an overall best grain diameter 

estimate for each image. These best estimates for each micrograph were listed earlier in column 

four of tables 3-10 through 3-13. Finally, three viewing directions were averaged over each 

metallography site (e.g., over coupons A, B, and C at the billet center) to obtain the best estimate 

of the mean grain diameter at a given radial depth. These latter estimates are listed in table 3-14. 

3-78
GRAIN DIAMETER ESTIMATES

 (Averaged over X and Y)

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

MCL ESTIMATE (microns)

P(L) ESTIMATE (microns) .

GFMA

V-DIE-A

V-DIE-B

WASP

TRENDLINE

y = 0.9461x

R2 = 0.9796

GRAIN DIAMETER ESTIMATES

 (Averaged over X and Y)

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40

MCL ESTIMATE (microns)

P(L) ESTIMATE (microns) .

GFMA

V-DIE-A

V-DIE-B

WASP

TRENDLINE

y = 0.9461x

R2 = 0.9796

GRAIN DIAMETER ESTIMATES

 (Averaged over X and Y)

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

MCL ESTIMATE (microns)

P(L) ESTIMATE (microns) .

GFMA

V-DIE-A

V-DIE-B

WASP

TRENDLINE

y = 0.9461x

R2 = 0.9796

GRAIN DIAMETER ESTIMATES

 (Averaged over X and Y)

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40

MCL ESTIMATE (microns)

P(L) ESTIMATE (microns) .

GFMA

V-DIE-A

V-DIE-B

WASP

TRENDLINE

y = 0.9461x

R2 = 0.9796

FIGURE 3-70. COMPARISONS OF AVERAGE GRAIN DIAMETERS AS ESTIMATED BY 

THE MCL AND P(L) METHODS FOR EACH OF THE 4 x 12 MICROGRAPHS ANALYZED 

(The right-hand panel is a blowup of the small diameter region where the IN718 values lie.) 

TABLE 3-14. BEST ESTIMATES OF AVERAGE GRAIN DIAMETERS AT RADIAL SITES 

IN THE FOUR Ni ALLOY METALLOGRAPHY SLABS 

Average Grain Diameter in Microns 

Site WASP GFM-A V-Die-A V-Die-B 

0′′ 115.39 33.52 23.85 25.12 

2′′ 97.40 27.35 22.01 20.23 

3′′ 78.50 22.53 15.88 16.44 

4′′ 47.36 16.20 20.00 13.15 

Note: Site coordinate is the distance from the billet center. 

As will be discussed in section 3.1.5.4, the grain-sizing results in tables 3-10 through 3-13 were, 

for the most part, reasonably well correlated with the earlier UT results. For a given strip 

coupon, sites with larger grain diameters tended to have higher attenuations and higher FOM 

values. The notable exception was the V-Die-A strip. For that specimen, attenuation and FOM 

values tended to be larger near the OD than near the billet center (see figures 3-50 and 3-51). 

However, metallography performed on the adjacent rough-cut slab found average grain 

diameters to be slightly larger near the billet center than near the billet OD (see table 3-14). 

A second round of metallographic studies was carried out on the V-Die-A rough-cut specimen to 

check the original results. The locations of the 12 metallographic coupons used in the original 

study and the four coupons used in the new study are shown in figures 3-71(a) and 3-71(b), 

respectively. The four new coupons had different dimensions in the hoop direction (ranging 

from 1/4′′ to 5/8′′) so that there was no possibility of misidentifying their locations within the 

V-Die-A rough-cut specimen. The coupons were polished and etched to reveal views of the 

microstructure as seen looking radially inward from the billet OD. The grain structures are 

illustrated in figure 3-72. Average grain sizes were deduced using the MCL and P(L) methods as 

before. The results of the old and new grain size measurements are compared in table 3-15 and 

3-79
are quite similar. The quoted values are averages of the MCL and P(L) estimates using the 

combined XY data, and for the old results, averages over the three viewing directions at a given 

radial site are listed. One sees that for both V-Die-A metallographic studies, the average grain 

size tends to be slightly larger near the billet center than near the billet OD, in apparent conflict 

with the UT data. 

A

B

C

D

E

F

G

H

I

J

K

L

10”

Face polished for axial view.

Face polished for radial view.

Face polished for hoop view.

(polished surface shaded blue)

2”

(a)

(b) 3/16” 3/16” 3/16” 3/16”

Center of billet

Color-shaded regions

show new coupon locations

3/16” 3/16” 3/16” 3/16”

Center of billet

Color-shaded regions

show new coupon locations

0” 3” 2” 4”

A

B

C

D

E

F

G

H

I

J

K

L

10”

Face polished for axial view.

Face polished for radial view.

Face polished for hoop view.

(polished surface shaded blue)

2”

A

B

C

D

E

F

G

H

I

J

K

L

A

B

C

D

E

F

G

H

I

J

K

L

10”

Face polished for axial view.

Face polished for radial view.

Face polished for hoop view.

(polished surface shaded blue)

2”

(a)

(b) 3/16” 3/16” 3/16” 3/16”

Center of billet

Color-shaded regions

show new coupon locations

3/16” 3/16” 3/16” 3/16”

Center of billet

Color-shaded regions

show new coupon locations

(b) 3/16” 3/16” 3/16” 3/16”

Center of billet

Color-shaded regions

show new coupon locations

3/16” 3/16” 3/16” 3/16”

Center of billet

Color-shaded regions

show new coupon locations

0” 3” 2” 4”

FIGURE 3-71. (a) LOCATIONS AND DESIGNATIONS OF THE 12 ORIGINAL 

METALLOGRAPHY COUPONS FROM EACH ROUGH-CUT SPECIMEN AND 

(b) LOCATIONS AND SHAPES OF THE FOUR NEW METALLOGRAPHY 

COUPONS FOR V-DIE-A 

0” from billet center

3” from billet center

2” from billet center

4” from billet center

FIGURE 3-72. LOCAL MICROSTRUCTURES OF THE FOUR NEW METALLOGRAPHY 

COUPONS FROM THE V-DIE-A ROUGH-CUT SLAB 

(Grain size estimates are deduced from similar photographs covering larger areas.) 

3-80
TABLE 3-15. BEST ESTIMATES OF AVERAGE GRAIN DIAMETERS AT 

RADIAL SITES IN THE V-DIE-A METALLOGRAPHY SLAB 

Average Grain Diameter 

(microns) 

Site 

V-Die-A 

4 New Coupons 

V-Die-A 

12 New Coupons 

0′′ 23.31 23.85 

2′′ 25.35 22.01 

3′′ 116.36 15.88 

4′′ 15.18 20.00 

Note: Site coordinate is the distance from the billet center. 

From the original C-scans of the uncut billets and the measurements on the rectangular coupons, 

it was known that backscattered grain noise levels and other UT properties vary with position. 

The metallographic studies show that the billet microstructures vary with position as well and are 

presumably responsible for the UT property variations. One possible explanation for the V-DieA anomaly would be that microstructural variations with position are sufficiently large in the 

vicinity of the V-Die-A strip coupon, or the microstructures of the UT property strip and the 

metallography slab are significantly different. To test this hypothesis, the remnants of the strip 

coupons and metallography slabs were scanned to create C-scan images of backscattered noise 

levels for comparison with the original strip coupon noise images shown in figures 3-19 

through 3-22. This task was complicated by three factors: (1) the UT inspection system used in 

the original noise scans had been replaced with a new system from a different manufacturer, 

making direct absolute noise level comparisons impossible; (2) in the original scans, the depth 

zone from which grain noise was acquired extended beyond the 0.5′′ thickness available in the 

metallography slab; and (3) the surfaces of the metallography slabs were not smooth, being 

rough EDM cuts for the IN718 specimens and rough saw cuts for the Waspaloy specimen. 

Nonetheless, it was hoped that the new scans would reveal how grain noise varied with position 

in the metallography slabs for comparison with the variations seen earlier in the UT property 

strips. 

The same 15-MHz focused transducer employed earlier was used for the new scans, but the 

water path was adjusted to focus the beam about 0.3′′ below the entry surface (side 2 of each 

specimen) and the depth gate for noise acquisition was narrowed to about 0.1′′-0.5′′. Useable 

grain noise images for the Waspaloy metallography slab could not be obtained because of strong 

scattering from the saw-cut ridges on the entry surface. However, useable grain noise images 

were obtained for the three IN718 cases, and these are shown in figures 3-73 through 3-75. Note 

that each figure contains three images. The rightmost two images are the new C-scans of the 

metallography slab and strip coupon remnants, respectively, with both scans carried out at the 

same gain. The holes in the metallography slab where coupons were removed are clearly visible. 

The leftmost image in each figure is the original backscattered grain noise C-scan image 

(see figures 3-19 through 3-22) of the 10′′ long strip coupon with the amplitude-to-color mapping 

altered so that the lower half of the image resembles the new scan of the 4.5′′ long strip coupon 

3-81
remnant. For each of the three IN718 cases, the old and new images of the lower half of the strip 

coupon show similar variations of noise with position, as expected. For GFM-A and V-Die-B, 

the noise patterns in the original strip coupon and the metallography slab remnant are reasonably 

similar, with the noise tending to be largest near the billet center and decreasing as the OD is 

approached. For the V-Die-A case, however, there are some noticeable differences between the 

noise patterns in the property strip and the metallography slab, particularly near those areas 

where the small metallography coupons were removed. Of the four metallography sites (0′′, 2′′, 

3′′, and 4′′), the noise level in the original V-Die-A property strip is clearly highest at 4′′ from the 

billet center. For the corresponding metallography slab, however, the noise level at 0′′ appears to 

be slightly higher than those at 2′′, 3′′, and 4′′. One concludes that the microstructures of the strip 

coupon and metallography slab are likely more dissimilar for V-Die-A than for the other two 

IN718 cases. 

Low 

Amp.

1

3 6

4

1

3 6

4

1

3 6

4

Original Billet 

“Slice” Coupon

GFM-A

Remainder of 

Rough-Cut 

Metallography Slab

Remainder of 

“Slice” Coupon

Inspections Through 

Side 2 (Axial)

High 

Amp.

Low 

Amp.

1

3 6

4

1

3 6

4

1

3 6

4

Original Billet 

“Slice” Coupon

GFM-A

Remainder of 

Rough-Cut 

Metallography Slab

Remainder of 

“Slice” Coupon

Inspections Through 

Side 2 (Axial)

High 

Amp.

FIGURE 3-73. BACKSCATTERED GRAIN NOISE C-SCANS OF THE GFM-A STRIP 

COUPON AND METALLOGRAPHY SLAB 

Thus, it appears that the variation of microstructure with position within the studied region of the 

V-Die-A billet was sufficiently large that grain sizes within the rough-cut metallography 

specimen were not indicative of those in the companion UT specimen at similar radial depths. 

Cutting new metallography coupons from the V-Die-A UT property specimens themselves were 

considered. However, this was not done because of (1) a desire to keep those specimens intact 

and (2) the fact that the GFM-A and V-Die-B metallography coupons provided sufficient data to 

quantify the relationship between grain size and UT properties in IN718 material. 

3-82
Low 

Amp.

1

3 6

4

1

3 6

4

1

3 6

4

Original Billet 

“Slice” Coupon

V-Die-A

Remainder of 

Rough-Cut 

Metallography Slab

Remainder of 

“Slice” Coupon

Inspections Through 

Side 2 (Axial)

High 

Amp.

Low 

Amp.

1

3 6

4

1

3 6

4

1

3 6

4

Original Billet 

“Slice” Coupon

V-Die-A

Remainder of 

Rough-Cut 

Metallography Slab

Remainder of 

“Slice” Coupon

Inspections Through 

Side 2 (Axial)

High 

Amp.

FIGURE 3-74. BACKSCATTERED GRAIN NOISE C-SCANS OF THE V-DIE-A STRIP 

COUPON AND METALLOGRAPHY SLAB 

Low 

Amp.

1

3 6

4

1

3 6

4

1

3 6

4

Original Billet 

“Slice” Coupon

V-Die-B

Remainder of 

Rough-Cut 

Metallography Slab

Remainder of 

“Slice” Coupon

Inspections Through 

Side 2 (Axial)

High 

Amp.

Low 

Amp.

1

3 6

4

1

3 6

4

1

3 6

4

Original Billet 

“Slice” Coupon

V-Die-B

Remainder of 

Rough-Cut 

Metallography Slab

Remainder of 

“Slice” Coupon

Inspections Through 

Side 2 (Axial)

High 

Amp.

FIGURE 3-75. BACKSCATTERED GRAIN NOISE C-SCANS OF THE V-DIE-B STRIP 

COUPON AND METALLOGRAPHY SLAB 

3-83
3.1.5.4 Correlations Between UT Properties and Average Grain Diameters. 

Figure 3-76 displays the manner in which average grain diameter depends on radial position in 

each of the four strip coupons. The best estimates of the average grain diameters, as viewed 

from the radial, axial, and hoop directions, have been plotted versus radial depth from the billet 

center. For the V-Die-A strip, results are shown for both the original set of 12 metallography 

coupons and the auxiliary set of 4 newer coupons, with the latter results labeled radial-new in the 

figure. 

Figure 3-76 can be compared with similar plots in figures 3-50 and 3-51, showing the 

dependence of UT attenuation and noise FOM on the radial coordinate of the measurement site. 

Comparing the three figures for a given strip specimen, one sees that increasing measured grain 

size is generally associated with increasing attenuation and increasing FOM. The lone exception 

is the V-Die-A strip. For that case, the attenuation and noise tend to be highest near the 3′′ or 4′′

sites, while the measured grain size is highest near the billet center. For the V-Die-A case, there 

is reason to believe that the microstructure of the metallography slab is somewhat different than 

that of the property measurement strip, and this likely accounts for the minor divergence between 

the trends of the UT properties and the measured grain diameters. 

GFM-A Metallography

0

5

10

15

20

25

30

35

40

0" 1" 2" 3" 4"

Measurement Site

(Distance from Billet Center)

Ave. Grain Diameter .

(microns)

Axial

Radial

Hoop

Waspaloy Metallography

0

20

40

60

80

100

120

140

0" 1" 2" 3" 4"

Measurement Site

(Distance from Billet Center)

Ave. Grain Diameter .

(microns)

Axial

Radial

Hoop

V-Die-B Metallography

0

5

10

15

20

25

30

0" 1" 2" 3" 4"

Measurement Site

(Distance from Billet Center)

Ave. Grain Diameter .

(microns)

Axial

Radial

Hoop

V-Die-A Metallography

0

5

10

15

20

25

30

0" 1" 2" 3" 4"

Measurement Site

(Distance from Billet Center)

Ave. Grain Diameter .

(microns)

Axial

Radial

Hoop

Radial-New

GFM-A Metallography

0

5

10

15

20

25

30

35

40

0" 1" 2" 3" 4"

Measurement Site

(Distance from Billet Center)

Ave. Grain Diameter .

(microns)

Axial

Radial

Hoop

Waspaloy Metallography

0

20

40

60

80

100

120

140

0" 1" 2" 3" 4"

Measurement Site

(Distance from Billet Center)

Ave. Grain Diameter .

(microns)

Axial

Radial

Hoop

V-Die-B Metallography

0

5

10

15

20

25

30

0" 1" 2" 3" 4"

Measurement Site

(Distance from Billet Center)

Ave. Grain Diameter .

(microns)

Axial

Radial

Hoop

V-Die-A Metallography

0

5

10

15

20

25

30

0" 1" 2" 3" 4"

Measurement Site

(Distance from Billet Center)

Ave. Grain Diameter .

(microns)

Axial

Radial

Hoop

Radial-New

FIGURE 3-76. BEST ESTIMATES OF GRAIN DIAMETERS VERSUS RADIAL POSITION 

3-84
Measured attenuation and FOM values are plotted versus the mean grain diameter in figures 3-77 

and 3-78, respectively. The UT property values are specifically those at 7.5 MHz. One can 

make similar plots at other frequencies using tables 3-6 and 3-7 and tables 3-10 through 3-13. 

Each plotted point in figures 3-77 and 3-78 represents a UT measurement made at one site on 

one billet specimen for one particular inspection direction. Thus, in the upper left panel of each 

figure there are 12 points shown for each strip specimen: four inspection sites per specimen and 

three inspection directions per site. The UT result for a given sound propagation direction has 

been plotted against the grain diameter measured using metallography coupons whose viewing 

direction was parallel to that of the propagation direction. For example, at the billet center (0′′

site) the attenuation or FOM value for radial propagation is plotted against the measured grain 

diameter for metallography coupon B. For the V-Die-A case, the grain diameters are those 

measured using the original suite of 12 metallography coupons. Because the correspondence 

between the grain diameters and UT properties is suspect for the V-Die-A strip, the V-Die-A 

results have been omitted from the lower two panels of figures 3-77 and 3-78. The lower panels 

show a much stronger correlation between grain diameter and UT properties than is seen in the 

upper panels, which include the V-Die-A results. 

Correlation Between Measured Attenuation at 7.5 

MHz and Measured Average Grain Diameter 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 25 50 75 100 125 150

Grain Diameter (microns)

Attenuation (dB/inch)

WASPALOY

GFMA

V-DIE-A

V-DIE-B

Model: Pure Ni

Correlation Between Measured Attenuation at 7.5 

MHz and Measured Average Grain Diameter 

0.0

0.1

0.2

0.3

0.4

0.5

0 10 20 30 40

Grain Diameter (microns)

Attenuation (dB/inch)

WASPALOY

GFMA

V-DIE-A

V-DIE-B

Model: Pure Ni

Correlation Between Measured Attenuation at 7.5 

MHz and Measured Average Grain Diameter 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 25 50 75 100 125 150

Grain Diameter (microns)

Attenuation (dB/inch)

WASPALOY

GFMA

V-DIE-B

Model: Pure Ni

Correlation Between Measured Attenuation at 7.5 

MHz and Measured Average Grain Diameter 

0.0

0.1

0.2

0.3

0.4

0.5

0 10 20 30 40

Grain Diameter (microns)

Attenuation (dB/inch)

WASPALOY

GFMA

V-DIE-B

Model: Pure Ni

Correlation Between Measured Attenuation at 7.5 

MHz and Measured Average Grain Diameter 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 25 50 75 100 125 150

Grain Diameter (microns)

Attenuation (dB/inch)

WASPALOY

GFMA

V-DIE-A

V-DIE-B

Model: Pure Ni

Correlation Between Measured Attenuation at 7.5 

MHz and Measured Average Grain Diameter 

0.0

0.1

0.2

0.3

0.4

0.5

0 10 20 30 40

Grain Diameter (microns)

Attenuation (dB/inch)

WASPALOY

GFMA

V-DIE-A

V-DIE-B

Model: Pure Ni

Correlation Between Measured Attenuation at 7.5 

MHz and Measured Average Grain Diameter 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 25 50 75 100 125 150

Grain Diameter (microns)

Attenuation (dB/inch)

WASPALOY

GFMA

V-DIE-B

Model: Pure Ni

Correlation Between Measured Attenuation at 7.5 

MHz and Measured Average Grain Diameter 

0.0

0.1

0.2

0.3

0.4

0.5

0 10 20 30 40

Grain Diameter (microns)

Attenuation (dB/inch)

WASPALOY

GFMA

V-DIE-B

Model: Pure Ni

FIGURE 3-77. RELATIONSHIP BETWEEN AVERAGE GRAIN DIAMETER AND 

MEASURED ATTENUATION (AT 7.5 MHz) FOR EACH OF THE FOUR STRIP COUPONS 

(Each right-hand panel shows a blowup of the low-diameter region of the left-hand panel. 

V-Die-A data are included in the upper two panels and omitted from the lower two panels.) 

3-85
For comparison, figures 3-77 and 3-78 also display predicted curves for equiaxed pure Ni 

microstructures with randomly oriented grains. As before, attenuation and FOM values, as 

functions of the mean grain diameter, were predicted using the formalisms of references 40-44, 

assuming an exponential P(L) curve in each case. For the range of diameters shown in 

figures 3-77 and 3-78, the predicted attenuation and FOM values at 7.5 MHz increase 

systematically with grain diameter. At larger grain diameters, predicted attenuation and FOM 

both decrease and eventually approach zero in the limit where the grain diameter exceeds the 

specimen thickness. The measured attenuation and FOM values of the Ni alloy specimens tend 

to increase with grain diameter in manners that are similar in form to those predicted by the 

models. However, the average rise rates for the measured data, which are slightly different for 

the IN718 and Waspaloy cases, are somewhat lower than those predicted for pure Ni. This may 

indicate that the single-crystal elastic constants of the two alloys are significantly different from 

those of pure Ni. Alternatively, it may be connected with the decision to exclude twin 

boundaries from the grain size analysis; this choice effectively increases the measured grain 

diameters and thus alters the rise rates of the attenuation-versus-grain size and FOM-versus-grain 

size curves. 

Correlation Between Measured Noise FOM at 7.5 

MHz and Measured Average Grain Diameter 

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0 25 50 75 100 125 150

Grain Diameter (microns)

Noise FOM (cm^-0.5)

WASPALOY

GFMA

V-DIE-A

V-DIE-B

Model: Pure Ni

Correlation Between Measured Noise FOM at 7.5 

MHz and Measured Average Grain Diameter 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0 10 20 30 40

Grain Diameter (microns)

Noise FOM (cm^-0.5)

WASPALOY

GFMA

V-DIE-A

V-DIE-B

Model: Pure Ni

Correlation Between Measured Noise FOM at 7.5 

MHz and Measured Average Grain Diameter 

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0 25 50 75 100 125 150

Grain Diameter (microns)

Noise FOM (cm^-0.5)

WASPALOY

GFMA

V-DIE-B

Model: Pure Ni

Correlation Between Measured Noise FOM at 7.5 

MHz and Measured Average Grain Diameter 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0 10 20 30 40

Grain Diameter (microns)

Noise FOM (cm^-0.5)

WASPALOY

GFMA

V-DIE-B

Model: Pure Ni

Correlation Between Measured Noise FOM at 7.5 

MHz and Measured Average Grain Diameter 

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0 25 50 75 100 125 150

Grain Diameter (microns)

Noise FOM (cm^-0.5)

WASPALOY

GFMA

V-DIE-A

V-DIE-B

Model: Pure Ni

Correlation Between Measured Noise FOM at 7.5 

MHz and Measured Average Grain Diameter 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0 10 20 30 40

Grain Diameter (microns)

Noise FOM (cm^-0.5)

WASPALOY

GFMA

V-DIE-A

V-DIE-B

Model: Pure Ni

Correlation Between Measured Noise FOM at 7.5 

MHz and Measured Average Grain Diameter 

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0 25 50 75 100 125 150

Grain Diameter (microns)

Noise FOM (cm^-0.5)

WASPALOY

GFMA

V-DIE-B

Model: Pure Ni

Correlation Between Measured Noise FOM at 7.5 

MHz and Measured Average Grain Diameter 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0 10 20 30 40

Grain Diameter (microns)

Noise FOM (cm^-0.5)

WASPALOY

GFMA

V-DIE-B

Model: Pure Ni

FIGURE 3-78. RELATIONSHIP BETWEEN AVERAGE GRAIN DIAMETER AND 

MEASURED NOISE FOM (AT 7.5 MHz) FOR EACH OF THE FOUR STRIP COUPONS 

(Each right-hand panel shows a blowup of the low-diameter region of the left-hand panel. 

V-Die-A data are included in the upper two panels and omitted from the lower two panels.) 

3-86
3.1.5.5 Estimating Single-Crystal Elastic Constants of Two Ni Alloys. 

The single-crystal elastic constants of IN718 and Waspaloy are not presently known. The 

possibly of deducing them from the UT and metallography study was not envisioned in the 

original ETC Phase II proposal, but has emerged as a beneficial windfall. 

If one assumes that the grain sizes have been properly measured from an attenuation and 

scattering point of view, then an approach can be crafted to estimate the single-crystal elastic 

constants for each alloy from the UT and metallography data in hand. The approach is illustrated 

in figure 3-79 using data from the IN718 GFM-A specimen measured at a site 2′′ from the billet 

center. The starting points are model expressions that relate UT wave speeds [46] and 

attenuation [43] to details of the microstructure. It was assumed that IN718 and Waspaloy have 

single-phase cubic crystalline structures like that of pure Ni. The three independent elastic 

constants for a cubic-phase microstructure {C11, C12, C44} were varied until the predicted 

longitudinal and shear-wave velocities for polycrystalline specimens and the predicted 

attenuation near 7.5 MHz matched the measured results. Three averaging schemes for relating 

polycrystalline sound velocities to elastic constants were investigated (the so-called Voigt, 

Reuss, and Hill averages), and each gave rise to slightly different results. The upper portion of 

figure 3-79 lists the experimental velocity and attenuation values used in the by-hand fitting 

process, and the predicted quantities for a near-optimal set of elastic constants. There, the Hill 

averaging scheme was used for the predicted velocities. Model attenuation values were again 

obtained using the on-line attenuation calculator developed by Joe Turner [40], which requires 

the density, elastic constants, and average grain diameter as inputs. The lower portion of 

figure 3-79 compares the measured and predicted attenuation-versus-frequency curves for the 

fitted set of IN718 elastic constants, namely {C11, C12, C44} = {242, 140, 110 GPa}. For this 

set, the measured and predicted velocity and attenuation values are in reasonably good agreement 

with one another. Slightly better agreement can likely be achieved by fine-tuning the elastic 

constant values, and future study along those lines is planned. 

The preliminary fitting study was completed using the UT property and metallography data for 

the Waspaloy, GFM-A, and V-Die-B specimens. The results are summarized in table 3-16. At 

each measurement site, UT property and grain size results for the three propagation directions 

have been averaged to estimate corresponding values for a perfectly isotropic specimen. The 

averaged grain diameters and attenuation values at each site, which are inputs to the fitting 

procedure, are listed in the table. Other inputs are the measured density, longitudinal-wave 

velocity, and shear-wave velocity, which can be found in tables 3-2, 3-4, and 3-5, respectively. 

The single-crystal elastic constants deduced by fitting to the measured data at each site are listed 

in the rightmost nine columns of table 3-16. Separate results are shown for each of the three 

model approaches (Voigt, Reuss, and Hill) that relate average sound velocities to the elastic 

constants and density of the specimen. At a given measurement site, the three approaches led to 

roughly similar sets of elastic constants, typically differing by several percent from one another. 

Site-to-site differences in the {C11, C12, C44} values for a fixed choice of the velocity 

averaging scheme (e.g., the Hill average) are on the same order, namely, from 0%-10% in most 

cases. In addition, table 3-16 shows that the average of the deduced elastic stiffness constants 

value oversites in GFM-A is reasonably similar to the average oversites in V-Die-A. This is 

3-87
as expected since both specimens are of the same (nominal) IN718 alloy. For comparison, 

the single-crystal elastic constants of pure Ni are C11 = 250 GPa, C12 = 160 GPa, and 

C44 = 118.5 GPa. 

FIGURE 3-79. EXAMPLE OF DETERMINING IN718 SINGLE-CRYSTAL ELASTIC 

CONSTANTS BY FITTING TO MEASURED VELOCITY AND ATTENUATION DATA 

0.00

0.02

0.04

0.06

0.08

0.10

2 3 4 5 6 7 8 9 10 11 12 13 14

Frequency (MHz)

Attenuation (Np/cm)

Raw data for Axial - 2"

Raw data for Hoop - 2"

Raw data for Radial - 2"

JT predicted attenuation

Joe Turner prediction for Attenuation was estimated using the 

elastic constants determined by Hill averaging procedure 

(c12=140 c11=242 c44=110)

Specimen: GFMA

Measurement Site: 2” from billet center

Measured Ave. Grain Diameter (a): 27.35 microns

Measured Longitudinal Velocity (a): 0.5808 cm/usec

Predicted Longitudinal Velocity: 0.5811 cm/ usec (Hill Ave.)

Measured Shear-wave Velocity (b): 0.3087 cm/usec

Predicted Shear-Wave Velocity: 0.3087 cm/usec (Hill Ave.)

Measured Attenuation at 7.5 MHz (a): 0.0101 N/cm = 0.223 dB/inch

Predicted Attenuation at 7.5 MHz: 0.0105 N/cm = 0.232 dB/inch

Model Elastic Constants: C11 = 242 GPa

C12 = 140 Gpa

C44 = 110 Gpa

Notes: (a) Averaged over radial, axial, hoop directions.

(b) Measured for radial propagation.

Model predictions use the J. Turner

Attenuation Calculator with

C11, C12, C44 = 242, 140, 110 GPa.

Measured and Predicted Attenuation

for IN718 Specimen GFMA (2” Site)

0.00

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0.08

0.10

2 3 4 5 6 7 8 9 10 11 12 13 14

Frequency (MHz)

Attenuation (Np/cm)

Raw data for Axial - 2"

Raw data for Hoop - 2"

Raw data for Radial - 2"

JT predicted attenuation

Joe Turner prediction for Attenuation was estimated using the 

elastic constants determined by Hill averaging procedure 

(c12=140 c11=242 c44=110)

Specimen: GFMA

Measurement Site: 2” from billet center

Measured Ave. Grain Diameter (a): 27.35 microns

Measured Longitudinal Velocity (a): 0.5808 cm/usec

Predicted Longitudinal Velocity: 0.5811 cm/ usec (Hill Ave.)

Measured Shear-wave Velocity (b): 0.3087 cm/usec

Predicted Shear-Wave Velocity: 0.3087 cm/usec (Hill Ave.)

Measured Attenuation at 7.5 MHz (a): 0.0101 N/cm = 0.223 dB/inch

Predicted Attenuation at 7.5 MHz: 0.0105 N/cm = 0.232 dB/inch

Model Elastic Constants: C11 = 242 GPa

C12 = 140 Gpa

C44 = 110 Gpa

Notes: (a) Averaged over radial, axial, hoop directions.

(b) Measured for radial propagation.

Model predictions use the J. Turner

Attenuation Calculator with

C11, C12, C44 = 242, 140, 110 GPa.

Measured and Predicted Attenuation

for IN718 Specimen GFMA (2” Site)

The initial efforts to extract single-crystal elastic constants from the measured data appear 

promising. Further work is planned to develop software that will automate the fitting process, 

resulting in more optimal fits at each measurement site. The introduction of FOM data into the 

fitting process will also be explored. In addition, efforts will be undertaken to test the approach 

using polycrystalline specimens for which the single-crystal elastic constants are known. For 

example, equiaxed, untextured specimens of pure Ni could be procured and studied. Efforts 

along these lines are currently underway, and preliminary findings have been published [47]. 

Additional investigations are in progress, and it is expected that results will be published in the 

scientific literature at an appropriate time. 

3-88
TABLE 3-16. PRELIMINARY VALUES OF DEDUCED SINGLE-CRYSTAL ELASTIC 

CONSTANTS FOR THE V-DIE-B, GFM-A, AND WASPALOY BILLET SPECIMENS 

V-Die-B Deduced Elastic Constants 

Average over radial, axial, hoop Voigt Average Reuss Average Hill Average 

Site 

Grain Dia. 

(microns) 

Attenuation 

(dB/inch) c12 c11 c44 c12 c11 c44 c12 c11 c44 

0′′ 25.12 0.1038 137 248 94 135 252 102 136 250 99 

2′′ 20.23 0.1151 140 242 97 138 246 113 138 246 104 

3′′ 16.44 0.0534 139 244 96 137 248 109 137 248 102 

4′′ 13.15 0.0210 130 262 87 130 262 90 131 260 90 

Average over sites: 136.5 249 93.5 135 252 103.5 135.5 251.0 98.8

GFM-A Deduced Elastic Constants 

Average over radial, axial, hoop Voigt Average Reuss Average Hill Average 

Site 

Grain Dia. 

(microns) 

Attenuation 

(dB/inch) c12 c11 c44 c12 c11 c44 c12 c11 c44 

0′′ 33.52 0.3132 139 244 96 137 248 109 138 246 104 

2′′ 27.35 0.2229 142 238 99 139 244 117 140 242 110 

3′′ 22.53 0.1213 140 242 97 138 246 113 139 244 107 

4′′ 16.20 0.0827 145 232 102 141 240 130 142 238 118 

Average over sites: 141.5 239 98.5 138.75 244.5 117.25 139.8 242.5 109.8

Waspaloy Deduced Elastic Constants 

Average over radial, axial, hoop Voigt Average Reuss Average Hill Average 

Site 

Grain. Dia. 

(microns) 

Attenuation 

(dB/inch) c12 C11 c44 c12 c11 c44 c12 c11 c44 

0′′ 115.38 3.5626 143 264 101 141 268 109 142 266 106 

2′′ 97.40 2.8959 144 262 102 141 268 109 142 266 106 

3′′ 78.50 1.5187 143 264 101 141 268 109 142 266 106 

4′′ 47.36 0.4407 143 264 101 141 268 109 142 266 106 

Average over sites: 143.25 263.5 101.25 141 268 109 142.0 266.0 106.0

Note: The measured average grain diameter and attenuation (at 7.5 MHz) used in the fitting process are listed in 

the second and third columns from the left. Deduced elastic constants are in GPa units. 

3.2 SUMMARY. 

As part of the ongoing effort to quantify and improve inspection practices, the ETC has 

investigated the UT properties of Ni alloy, cylindrical billets used in the fabrication of rotating 

jet engine components. A primary goal of the study was to determine, for representative 

10′′ diameter billets, the manner in which longitudinal velocity, attenuation, and backscattered 

grain noise capacity depend on position and inspection direction. UT attenuation and 

backscattered noise capacity are frequency-dependent. Currently, most Ni alloy billet 

inspections are performed using 5-MHz transducers, but it may be desirable in the future to 

improve sensitivity by increasing the inspection frequency. In the Ni survey study, it was 

endeavored to measure attenuation and noise capacity within the frequency range of greatest 

present and future interest, namely, from about 5 to 12.5 MHz. 

3-89
UT property measurements are most easily carried out on rectangular specimens, which were 

used in this study. To construct a coherent picture of how properties vary with position and 

sound propagation direction, a two-stage, coupon-cutting scheme was adopted. Rectangular strip 

coupons with approximate dimensions of 2′′ x 2′′ x 10′′ were cut along billet diameters and used 

for UT measurements in the axial and hoop directions. The strip coupons were later cut into 1′′

thick slices for UT measurements in the radial direction. It was decided by consensus that two 

Ni alloys would be studied: IN718 and Waspaloy. Two different forging methods known as 

GFM and V-Die are commonly used to transform IN718 cast ingot into finished billet, and it was 

desired that both be studied. Budgetary considerations allowed for the study of a total of four 

strip coupons and their associated slices. 

The UT properties of Ni alloy billets are dependent in part on the casting and forging history. A 

lack of cylindrical symmetry during forging leads to noncylindrically symmetric UT properties, 

as evidenced by the noise-banding patterns that are seen in routine multizone inspections of 

billets. From earlier studies, it was known that changes in velocity and attenuation values are 

generally well correlated with changes in backscattered noise levels. Thus, backscattered noise 

patterns could be used to identify coupon sites with different UT properties. Noise-banding 

patterns seen in multizone inspections of 10′′ diameter candidate billets were used to select the 

sites where strip coupons were cut. The final choices and their designations were: 

• V-Die-A: Strip coupon from a high-noise band in an IN718 V-Die billet. 

• V-Die-B: Strip coupon from a low-noise band in the above V-Die billet. 

• GFM-A: Strip coupon from a high-noise band in an IN718 GFM billet. 

• Waspaloy: Strip coupon from high-noise band in a Waspaloy billet. 

The strip coupons were cut from the billets using EDM methods, and the sound entry surfaces 

for the UT measurements were ground smooth. The density of each strip was determined in the 

standard manner by dividing the measured mass by the computed volume, resulting in 

8.26 gm/cc for each of the IN718 strips and 8.21 gm/cm for the Waspaloy strip. 

Before detailed UT property measurements were performed, each strip coupon was scanned at 

normal incidence through the four large (2′′ x 10′′) surfaces using planar and focused transducers. 

The object was to produce C-scan images of back-wall echo TOF, back-wall echo amplitude, and 

backscattered grain noise amplitude. TOF images indicated that the sonic velocity was nearly 

constant within a given strip coupon. However, back-wall amplitude and backscattered noise 

images showed significant variations with position, principally with radial depth in the billet. 

Sites with low back-wall amplitude (i.e., high attenuation) tended to have high backscattered 

noise levels. Moreover, at a given radial position in a given strip coupon, the back-wall 

attenuation or grain noise level was nearly identical for axial and hoop sound propagation. Thus, 

the results of the preliminary scans were consistent with the hypothesis that the billet 

microstructures had approximately equiaxed, randomly oriented grains whose mean diameter 

varied systematically with position. It was also noted that the V-Die-A and V-Die-B strip 

coupons, which were cut from high- and low-noise bands of the same IN718 billet, differed 

significantly in their attenuation and grain noise levels near the coupon ends (OD regions) but 

had very similar values near their centers. Such a coalescing of UT property values at the billet 

3-90
center were shown in an earlier study using coupons cut from high- and low-noise bands of 

Ti-6-4 billets. It indicates that the asymmetries in the mechanical working of a billet, which led 

to noise banding and other property variations in the hoop direction, have a diminished effect 

near the billet center. 

The preliminary back-wall and backscattered noise C-scans revealed that the UT properties of 

the strip coupons were primarily dependent on radial depth and were approximately symmetric 

about the billet center. It was, thus, decided that detailed property measurements for each strip 

coupon would be made at sites along a single billet radius, rather than along a full diameter. Five 

sites were selected, which were located 0′′, 1′′, 2′′, 3′′, and 4′′ from the billet center. At each site 

velocity, attenuation and grain noise capacity were carefully measured in each of the three 

principal directions. For sonic beam propagation in the axial and hoop directions, UT 

measurements were made on the intact strip coupons. For measurements in the radial direction, 

the strip coupons were cut into approximately 1′′ thick slices centered at the five sites. 

Except for a limited number of shear-wave velocity measurements, all UT property 

measurements were made in pulse/echo mode with the specimens immersed in distilled water. 

Detailed velocity measurements were made using 0.25′′ diameter planar transducers at normal 

incidence. The velocity at each measurement site was determined from the average time delay 

between successive back-wall echoes. Early measurements for selected cases using both 5- and 

10-MHz planar transducers found very little difference between the two measured velocities, i.e., 

typically less than one-tenth of 1 percent. Consequently, the dependence of velocity on 

frequency was deemed to have little impact on practical inspections, and subsequent velocity 

measurements were made using only the 10-MHz transducer. 

Two competing immersion methods for detailed attenuation measurements were explored. One 

was based on comparing the first back-wall echo in the metal specimen to that in an FQ 

reference block of similar thickness, and a second was based on comparing the first three metal 

back-wall echoes with one another. Each method yielded an attenuation-versus-frequency curve 

within the bandwidth of the 10-MHz, 0.25′′ diameter broadband planar transducer that was used. 

It was found that good agreement between the two methods could be obtained if (1) the spectra 

of the back-wall echoes being used to determine the valid frequency range for the measurement 

were examined; (2) the results where the sonic beam interacted appreciably with the lateral edges 

of the coupon were discounted; and (3) the same equipment gain setting was used when 

acquiring all echoes that contributed to the measured attenuation at a given site. After these 

preliminary measurements were completed, the second method was selected as the primary 

measurement modality since it was believed to be more accurate for low-attenuation cases. 

The goal of the detailed backscattered noise measurements was to determine the frequencydependent FOM parameter that quantifies the noise-generating capacity of the microstructure. 

The FOM value depends solely on the microstructure of the metal and, ideally, is independent of 

the means used to measure it. At each measurement site, a well-characterized 15-MHz, F7, 

focused transducer was scanned above the metal specimen and several hundred noise A-scans 

were acquired and stored. A back-wall echo from an FQ block served as a reference echo, which 

3-91
was analyzed to deduce a measurement system efficiency factor. The FOM-versus-frequency 

curve was then extracted from the stored noise data using the independent scatterer noise model, 

which relates the average noise spectrum to the FOM and various measurement system 

parameters. The results are considered to be trustworthy in the frequency range where the 

attenuation is accurately known and the average backscattered noise spectrum is appreciable. 

The backscattered noise study afforded an opportunity to test some predictions of the 

independent scatterer noise model. In preliminary studies, the model was used to predict the 

depth dependence of the rms noise level from measured FOM-versus-frequency curves. The rms 

noise-versus-depth curves measured with focused probes typically display a pronounced 

maximum near the focal depth. The model was found to do a good job of predicting the overall 

shape and location of the focal maximum. However, the measured noise levels tended to be 

above the predicted result for times that were either well before or well beyond the focal 

maximum. One possible explanation, to be explored in future work, is that the departures from 

model predictions are due to multiple scattering events and the manner in which they contribute 

to the observed noise in and away from the focal zone. The determination of the grain noise 

FOM from measured noise data requires analysis of backscattered noise RF waveforms within 

some designated time gate. Various gate choices were explored, but for the quoted results, a 

time gate that bracketed the focal zone was used. This choice was made because the noise model 

is believed to be most reliable in that region and because the grain noise level is highest there, 

thus reducing the impact of residual electronic noise. The deduced FOM value also depends on 

the assumed rate at which energy is lost by the sonic beam during propagation. The sensitivity 

of deduced FOM values to uncertainty in the energy loss attenuation was also explored. For the 

quoted results, it was assumed the energy loss attenuation to be equivalent to the attenuation 

measured in the back-wall echo study. In general, the attenuation of surface echoes has 

contributions due to both energy loss and beam distortion. However, for fine-grained metals like 

the specimens in this study, the attenuation of surface echoes is believed to be dominated by the 

energy loss effect. 

For each of the three properties of interest, 60 measurement trials were conducted, corresponding 

to four billet strip specimens, five measurement sites per strip, and three inspection directions per 

site. 

For the set of 60 measurements, longitudinal velocities were found to range over 0.5805 ±0.0009 

cm/μsec (or ±0.15%) for the IN718 specimens and over 0.6008 ±0.0006 cm/μsec (or ±0.10%) 

for the Waspaloy specimens. No obvious trends were observed in the manner in which velocity 

varied with position. The minor velocity variations within each of the four billet strips may 

simply reflect the inherent errors in the measurement process. In practice, the small velocity 

variations seen in these Ni alloy billets, in and of themselves, are expected to have no significant 

impact on inspectability. The sonic velocity influences the radiation pattern within the billet that 

is produced by the insonifying transducer. Sizeable velocity inhomogeneities can shift the focal 

depth and alter the degree of focus at the focal zone. For the specimens studied, the velocity 

variations are very small and are unlikely to cause any noticeable changes in the radiation 

pattern. 

3-92
In contrast to the velocity behavior, attenuation and FOM values varied significantly with both 

frequency and position. In particular, it was found that 

• the attenuation and FOM values varied significantly with radial position in each of the 

billet strips studied, being largest near the billet center (0′′) for Waspaloy, GFM-A, and 

V-Die-B, and largest near the OD for V-Die-A. 

• on average, at a given measurement site in a given strip, there was little difference 

between the UT properties for the axial, radial, and hoop directions. 

• averaged over radial position, the attenuation or FOM value increased for the sequence: 

V-Die-B, V-Die-A, GFM-A, and Waspaloy. 

• attenuation and FOM values tended to be well correlated: sites with higher (lower) 

attenuation also had higher (lower) FOM. 

The attenuation and FOM values varied widely over the suite of specimens, with values for both 

properties being much larger for Waspaloy than for IN718. Averaged over all the coupons and 

all inspection directions, the Waspaloy attenuation at 5 MHz (0.56 dB/inch) was 15 times larger 

than the IN718 attenuation (0.036 dB/inch). At 7.5 MHz, the average Waspaloy attenuation was 

16 times larger (2.41 dB/inch versus 0.15 dB/inch). The average FOM values for Waspaloy at 5 

and 7.5 MHz were larger than the corresponding averages for IN718 by factors of 3 and 4, 

respectively. Within a given billet strip coupon, attenuation values at these frequencies varied by 

as much as an order of magnitude with position, and FOM values varied by as much as a factor 

of 3. 

Models for simulating inspections often assume that the attenuation can be approximated as a 

power law in frequency: 

Attenuation in N/cm units = C (frequency in MHz) p 

For this reason, each measured attenuation-versus-frequency curve was fit to such a power law. 

For the Ni billet specimens in the frequency range of interest, power-law fits in virtually all cases 

yielded powers in the 3 < p < 4 range. For some sites in the Waspaloy strip specimen, the 

attenuation values were too large to permit accurate measurement above about 8 MHz for the 

transducer and sample thicknesses used. However, the summary tables and graphs in this study 

are complete for 5 MHz (the center frequency for existing Ni billet inspections) and for 7.5 MHz 

(the likely center frequency for a next-generation inspection with improved sensitivity). 

For each of the Ni alloy billet specimens studied, the variations of properties with position were 

such that the attenuation and noise FOM tended to increase or decrease together as the 

measurement site was changed. This behavior is expected for specimens with equiaxed grains—

as the mean grain size increases, the attenuation and noise FOM both increase. However, this 

trend is not always seen in jet engine materials. For example, for the Ti alloy billet specimens 

studied under ETC Phase I, the direction of maximum back-wall attenuation was usually the 

direction of minimum grain noise, leading to an inverse relationship between attenuation and 

noise amplitudes. These two different trends were attributed to the role of recrystallization 

3-93
during the billet formation process. In Ni alloys, there is a significant amount of recrystallization 

that occurs due to mechanical working of the billet during its formation. This evidently led to 

nearly equiaxed grains in the Ni billet specimens under study. In contrast, very little or no 

recrystallization occurs during the working of Ti alloy billets. Hence, the large-scale 

macrograins present in the Ti ingot become elongated in the axial direction, and these elongated 

macrograins cause sonic beam distortions that led to an inverse relationship between noise FOM 

and attenuation. 

The systematic variation of attenuation and FOM with position will make some locations within 

billets more difficult to inspect than others. For a given defect, SNR will vary with defect 

position, generally being smaller at locations having larger FOM values. The UT properties 

reported in this study can be used as inputs to model calculations that simulate billet inspections 

and estimate SNR values. 

A second goal of the study was to correlate the measured UT properties with the local billet 

microstructures. To this end, small metallography coupons were cut from a 1/2′′ thick slab that 

was adjacent in the billet to the strip coupon used for UT property measurements. For each of 

the four strip coupons studied, there were 12 metallography coupons: three each from sites 

located 0′′, 2′′, 3′′, and 4′′ from the billet center. The collection of 12 coupons provided axial, 

radial, and hoop views of the microstructure at each of the four inspection sites. 

Grain size information was deduced from micrographs of the polished and etched metallography 

coupons. Tracings of the grain boundaries were made from apparent twin boundaries that were 

identified and excluded since it is believed that such boundaries do not play a major role in 

determining UT attenuation and backscattered noise levels. The grain boundary tracings were 

then digitized and analyzed. Two different approaches, known as the MCL and P(L) methods, 

were used to deduce average grain diameters from a given image. Each method found the grains 

to be relatively equiaxed at a given site, i.e., there was no general tendency for the grains to be 

elongated in the radial, axial, or hoop directions. Overall, the average diameter estimates 

provided by the two methods were very similar, with the P(L) estimate tending to be about 5% 

below the MCL method. The results of the two methods were averaged to obtain best estimates 

of the grain diameters at the various measurement sites. Average grain diameters ranged from 

about 13-34 microns in the IN718 specimens and from about 47 to 115 microns in the Waspaloy 

specimens. 

For each billet strip studied, the measured average grain diameter tended to vary systematically 

with radial position. For the most part, the measured grain diameters were well correlated with 

the UT properties; for a given strip coupon, sites with larger grain diameters tended to have 

higher attenuations and higher FOM values. The notable exception was the V-Die-A strip. 

Follow-up studies revealed that backscattered noise levels at corresponding points in the V-DieA strip coupon and the adjacent metallography slab were dissimilar, suggesting different mean 

grain sizes. The fact that grain diameters measured in the V-Die-A metallography slab were not 

representative of those in the property strip is likely responsible for the lack of correlation 

between the UT properties and grain diameters for that specimen. Setting aside the V-Die-A 

results, the remaining data served to quantify the relationship between grain diameter on the one 

hand and UT attenuation and backscattered noise FOM on the other. For both IN718 and 

3-94
Waspaloy, the tables and graphs in this study allowed one to deduce attenuation and FOM values 

from knowledge of the mean grain diameter and vice versa. The significant grain diameter 

variations, with radial depth in the billet strips, were documented. There are also significant 

microstructural variations in the hoop and axial directions, as indicated by the noise-banding 

patterns. 

In the future, it may be possible to estimate the average grain diameter at a given site in a billet 

under inspection from systematic measurements of backscattered grain noise. One would have 

to measure the backscattered noise level as a function of depth between the billet OD and the 

interior site in question by varying the focal depth of the interrogating sound beam. For analysis 

purposes, the depth region would be partitioned into thin layers and the grain size within each 

layer would be estimated in a bootstrapped fashion. Beginning with the layer closest to the OD, 

one would estimate the local average grain diameter by locating that value for which measured 

noise characteristics best matched the model predictions. The grain diameter in the first layer 

would determine attenuation losses that impact the analysis of noise from the second and deeper 

layers. One could systematically proceed layer by layer, estimating the mean grain size in each 

layer that best accounted for the observed noise, taking into consideration the accumulating 

effects of attenuation as one proceeded deeper into the billet. Tables 3-6, 3-7, and 3-10 through 

3-14, which relate attenuation and noise FOM to grain size, would be instrumental in such a 

process. 

It was noted that the measured attenuation and FOM values of the Ni alloy specimens tend to 

increase with grain diameter in manners that are very similar in form to those predicted by the 

models for pure Ni microstructures containing randomly oriented, equiaxed grains. However, 

the average rise rates for the measured data, which are slightly different for the IN718 and 

Waspaloy cases, are lower than those predicted for pure Ni. This indicated that the single-crystal 

elastic constants of the two alloys are somewhat different from those of pure Ni. 

The single-crystal elastic constants of IN718 and Waspaloy are not presently known, but an 

approach for deducing them from the UT and metallography work is being explored. This may 

prove to be an added benefit of the ETC-sponsored research that was not envisioned in the 

original proposal. The approach is based on varying the three independent elastic constants for a 

cubic-phase microstructure {C11, C12, and C44} until model-predicted velocities and 

attenuation best match the experimental values. The initial efforts to extract single-crystal elastic 

constants from the measured IN718 and Waspaloy data appear promising, and further work is 

planned to refine and test the approach. 

3-95/3-96
4. SYNTHETIC INCLUSION SAMPLES. 

4.1 INTRODUCTION. 

Ni alloys such as IN718, Waspaloy, IN901, IN100, and René 95 are used extensively in hightemperature jet engine applications. The basic UT properties of unflawed Ni alloy billets were 

the subject of the previous section. Equally important are the sonic properties of defects in these 

materials, since detectability is determined in part by the difference in UT impedance (sound 

velocity times density) between the defect and the host metal. Multiple defect types exist in 

forged billets, including dirty white spots, clean white spots, segregation, and slag. In powder 

metal alloys, ceramic particles, porosity, and powder-lot anomalies are seen. Typically, dirty 

white spots contain a microstructure of Nb-lean alloy with fine oxides and nitrides. Typical 

segregation defects consist of Nb-rich regions in billets that produce undesirable, brittle phases. 

Slag defects are Ca-rich ceramic phases originating from the ESR melting step and typically 

result from poor ingot surface preparation. 

This section describes the selection and manufacture of synthetic defect seeds and their 

embedding in IN718 material. Micrographs of the seeds themselves and of destructive analysis 

of some of the blocks are included. UT data in the form of velocity measurements and C-scan 

images were used to develop conclusions regarding defect detection in production inspections. 

4.2 SYNTHETIC DEFECTS. 

Three types of synthetic defects were made for this study: dirty white spot, clean white spot, and 

freckle. White spots are Nb- and Ti-lean regions in IN718. There are two basic types: dirty 

white spots and clean white spots. So-called dirty white spots are generally contaminated dropin of the crown or torus above the melt during vacuum arc remelting. Clean white spots fall into 

one of several categories: discrete white spots occur by drop-in from crown or torus, dendritic 

white spots occur by drop-in from shrinkage, and solidification white spots are thought to occur 

as a result of an instability in solidification conditions during the final VAR process. Freckles 

are generally considered to be macrosegregation as a result of instabilities in the convective flow 

of Nb-rich liquid during solidification. The freckle regions typically possess higher volume 

fractions of the Laves phase and/or δ-Ni3Nb. 

The synthetic defect seeds were chosen to lie within the composition range of defects found in 

production. Jackman, et al. [48] described the white spot compositions; Wlodek and Field [49] 

described the freckle compositions and phase analyses. The chemical composition of the seeds 

and the IN718 parent material are presented in table 4-1. 

4-1
TABLE 4-1. COMPOSITIONS, IN WEIGHT PERCENT, OF SYNTHETIC DEFECTS 

Defect Ni Fe Cr Nb Ti Al Mo C O B N 

White Spot 55.9 19.2 17.7 3.0 0.6 0.4 3.2 0.008 0 0.004 0 

Freckle 49.4 14.5 17.1 13.2 1.7 0.7 3.4 0.14 0 0.004 0 

Dirty White Spot 55.8 18.7 16.7 2.9 2.2 0.6 3.1 0.008 0.2 0.004 0.2 

IN718 52.8 18.5 19.0 5.1 1.1 0.5 3.0 

Note: Synthetic defects fabricated for this work are shown along with the IN718 base material. 

The white spot and freckle compositions were prepared by vacuum induction and casting into a 

rectangular slab mold. The dirty white spot composition was prepared by VAR, where TiO2 and 

TiN were used to achieve the prescribed oxygen and nitrogen contents. 

After casting, the defect slabs were HIPped at 1100°C, 210 MPa for 3 hours to close any 

solidification shrinkage. Then the defect slabs were cut in half, and one-half of each defect slab 

was press forged 50% at 982°C. This forging step was to create within the synthetic defects the 

microstructure that would be present in a production defect after billetizing or forging and to 

assure that the synthetic defect would have directional features consistent with the forgings into 

which they would be placed. 

Small samples with smooth surfaces were machined for speed of sound and density 

measurements from cast plus HIPped defect material and also from cast, HIP, and forged 

material. Optical microscopy and electron backscatter diffraction analysis have been performed 

to determine the constituent phases of the synthetic inclusions of the selected compositions. 

Figures 4-1 and 4-2 display microstructures of the synthetic white spot composition. Figure 4-1 

displays the as-cast plus HIP microstructure, which contains large grains, some larger than the 

field of view, with linear arrays of particles that may represent regions of interdendritic 

microsegregation. Figure 4-2 displays the microstructure of the white spot composition after 

forging at 982°C. The grain boundaries are jagged, suggesting some warm-work that has not 

been fully recovered, with some vertical slip lines present in the field of view. 

Figures 4-3 and 4-4 display the microstructure of the synthetic freckle composition. Figure 4-3 

displays the as-cast plus HIP microstructure, which contains primary Ni dendrites and an 

interdendritic eutectic of fine-scale Ni and Cr2Nb-type C14 Laves phase, identified by electron 

backscatter diffraction. Figure 4-4 displays the microstructure of the freckle composition after 

forging at 982°C; the microstructural features are compacted relative to those of figure 4-3. 

4-2
FIGURE 4-1. MICROSTRUCTURE OF WHITE SPOT CASTING 

FIGURE 4-2. MICROSTRUCTURE OF WHITE SPOT FORGING 

4-3
FIGURE 4-3. MICROSTRUCTURE OF FRECKLE INGOT 

(The microstructure contains primary gamma-Ni dendrites and a gamma-Laves eutectic.) 

FIGURE 4-4. MICROSTRUCTURE OF FRECKLE FORGING, EXHIBITING REFINEMENT 

DUE TO FORGING 

4-4
Figures 4-5 and 4-6 display the microstructure of the synthetic dirty white spot composition. 

Figure 4-5 displays the as-cast plus HIP microstructure, which contains large grains, larger than 

the field of view, with clumps of smaller particles that arose from the intentional TiO2 and TiN 

additions. 

FIGURE 4-5. MICROSTRUCTURE OF DIRTY WHITE SPOT BUTTON 

FIGURE 4-6. MICROSTRUCTURE OF DIRTY WHITE SPOT FORGING 

4-5
Figure 4-6 displays the microstructure of the dirty white spot composition after forging at 982°C. 

The grain boundaries are seen to be jagged, similar to those in the forged white spot of 

figure 4-2. In addition, (vertical) slip lines are present in the field of view and clumps of dark 

particles are present. 

For each of the three defect compositions, rectangular coupons were fabricated from the HIPped 

and forged material. These were used for round-robin measurements of density and velocity 

carried out at GE (Schenectady, NY), P&W (West Palm Beach, FL), and ISU (Ames, IA). The 

velocity results are listed in table 4-2. Measurements made at different installations were 

generally found to be in good agreement. Shear-wave measurements, made using contact 

transducers, were generally difficult due to the inhomogeneity of the specimens and their 

relatively small size. For a given propagation direction, the shear-wave speed often depended 

significantly on the polarization direction of the wave. For the measurements performed at ISU 

and P&W, the polarization direction was varied to locate the slowest and fastest shear-wave 

speeds, which were tabulated. In some cases (for example, in the side 3 direction of dirty white 

spot specimen), reliable shear-wave speed measurements were not feasible due to the specimen 

and probe dimensions. The reflectivity of a defect in a host billet is expected to depend upon the 

impedance mismatch between the defect and host metal, with UT impedance defined as the 

product of sound velocity and density. Thus, in addition to sound velocity, density was also 

measured for each of the three rectangular synthetic defect specimens. Measured densities were 

8.267, 8.246, and 8.227 gm/cc for white spot, freckle, and dirty white spot specimens, 

respectively. 

TABLE 4-2. RESULTS OF UT VELOCITY MEASUREMENTS ON RECTANGULAR 

COUPONS OF SYNTHETIC Ni BILLET DEFECT MATERIAL 

ISU GE P&W 

Specimen Side 

Thickness 

(mm) 

L-wave 

Velocity 

(mm/μsec) 

S-wave 

Velocity 

(mm/μsec) 

L-wave 

Velocity 

(mm/μsec) 

S-wave 

Velocity 

(mm/μsec) 

L-wave 

Velocity 

(mm/μsec) 

S-wave 

Velocity 

(mm/μsec) 

1 10.684 5.60 3.07-3.31 5.65 5.61 3.14-3.30 

2 12.410 5.66 2.98-3.35 5.75 5.68 3.07-3.32 

White 

Spot 

3 12.514 5.74 2.94-3.41 5.78 

3.27 

5.75 3.12-3.35 

1 10.558 5.78 3.12-3.16 5.79 5.79 3.08-3.18 

2 12.555 5.84 3.06-3.13 5.85 5.84 3.05-3.16 

Freckle 

3 12.419 5.84 3.05-3.15 5.86 

3.18 

5.85 3.06-3.14 

1 4.312 5.88 3.12-3.17 6.03 5.91 

2 7.268 5.90 5.97 5.92 

Dirty 

White 

Spot 3 7.323 5.94 6.07 

2.99 

5.95 

4.3 FLAT-BOTTOM HOLE BLOCK. 

A #1 FBH block was designed and machined from the sample billet that was selected for 

manufacturing the synthetic defect seed blocks. The geometry of this block is shown in 

figure 4-7. Five #1 FBHs were drilled 1/2′′ deep in alloy 718 to provide a reference standard for 

4-6
the UT measurement of noise levels in the IN718 billet material and the measurement of defect 

signal amplitudes in the seeded blocks. 

2"

1 1/2"

4"

.500

1"

1" 1/2"

2"

1 1/2"

4"

.500

1"

1" 1/2"

FIGURE 4-7. #1 FBH IN718 BLOCK DESIGN 

The F number is used to describe the focusing characteristics of a given UT transducer, with F/n 

denoting a transducer whose ratio of focal length in water to element diameter n. C-scan images 

were acquired from the FBH block using a 10-MHz F/7 transducer, as shown in figure 4-8. The 

mean material noise level, on an 8-bit scale (0-255), was 42.6 at a receiver attenuation setting of 

-6 dB or equivalently 4.26 at -26 dB. The five FBH targets scanned left to right at -26 dB gave 

8-bit amplitudes of 209, 215, 214, 220, and 208, respectively. The average FBH amplitude level 

is 213.2, with error bars of +6.8 and -5.2. This yields an average SNR of 213.2: 4.26, or 

approximately 50:1, and also suggests the holes are very consistent. 

The five images of the FBHs in figure 4-8 show that they do not lie along a straight line. 

However, the entry points of the drilled holes did lie along a straight line on the block surface. 

This suggests that the drill bits wandered in their progress from the block surface to the 1/2′′

depth. 

4-7
FIGURE 4-8. ULTRASONIC C-SCAN OF #1 FBH IN718 BLOCK 

4.4 TRIAL SEED BLOCK DESIGN. 

Prior to the manufacture of the final blocks containing the seeded defects, four smaller blocks 

were fabricated and studied. These were used to assess the fabrication procedure and to evaluate 

the bonding of the synthetic seeds to the IN718 parent material. In addition, the UT response of 

the bond between the cap and the base portions of the IN718 block was investigated using these 

test blocks. The basic test block design is shown in figure 4-9. 

Following this design, four separate test blocks were made. Three contained only one type of 

defect each, i.e., either freckle, dirty white spot, or white spot compositions. The fourth block 

contained two defects of each seed type. The initial IN718 blocks were machined to size and 

ultrasonically inspected to ensure that no prior flaws existed in the parent material. The caps 

were cut off using wire EDM and ground flat. Seed pockets were drilled in the block base and 

mating surfaces cleaned. The blocks and seeds were assembled and electron beam (EB)-welded 

in a vacuum. Next, they were HIPped at 970°C for 3 hours at 210 MPa. The HIP temperature 

was selected to be just below the delta-Ni3Nb solvus temperature. This prevented grain growth 

by delta-phase pinning of the grain boundaries and was a sufficiently high temperature that largescale delta precipitation was not possible. 

4-8
2 1/2"

1"

2 3/4"

1/2"

3/4" 1 1/4" 3/4"

3/4"

3/4"

1"

3/4"

3/4"

1/2"

I.D.

1

2

3

4

5

6

Seed

 1 .182” dia .125” lg.

 2 .182” dia. .195” lg.

 3 .182” dia. .195” lg.

 4 .088” dia. .195” lg.

 5 .088” dia. .125” lg

 6 .182” dia. .195” lg.

Cap

FIGURE 4-9. TRIAL BLOCK DESIGN 

Seeds were numbered 1 through 6 in each block, as shown in figure 4-9. The blocks were 

denoted freckle, white spot, dirty white spot, and composite based on seed type. UT C-scans of 

the blocks were made using a 10-MHz F/4 transducer, and the resulting images are shown in 

figure 4-10. The UT reflections from the front and back surfaces of each seed in each block 

were examined. For well-bonded, weakly scattering inclusions, one expects the echoes from the 

front and back surfaces of the defect to have similar amplitudes and opposite phases. Cases with 

anomalous differences in the amplitudes of the front- and back-surface reflections, or those 

having high amplitude signals between the arrival times of the front- and back-surface 

reflections, were noted. The noteworthy cases are listed in table 4-3. Based on the UT data, the 

abnormal seeds were cut out of the test blocks by EDM and examined using optical microscopy. 

4-9
1 2

3 4

min max

FIGURE 4-10. ULTRASONIC C-SCANS OF TRIAL BLOCKS 

(C-scan images of the freckle (1), white spot (2), dirty white spot (3), and 

Composite (4) test blocks using a 10-MHz F/4 transducer.) 

TABLE 4-3. ANOMALOUS INDICATIONS FROM IN718 TRIAL BLOCKS 

Seed UT Observation 

Freckle 1 Abnormally strong front and back pulses 

Freckle 2 Abnormally strong front and back pulses, back pulse much 

higher than front 

Freckle 3 Abnormally strong front and back pulses 

White Spot 1 Internal structure 

White Spot 2 Internal structure, big void back wall, that verberated 

White Spot 3 Internal structure, big void back wall, that verberated 

Dirty White Spot 1 Back-wall or crack in seed, back wall could be internal crack 

Dirty White Spot-6 Abnormally strong front pulse, strongly voided front 

Composite 1 (Freckle) Back-wall echo had high amplitude and was disorganized in 

time domain 

Composite 2 (Dirty White 

Spot) Internal structure 

Composite 3 (White Spot) Internal structure 

Note: C-scan data for these seeds gave higher than expected ultrasonic reflections. These seeds were cut out and 

destructively analyzed. 

4-10
In general, the interfaces between the synthetic flaws and the host metal, and those between the 

cap and base portions of the host metal itself, were found to be tight, conforming, absent of new 

phases, and void-free. Grain growth was seen across the cap/base interface, suggesting local 

mass transport. Such host/defect and host/host boundaries would generally be considered wellbonded. In some samples, local areas of poor bonding were identified, and representative 

examples are described below. 

4.4.1 IN718-IN718 Interface. 

As a baseline, figure 4-11 displays the microstructure of an IN718 to IN718 bond (cap to base). 

This particular bond is from the block containing dirty white spots and is typical of similar 

interfaces in all blocks. With reference to the figure, the as-HIPped microstructure contains 

small Ni3Cb delta plates in the region away from the bond and small particles, presumably delta 

phase, near the bond line. The boundary grains are small and appear to cross the bond line, a 

sign of a good metallurgical bond from diffusion across this interface. 

FIGURE 4-11. IN718 TO IN718 BOND LINE IN THE DIRTY WHITE SPOT 

TEST BLOCK 

4.4.2 Freckle-IN718 Interface Defects. 

Small discontinuities could typically be found on freckle-IN718 interfaces. Figure 4-12 shows a 

low-magnification view of the interface between IN718 and freckle seed 1; at this magnification, 

no discontinuities are apparent. At a higher magnification, figure 4-13 shows a portion of the 

interface between freckle seed 2 and IN718; there, small crack-like voids can be seen. Similar 

small voids could be found on all freckle seeds examined. 

4-11
FIGURE 4-12. BOND BETWEEN FRECKLE SEED 1 (BOTTOM) AND IN718 (TOP) 

FIGURE 4-13. DETAIL OF BOND LINE BETWEEN FRECKLE SEED 2 (BOTTOM) AND 

IN718 (TOP) SHOWING SMALL DISCONTINUITIES 

4.4.3 White Spot-IN718 Interface Defects. 

Figure 4-14 shows large interior grains within white spot seed 1, which may be the source of the 

internal structure noted by ultrasound. For white spot seed 2, figure 4-15 shows the IN718 

interface defect at a low magnification. There are small discontinuities present, particularly in 

the left half of the image. Additionally, a larger disbond was seen near the corner that circled the 

back surface of the seed. A similar ring disbond was also found around white spot seed 3. 

4-12
FIGURE 4-14. LARGE GRAIN STRUCTURE OF WHITE SPOT SEED 1 

(This grain size is typical of that seen for all the white spot defects.) 

FIGURE 4-15. IN718 (TOP) AND WHITE SPOT SEED 2 (BOTTOM) INTERFACE 

4.4.4 Dirty White Spot-IN718 Interface Defects. 

Figure 4-16 shows a detailed IN718 dirty white spot 6 bond line in which large ceramic particles 

present at the surface of the synthetic defect did not bond to the IN718 host. These ceramics are 

presumably those present in the ingot and forging created by the TiO2 and TiN additions. 

4-13
FIGURE 4-16. IN718 DIRTY WHITE SPOT 6 BOND LINE IN WHICH LARGE CERAMIC 

PARTICLES CREATE A NONBONDED REGION GREATER THAN 0.5 mm LONG 

4.5 FINAL BLOCK DESIGN. 

Based on experience from the trial block design, a new design was devised for the final blocks. 

White spot and dirty white spot grains are more than an order of magnitude larger than freckle 

grains (see figure 4-16). These grain sizes are consistent with the results of UT measurements 

(and with the difficulties in obtaining them). Seeds smaller than 0.185′′ (~5.0 mm) would not 

likely accommodate a statistically significant number of the larger grains. In addition, the 

measured reflectivities of the nonvoided seeds in the trial Ni-based blocks were relatively small, 

ranging from about 0.1% to 1% of that of an equal area FBH. 

As a result, the new design incorporated seeds on the order of a #3 FBH in diameter. Smaller 

seeds would be harder to fabricate, and direct UT velocity measurements on the seeds 

themselves (before insertion) would be difficult. In addition, smaller seeds would not likely 

provide accurate data in reflectivity studies because of competing grain noise. 

Some of the seeds in the trial blocks showed good bonding to the host material while others did 

not. This demonstrated that good seed bonding was possible but the HIP parameters might need 

adjustment. The equipment used could not produce pressures greater than 210 MPa and the 

temperature could not be increased without causing large grain growth. The only parameter 

available for adjustment was HIP time, which was increased from 3 to 8 hours to permit plastic 

flow of the synthetic flaws and the achievement of metallurgical bonding. 

Three IN718 blocks were machined with holes to accept seeds having nominal diameters of 

0.185′′ and nominal lengths of 0.205′′. Figures 4-17 through 4-19 show the block designs and 

seed positions. The seeds themselves were again fabricated from forged batches of material 

having freckle, white spot, and dirty white spot compositions. The finished IN718 blocks into 

4-14
which the seeds were inserted were assigned serial numbers beginning with letters FF, WF, and 

DWF for freckle, white spot, and dirty white spot seeds, respectively. 

2 1/2"

1"

2 3/4"

1/2"

3/4" 5/8" 3/4"

3/4"

3/4"

1"

5/8"

Sample Length Diameter

DWF-1 .203 .185

DWF-2 .201 .185

DWF-3 .199 .185

DWF-4 .200 .185

DWF-5 .205 .1855

DWF-6 .203 .185

1 2

4 3

5

6

DWF

2 1/2"

1"

2 3/4"

1/2"

3/4" 5/8" 3/4"

3/4"

3/4"

1"

5/8"

Sample Length Diameter

DWF-1 .203 .185

DWF-2 .201 .185

DWF-3 .199 .185

DWF-4 .200 .185

DWF-5 .205 .1855

DWF-6 .203 .185

1 2

4 3

5

6

DWF

FIGURE 4-17. DIMENSIONS AND SEED MAP FOR THE DIRTY WHITE SPOT 

SYNTHETIC SEED BLOCK 

4-15
2 1/2"

1"

2 3/4"

1/2"

3/4" 5/8" 3/4"

3/4"

3/4"

1"

5/8"

Sample Length Diameter

FF-1 .205 .185

FF-2 .205 .185

FF-3 .205 .185

FF-4 .205 .185

FF-5 .2055 .1855

FF-6 .205 .185

1 23

4 56

FF

2 1/2"

1"

2 3/4"

1/2"

3/4" 5/8" 3/4"

3/4"

3/4"

1"

5/8"

Sample Length Diameter

FF-1 .205 .185

FF-2 .205 .185

FF-3 .205 .185

FF-4 .205 .185

FF-5 .2055 .1855

FF-6 .205 .185

1 23

4 56

FF

FIGURE 4-18. DIMENSIONS AND SEED MAP FOR THE FRECKLE 

SYNTHETIC SEED BLOCK 

4-16
2 1/2"

1"

2 3/4"

1/2"

3/4" 5/8" 3/4"

3/4"

3/4"

1"

5/8"

Sample Length Diameter

WF-1 .205 .185

WF-2 .205 .185

WF-3 .205 .185

WF-4 .2045 .1845

WF-5 .205 .185

WF-6 .197 .1845

123

456

WF

2 1/2"

1"

2 3/4"

1/2"

3/4" 5/8" 3/4"

3/4"

3/4"

1"

5/8"

Sample Length Diameter

WF-1 .205 .185

WF-2 .205 .185

WF-3 .205 .185

WF-4 .2045 .1845

WF-5 .205 .185

WF-6 .197 .1845

123

456

WF

FIGURE 4-19. DIMENSIONS AND SEED MAP FOR THE WHITE SPOT 

SYNTHETIC SEED BLOCK 

The initial IN718 blocks were machined to size and ultrasonically inspected to ensure that no 

prior flaws existed in the parent material. The seeds were EDM’ed and machined to the lengths 

and diameters listed in table 4-4. The seed numbers in the table correspond to the seed numbers 

shown in the block drawings of figures 4-17 through 4-19. 

4-17
TABLE 4-4. SEED TYPES AND DIMENSIONS FOR SYNTHETIC BLOCKS 

Sample 

Length 

(inches) 

Diameter 

(inches) 

FF-1 0.205 0.185 

FF-2 0.205 0.185 

FF-3 0.205 0.185 

FF-4 0.205 0.185 

FF-5 0.2055 0.1855 

FF-6 0.205 0.185 

WF-1 0.205 0.185 

WF-2 0.205 0.185 

WF-3 0.205 0.185 

WF-4 0.2045 0.1845 

WF-5 0.205 0.185 

WF-6 0.197 0.1845 

DWF-1 0.203 0.185 

DWF-2 0.201 0.185 

DWF-3 0.199 0.185 

DWF-4 0.200 0.185 

DWF-5 0.205 0.1855 

DWF-6 0.203 0.185 

Prior to insertion into the IN718 host blocks, each seed was ultrasonically characterized to 

determine its longitudinal and shear-wave velocities in the axial (nominally 0.205′′) direction. 

Contact transducers were used having 1/4′′ diameters and center frequencies of 15 and 5 MHz for 

longitudinal and shear-wave measurements, respectively. The results are shown in table 4-5. 

For clean and dirty white spots, internal noise signals seen between the front- and back-surface 

echoes made measurements difficult; for shear-wave measurements, the SNR was not sufficient 

to accurately determine velocity. In addition, dirty white spot seeds had the poorest SNR for 

longitudinal measurements and the largest scatter in longitudinal velocity. Comparing tables 4-2 

and 4-5, one sees that the velocities measured on the seeds themselves are in good agreement 

with the earlier measurements using rectangular coupons of seed material. 

After the UT velocity measurements were completed, the blocks and seeds were assembled and 

EB-welded in a vacuum. The blocks were then HIPped at 970°C for 8 hours at 30 MPa. The top 

and bottom of each block was then lightly ground to remove any surface oxide and to provide 

clean, smooth surfaces for UT inspection. UT C-scans of the blocks were performed at 10 MHz 

with an F7 transducer. A representative C-scan is shown in figure 4-20. With reference to the 

figure, there is a high-amplitude spot on one of the inclusions in the block containing dirty white 

spots (upper row, center seed). The maximum amplitude from this inclusion was about twice 

4-18
that of the other dirty white spot inclusions and equaled about 53% of the response from a #1 

FBH in the calibration standard. 

TABLE 4-5. SYNTHETIC SEED DIMENSIONS AND SOUND VELOCITIES 

Sample 

Length 

(mm) 

Δ Time 

(L) (μsec) 

CL

(mm/ μsec) 

Δ Time 

(S) (μsec) 

CS

(mm/ μsec) 

FF-1 5.215 1.771 5.889 3.321 3.160

FF-2 5.210 1.798 5.795 3.293 3.164 

FF-3 5.203 1.766 5.892 3.305 3.149 

FF-4 5.203 1.766 5.892 3.283 3.170 

FF-5 5.204 1.771 5.877 3.285 3.168 

FF-6 5.204 1.776 5.860 3.267 3.186 

WF-1 5.198 1.764 5.896 Attempts to measure 

WF-2 5.212 1.788 5.830 shear data did not give 

WF-3 5.213 1.800 5.792 sufficient SNR for 

WF-4 5.210 1.814 5.744 precise velocity 

WF-5 5.223 1.776 5.882 determinations. 

WF-6 5.011 1.706 5.875 

DWF-1 5.160 1.792 5.759 Attempts to measure 

DWF-2 5.117 1.712 5.978 shear data did not give 

DWF-3 5.277 1.811 5.828 sufficient SNR for 

DWF-4 5.105 1.825 5.595 precise velocity 

DWF-5 5.063 1.532 6.610 determinations. 

DWF-6 5.155 1.793 5.750 

Note: freckle, white spot, dirty white spot seed dimensions, and longitudinal and shear velocities are listed. 

WF

DWF 

FF 

FIGURE 4-20. A 10-MHz F/7 C-SCAN OF BLOCKS CONTAINING 

SYNTHETIC DEFECTS 

(Clockwise from lower left, the three defect blocks contain freckle, dirty white spot, 

 and clean white spot defects, respectively. Image at lower right shows three of the 

 #1 FBH in the reference block.) 

4-19
Because of the abnormally large echo from one of the dirty white spot inclusions, it was decided 

to re-HIP all the blocks at the same conditions as before, viz. 970°C, 30 MPa, 8 hours. After this 

second HIP, the top and bottom surfaces of the blocks were again lightly machined and 

rescanned using the same 10-MHz F7 transducer. The resulting C-scan, shown in figure 4-21, 

was very similar to the earlier scan. Again, the same dirty white spot seed yielded an abnormally 

high peak amplitude. This amplitude was measured as 56% of the response from the average a 

#1 FBH in the calibration standard, quite close to the earlier result (53%). Thus, the re-HIP did 

not change the abnormally strong reflection from the dirty white spot in question. Since the 

block is believed to be well-bonded, it is possible that this strong reflection is from ceramic 

stringers that are present in dirty white spots. 

FIGURE 4-21. A 10-MHz C-SCAN OF BLOCKS WITH SYNTHETIC DEFECTS 

(Lower left block contains th ons, highly reflective seed is 

.6 SUMMARY

WF FF 

DWF

AFTER SECOND HIP 

e dirty white spot inclusi

essentially unchanged. (Block rotated 180° from previous scan.) Lower right of 

picture shows five #1 FBHs from the companion reference standard block.) 

4 . 

In this study, to produce samples containing synthetic defects, three defect types were selected. 

Bulk compositions corresponding to the three classes of IN718 defects—white spots, dirty white 

spots, and freckles—were synthesized using melting and forging processes. The microstructure 

and UT properties of the bulk synthetic flaws were characterized. A procedure was developed to 

embed small coupons (seeds) of these defect materials as synthetic flaws in IN718 blocks. Three 

blocks were manufactured containing such synthetic flaws. 

4-20
5. CONCLUSIONS. 

The fundamental studies program has provided information that will be useful to the 

nondestructive evaluation community in the design of inspection methods for nickel (Ni) billets. 

Key results include the following: 

• Ultrasonic (UT) velocity was found to be uniform for a given alloy. Since sonic velocity 

influences the UT beam radiation pattern, sizeable velocity variations can shift the focal 

depth and impact detectability. Given the uniformity of velocity measurements for 

IN718 and Waspaloy, it is unlikely that velocity variations will cause noticeable affects 

on detectability for these alloys. 

• In contrast to velocity, attenuation and backscattered grain noise capacity were found to 

vary significantly with frequency and position. Attenuation and grain noises were highly 

correlated, i.e., billet regions of high attenuation and grain noise were also found to have 

high grain noise capacity. Attenuation and noise capacity values were generally found to 

be larger in Waspaloy than IN718. 

• Grain size is the predominant factor controlling noise and attenuation in the Ni alloys 

studied in this program. In Ni processing, there is a significant amount of 

recrystallization that occurs due to mechanical working of the billet during the conversion 

process. This led to approximately equiaxed grains as determined by metallographic 

characterization performed during the program. As mean grain size increased, the 

attenuation and grain noise were also found to increase. 

• Billet conversion practice was also found to affect the attenuation and grain noise 

characteristics of the alloys studied. Values of both properties were found to be lower for 

so-called V-Die processing than GFM. 

• Attenuation and grain noise were found to be homogenous with little difference between 

the axial, radial, and hoop directions in a given measurement location. 

• At a given location in a billet, attenuation and grain noise capacity were found to be 

approximately isotropic with little difference between the axial, radial, and hoop 

directions. 

• Methods to produce synthetic defects with similar chemistries to naturally occurring 

defects were developed, and several specimens were fabricated that contain so-called 

white spot, dirty white spot, and freckle defects. 

5-1/5-2
6. REFERENCES. 

1. L. A. Jackman, G. E. Maurer, and S. Widge, “New Knowledge About ‘White Spots’ in 

Superalloys,” Advance Materials & Processes, Vol. 5, May 1993, pp. 18-25. 

2. P. Howard, D. Copley, and R. Gilmore, “Transducer Design for High Sensitivity 

Ultrasonic Inspection of Titanium,” in Rev. of Prog. in QNDE, Vol.16A, D. O. 

Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1997, pp. 893-900. 

3. T. A. Gray, “Application of Measurement Models to Specification of Ultrasonic 

Inspections,” in Rev. of Prog. in QNDE, Vol. 16B, D. O. Thompson and D. E. Chimenti, 

eds., Plenum Press, NY, 1997, pp. 2061-2068. 

4. R. B. Thompson, F. J. Margetan, I. Yalda, C.-P. Chiou, and P. D., Panetta, “Coupling 

Microstructure Outputs of Process Models to Ultrasonic Inspectability Predictions,” in 

Rev. of Prog. in QNDE, Vol. 17B, D. O. Thompson and D. E. Chimenti, eds., Plenum 

Press, NY, 1998, pp. 1847-1853. 

5. J. A. Umbach and R. A. Roberts, “Ultrasonic Phased Array Transducers-Model and 

Experiment,” presented at ASNT Fall Conference, October 1997. 

6. K. Smith and L. Brasche, “Successful Implementation of FAA Research Address the 

Disk Inspection Initiative,” in Proceedings of the 5th Annual FAA/NASA/DOD Aging 

Aircraft Conference, 2001. 

7. R. J. Filkins, J. P. Fulton, T. C. Patton, and J. D. Young, “Recent Advances and 

Implementations of Flexible Eddy Current Probe Technology,” in Rev. of Prog. in 

QNDE, Vol. 17B, D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1998, 

pp. 1809-1816. 

8. J. M. Amos and J. C. Chao, “Optimization of a Wide-Field Eddy Current Probe Using a 

Boundary Element Method Based Model,” in Rev. of Prog. in QNDE, Vol.17A, D. O. 

Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1998, pp. 275-282. 

9. J. C. Chao, N. Nakagawa, D. A. Raulerson, and J. C. Moulder, “Boundary Element 

Method Based Probe Design Model Validation,” in Rev. of Prog. in QNDE, Vol.16A, 

D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1997, pp. 967-972. 

10. W. Q. Meeker, V. Chan, R. B. Thompson, and C.-P. Chiou, “A Methodology for 

Predicting Probability of Detection for Ultrasonic Testing,” in Rev. of Prog. in QNDE, 

Vol. 20B, D.O. Thompson and D.E. Chimenti, eds., AIP, Melville, NY, 2001, pp. 1972-

1978. 

11. R. H. Burkel, C.-P. Chiou, T. K. Keyes, W. Q. Meeker, J. H. Rose, D. J. Sturges, R. B. 

Thompson, and W. T. Tucker, “A Methodology for the Assessment of the Capability of 

Inspection Systems for Detection of Subsurface Flaws in Engine Components,” FAA 

report DOT/FAA/AR-01/96, September 2002. 

6-1
12. www. Sandia.gov 

13. P. D. Panetta, F. J. Margetan, I. Yalda, and R. B. Thompson, “Ultrasonic Attenuation 

Measurements in Jet-Engine Titanium Alloys,” in Rev. of Prog. in QNDE, Vol. 15B, 

D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1996, p. 1525. 

14. P. D. Panetta, F. J. Margetan, I. Yalda, and R. B. Thompson, “Observation and 

Interpretation of Microstructurally Induced Fluctuations of Back-Surface Signals and 

Ultrasonic Attenuation in Titanium Alloys,” ibid., Vol. 16B, 1997, p. 1547. 

15. F. J. Margetan, P. D. Panetta, and R. B. Thompson, “Ultrasonic Signal Attenuation in 

Engine Titanium Alloys,” in Rev. of Prog. in QNDE, Vol. 17B, D. O. Thompson and D. 

E. Chimenti, eds., Plenum Press, NY, 1998, pp. 1469-1476. 

16. P. D. Panetta, R. B. Thompson, F. J. Margetan, and I. Yalda, “Ultrasonic Microstructural 

Characterization and Modeling of Attenuation in Ti-17 and Ti-6Al-4V Alloys,” 

Nondestructive Evaluation and Materials Properties, O. Buck and P. Liaw, eds., TMS, 

Warrendale, OH, 1998. 

17. P. D. Panetta, R. B. Thompson, and F. J. Margetan, “Use of Electron Backscatter 

Diffraction in Understanding Texture and the Mechanisms of Backscattered Noise 

Generation in Titanium Alloys,” in Rev. of Prog. in QNDE, Vol. 17A, D. O. Thompson 

and D. E. Chimenti, eds., Plenum Press, NY, 1998, pp. 89-96. 

18. R. B. Thompson, P. Panetta, and F. J. Margetan, “Relationship of the Ultrasonic 

Backscattering Coefficient of Titanium Alloys to Microstructure,” Materials and 

Processes Affordability: Keys to the Future, H. S. Kliger, B. J. Rasmussen, L. A. Pilato, 

and T. B. Tolle, eds., SAMPE, Covina, CA, 1998, pp. 1448-1457. 

19. F. J. Margetan, H. Wasan, and R. B. Thompson, “An Experimental Study of 

Microstructure-Induced Ultrasonic Signal Fluctuations in Jet-engine Titanium Alloys,” 

Rev. of Prog. in QNDE, Vol. 19B, D. O. Thompson and D. E. Chimenti, eds., AIP, 

Melville, NY, 2000, pp. 1433-1440. 

20. F. J. Margetan, T. A. Gray, and R. B. Thompson, “A Technique for Quantitatively 

Measuring Microstructurally Induced Ultrasonic Noise,” Review of Progress in QNDE, 

Vol. 10B, D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1991, pp. 1721-

1728. 

21. F. J. Margetan and R. B. Thompson, “Microstructural Noise in Titanium Alloys and its 

Influence on the Detectability of Hard-Alpha Inclusions,” Review of Progress in QNDE, 

Vol. 11, D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1992, pp. 1717-

1724. 

22. F. J. Margetan, R. B. Thompson, I. Yalda-Mooshabad, and Y. K. Han, “Detectability of 

Small Flaws in Advanced Engine Alloys,” ISU Center for NDE, July 1993. (This 221-

page report summarizes 3 years of U.S. Air Force sponsored research on the use of 

6-2
ultrasound to detect defects in titanium engine components and discusses at some length 

the underlying noise models that are used to deduce the FOM from measured noise data.) 

23. Edward J. Nieters, Robert S. Gilmore, David C. Copley, Patrick J. Howard, Robert C. 

Trzaskos, John D. Young, Michael E. Keller, and William J. Leach, “A Multizone 

Technique for Billet Inspection,” Review of Progress in QNDE, Vol. 14B, D. O. 

Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1995, p. 2137. 

24. P. J. Howard and R. S. Gilmore, “Ultrasonic Noise and the Volume of the Ultrasonic 

Pulse,” Review of Progress in QNDE, 15, D. O. Thompson and D. E. Chimenti, eds., 

Plenum Press, NY, 1996, p. 1495. 

25. P. Haldipur, F. J. Margetan, L. Yu, and R. B. Thompson, “Ultrasonic Attenuation 

Measurements in Jet-Engine Nickel Alloys,” Review of Progress in QNDE, Vol. 20, 

D. O. Thompson and D. E. Chimenti, eds., AIP, Melville, NY, 2001, pp. 1338-1345. 

26. A. Minachi, F. J. Margetan, and R. B. Thompson, “Reconstruction of a Piston Transducer 

Bean Using Multi-Gaussian Beams (MGB) and Its Applications,” in Review of Progress 

in QNDE, Vol. 17A, D. O. Thompson and D. E. Chimenti, eds., Plenum, NY, 1998, 

pp. 907-914. 

27. F. J. Margetan, R. B. Thompson, and T. P. Lerch, “An Analytical Approximation for a 

Common Ultrasonic Grain Noise Diffraction Integral,” Review of Progress in QNDE, 

Vol. 18A, D. O. Thompson and D. E. Chimenti, eds., Kluwer/Plenum, NY, 1999, p. 45. 

28. F. J. Margetan, R. B. Thompson, and I. Yalda-Mooshabad, “Modeling Ultrasonic 

Microstructural Noise in Titanium Alloys,” Review of Progress in QNDE, 12, D. O. 

Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1993, pp. 1735-1743. 

29. F. J. Margetan, R. B. Thompson, and I. Yalda-Mooshabad, “Backscattered 

Microstructural Noise in Ultrasonic Toneburst Inspections,” Journal of Nondestructive 

Evaluation, Vol. 13, Plenum Press, NY, 1994, pp. 111-136. 

30. F. J. Margetan, I. Yalda-Mooshabad, and R. B. Thompson, “Estimating Ultrasonic 

Signal-to-Noise Ratios for Inspections of Cylindrical Billets,” Review of Progress in 

QNDE, Vol. 13, D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1994, pp. 

1737-1744. 

31. F. J. Margetan, K. Y. Han, I. Yalda, Scot Goettsch, and R. B. Thompson, “The Practical 

Application of Grain Noise Models in Titanium Billets and Forgings,” Review of 

Progress in QNDE, Vol. 14, D. O. Thompson and D. E. Chimenti, eds., Plenum Press, 

NY, 1995, pp. 2129-2136. 

32. F. J. Margetan, I. Yalda and R. B. Thompson, “Predicting Gated-Peak Grain Noise 

Distributions for Ultrasonic Inspections of Metals,” Review of Progress in QNDE, Vol. 

15, D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1996, pp. 1509-1516. 

6-3
33. F. J. Margetan, I. Yalda, R. B. Thompson, J. Umbach, U. Suh, P. J. Howard, D. C. 

Copley, and R. Gilmore, “Ultrasonic Grain Noise Modeling: Recent Applications to 

Engine Titanium Inspections,” Review of Progress in QNDE, Vol. 16, D. O. Thompson 

and D. E. Chimenti, eds., Plenum Press, NY, 1997, pp. 1555-1562. 

34. F. J. Margetan, P. Haldipur, Linxiao Yu, R. B. Thompson, A. Degtyar, and H. Wasan, 

“Backscattered Ultrasonic Noise Measurements in Jet-Engine Nickel Alloys,” Review of 

Progress in QNDE, Vol. 20, D. O. Thompson and D. E. Chimenti, eds., AIP, Melville, 

NY, 2001, pp. 1314-1321. 

35. Anxiang Li, Ron Roberts, Pranaam Haldipur, Frank J. Margetan, and R. B. Thompson, 

“Computational Study of Grain Scattering Effects in Ultrasonic Measurements,” Review 

of Progress in QNDE, Vol. 22, D. O. Thompson and D. E. Chimenti, eds., AIP, Melville 

NY, in press. 

36. Pranaam Haldipur, F. J. Margetan, Linxiao Yu, and R. B. Thompson, “A Study of 

Ultrasonic Property Variations Within Jet-Engine Nickel Alloy Billets,” Review of 

Progress in QNDE, Vol. 21B, D. O. Thompson and D. E. Chimenti, eds., AIP, Melville, 

NY, 2002, pp. 1502-1509. 

37. P. Haldipur, F. J. Margetan, and R. B. Thompson, “Correlation Between Local Ultrasonic 

Properties and Grain Size Within Jet-Engine Nickel Alloy Billets,” Review of Progress in 

QNDE, Vol. 22, D. O. Thompson and D. E. Chimenti, eds., AIP, Melville, NY, in press. 

38. Pranaam Haldipur, “Model-Based Interpretation on Ultrasonic Data in Nickel-Based 

Superalloys,” Ph.D. Dissertation, Material Science Engineering Dept., Iowa State 

University, Ames, Iowa, in preparation. 

39. C. P. Chiou, Isaac Yalda, F. J. Margetan, and R. B. Thompson, “The Use of Ultrasonic 

Flaw and Noise Models in Designing Titanium Test Blocks,” Review of Progress in 

QNDE, 17B, D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1998, 

pp. 2069-2076. 

40. J. A. Turner, “On-line Attenuation Calculator,” may be accessed at http://emjaturner.unl.edu

41. J. A. Turner and R. L. Weaver, “Radioactive Transfer and Multiple Scattering of Diffuse 

Ultrasound in Polycrystalline Media,” J. Acoust. Soc. Am. V. 96, pp. 3675-3683, 1994. 

42. R. L. Weaver, “Diffusivity of Ultrasound in Polycrystals,” J. Mech. Phys. Solids 38, 55-

86, 1990. 

43. F. E. Stanke and G. S. Kino, “A Unified Theory for Elastic Wave Propagation in 

Polycrystalline Materials,” J. Acoust. Soc. Am. V. 75, pp. 665-681, 1984. 

44. J. H. Rose, “Ultrasonic Backscatter from Microstructure,” Review of Progress in QNDE, 

Vol. 11B, D. O. Thompson and D. E. Chimenti, eds., Plenum Press, NY, 1992, pp. 1677-

1684. 

6-4
45. Frank J. Margetan, Pranaam Haldipur, and Yanming Guo, “Determination of Grain 

Diameter Estimates From Grain-boundary Tracings,” a “How-To-Manual” which also 

demonstrates the use of the GmSize3 software, Center for NDE at Iowa State University, 

May 2002. 

46. R. B. Thompson, J. F. Smith, S. S. Lee, and G. C. Johnson, “A Comparison of Ultrasonic 

and X-Ray Determinations of Texture in Thin Cu and Al Plates,” Metallurgical 

Transactions A, Vol. 20A, November 1989, pp. 2431-2446. 

47. P. Haldipur, F.J. Margetan, and R.B. Thompson, “Estimation of Single-Crystal Elastic 

Constants From Ultrasonic Measurements on Polycrystalline Specimens,” Rev. of Prog. 

in QNDE, Vol. 23, D.O. Thompson and D.E. Chimenti, eds., AIP, Melville, NY, 2004, in 

press. 

48. Laurence A. Jackman, Gernant E. Maurer, and Sunil Widge, “White Spots in 

Superalloys,” In Superalloys 718, 625, 706, and Various Derivatives, E. A. Loria, ed., 

The Minerals, Metals & Materials Society, Warrendale, PA, 1994, pp. 153-166. 

49. S. T. Wlodek and R. D. Field, “Freckles” in Cast and Wrought Products, In Superalloys

718, 625, 706, and Various Derivatives, E. A. Loria, ed., 1994, The Minerals, Metals & 

Materials Society, Warrendale, PA, pp. 167-176. 

6-5/6-6
APPENDIX A—DEFECT CHARACTERIZATION OF NATURALLY 

OCCURRING ANOMALIES 

In addition to work with synthetic defects, five naturally occurring anomalies were ultrasonically 

and metallographically characterized in cooperation with members of the Specialty Metals 

Processing Consortium. Members of the consortium provided specimens and production 

inspection information for five indications. Samples were ultrasonically characterized at Iowa 

State University and metallographically characterized by the Original Equipment Manufacturer 

members of the Engine Titanium Consortium. The results of the characterization are provided in 

this appendix. Details of the ultrasonic characterization procedure are provided in Appendix B. 

A summary of the anomaly characterization is provided in table A-1. 

TABLE A-1. ANOMALY CHARACTERIZATION DATA SUMMARY 

Indication 

Number Material Description 

UT Response 

(%Full) Screen Height Anomaly Type 

1 IN 718 VIM/VAR NA Entrapped slag 

2 IN 718 VIM/ESR/VAR 80% +2.5 dB Entrapped slag 

3 IN 718 VIM/ESR/VAR 80% +2.5 dB Entrapped slag 

4 Waspaloy VIM/ESR/VAR 100% Nonmetallic inclusion 

5 Waspaloy VIM/ESR/VAR 100% Nonmetallic inclusion 

A-1
Ni Billet Indication #1 – General Information

• Found in 10" triple melt Inconel 718 billet

• Indication determined to be entrapped slag

Cube Orientation

T

F

R

L UT beam direction: 

from T to B

B

BB Billet 

axis

Polishing direction:

from R to L

R

B

F BB

T

Micrograph Orientation Cube Orientation

T

F

R

L UT beam direction: 

from T to B

B

BB Billet 

axis

Polishing direction:

from R to L

R

B

F BB

T

Micrograph Orientation

Ni Billet Indication # 1 – Sample sectioning and naming convention

A-2
Ni Real Indication #1: Image Orientation

T R R

R R

R

BB

BB

B

BB

BB

BB

FL

L

L

F

F

F F

B

L

L

L

B B B

T T T

T

Transducer P2: Panametrics V307 5MHz/1.0” dia. 8” PTF

Transducer PA: Megasonics IHM 20MHz/0.25” dia. 2” SF

Ni Billet Indication # 1 – Ultrasonic Measurements

5 MHz C-Scan Images (P2 transducer)

Side T

Gain=43 dB

Side B

Gain=43 dB

Side R

Gain=37 dB

Side L

Gain=37 dB

All are 200 by 200 pixels at 5 mils scan increments

Sides BB and F have no indication

Ni Billet Indication # 1 – Ultrasonic Measurements

A-3
20 MHz C-Scan Images (PA transducer)

Side T

Gain=30 dB

Side B

Gain=30 dB

Side R

Gain=24 dB

Side L

Gain=28 dB

All are 500 by 500 pixels at 2 mils scan increments

Side BB

Gain=43? dB

Side F

Gain=38 dB

Arrows: Surface air bubble

Ni Billet Indication # 1 – Ultrasonic Measurements

A-4
NOTE: the numbers on the right-hand side indicate the magnification 

(10x) and the polishing depth of slice measured from original top 

surface of R side (0.830” and 0.831”) 

Ni Billet Indication # 1 – 10X magnification

Ni Billet Indication # 1 – 10X magnification

A-5
Ni Billet Indication # 1 – 10X magnification

Ni Billet Indication # 1 – 10X magnification

Slice 0.839” is where the chemical analysis was performed

A-6
Ni Billet Indication # 1 – 10X magnification

Due to technical problem, after slice 0.846” the sectioning jumps directly 

to the last slice 0.861” at which Indication cross section was no longer seen.

Ni Billet Indication # 1 – 10X magnification

A-7
Slice 830

Slice 831

Slice 832

Slice 833

Ni Billet Indication # 1 – 100X composite micrographs

Slice 835

Slice 836

Slice 838

Slice 839

Ni Billet Indication # 1 – 100X composite micrographs

A-8
Slice 841

Slice 842

Slice 844

Slice 846

Ni Billet Indication # 1 – 100X composite micrographs

Ni Billet Indication # 1 – SEM analysis performed at slice 0.839”

Inco 718 triple melt

Matrix scan

A-9
Ni Billet Indication # 1 – SEM analysis performed at slice 0.839”

SEM analysis locations – 200X photograph

Location 1 – “island”

Location 2 – “big particle”

Location 3 – “general area” Location 4 – “grey area”

Ni Billet Indication # 1 – SEM analysis performed at slice 0.839”

Inco 718 triple melt

Location 1 – “island scan”

A-10
Ni Billet Indication # 1 – SEM analysis performed at slice 0.839”

Inco 718 triple melt

Location 2 – “large particle”

Ni Billet Indication # 1 – SEM analysis performed at slice 0.839”

Inco 718 triple melt

Location 2 – “white specks in large particle”

A-11
Ni Billet Indication # 1 – SEM analysis performed at slice 0.839”

Inco 718 triple melt

Location 3 – “general area”

A-12
Ni Billet Indication #2 – General Information

• Found in 10" triple melt Inconel 718 billet

• Amplitude was 80% plus 2.5 dB with a #2 FBH equal to 80%

• Depth was 2.7" and the length was 0.375 " in billet.

• Indication determined to be entrapped slag

Cube Information

 Dimension close to 1” cube with error around 20 mils all sides. 

 UT scans were performed at ISU using a Utex WinSpect system.

 UT data include single A-scans at peak positions, C-scan images, and volumetric Ascans at C-scan scan grids using both 5 and 15 MHz transducers.

 Additional A-scans were obtained from fused quartz block as reference waveforms. 

Billet/Cube Orientation

T, 1 F, 6 Billet axis

R, 4

UT beam direction L, 2

B, 3

BB, 5

Ni Billet Indication # 2 – Ultrasonic Measurements

A-13
Transducers:

PB: Panametrics V319 15MHz/0.5” dia. 90mm PTF; 

P2: Panametrics V307 5MHz/1.0” dia. 8” PTF

Scan Parameters: 

All scan files used 250 MHz sampling rate, 1 ms gate width and 4 time averages.

P2 files are 71x71 pixels (scan positions) at 10 mils spatial increment. 

PB files are 141x141 pixels (scan positions) at 5 mils spatial increment.

Cube UT Scan Information

Ni Billet Indication # 2 – Ultrasonic Measurements

P2 (5MHz) C-scan Images

Sides 5 and 6 were masked by P2’s internal structure in time trace

5

5 4

1

4

6 5 5

6 2 2

6 6

Side 1

Gain=32dB

Side 2

Gain=39 dB

Side 3

Gain=31 dB

Side 4

Gain=39 dB

Side 5

Gain=49 dB

Side 6

Gain=33 dB

1 1

1

2 3

2

4

4

3

3

3

Multizone Color Code

0 128 255=1v

Ni Billet Indication # 2 – Ultrasonic Measurements

A-14
PB (15MHz) C-scan Images

5

5 4

1

4

6 5 5

6 2 2

6 6

Side 1

Gain=32dB

Side 2

Gain=39 dB

Side 3

Gain=31 dB

Side 4

Gain=39 dB

Side 5

Gain=49 dB

Side 6

Gain=33 dB

1 1

1

2 3

2

4

4

3

3

3

Multizone Color Code

0 128 255=1v

Ni Billet Indication # 2 – Ultrasonic Measurements

A-15
Slice 1

Slice 2

Slice 3

500 μm

Ni Billet Indication # 2 – 100X composite micrograph

Slice 4

Slice 5

Slice 6

500 μm

Slice 4

Slice 5

Slice 6

500 μm

A-16
Slice 7

Slice 8

Slice 9

500 μm

Ni Billet Indication # 2 – 100X composite micrograph

Slice 12

Slice 11

Slice 10

A-17
Slice 13

Slice 14

Slice 15

500 μm

Ni Billet Indication # 2 – 100X composite micrograph

Slice 16

Slice 17

Slice 18

A-18
Slice 19

Slice 21

Slice 20

500 μm

Ni Billet Indication # 2 – 100X composite micrograph

Slice 22

Slice 24

Slice 23

A-19
Slice 25

Slice 27

Slice 26

500 μm

Ni Billet Indication # 2 – 100X composite micrograph

Slice 28

Slice 30

Slice 29

A-20
Slice 31

Slice 33 No indication present

Slice 32

500 μm

Ni Billet Indication # 2 – 100X composite micrograph

Slice 34

Slice 36

Slice 35

A-21
Slice 37 No indication present

Slice 39

Slice 38 No indication present

500 μm

Ni Billet Indication # 2 – 100X composite micrograph

Ni Billet Indication # 2 – SEM analysis

A-22
Ni Billet Indication # 2 – SEM analysis

Ni Billet Indication # 2 – SEM analysis

A-23
Ni Billet Indication # 2 – SEM analysis

Ni Billet Indication # 2 – SEM analysis

Location C

A-24
Ni Billet Indication # 2 – SEM analysis

Location C

A-25
Ni Billet Indication #3 – General Information

• Billet: double melt 718, 10” diameter

• Calibration: #3 flat-bottom hole at 80% of full screen height

• Indication Amplitude: 80% of full screen height

• Depth: 5.265”

• Length: 0.35”

• Source: Dirty white spot

Ni Billet Indication # 3 – Sectioning and naming convention

Top=1

Billet axis Front=6

Right=4

Left=2 UT beam direction

BOT=3

Back=5

Cube Block Orientation and Naming System

Top=1

Billet axis Front=6

Right=4

Left=2 UT beam direction

BOT=3

Back=5

Cube Block Orientation and Naming System

A-26
50 MHz

Side 1 Side 3

15 MHz

Side 1 Side 2 Side 3 Side 4 Side 5 Side 6

0.4” X 0.4”

Gain=35 dB

Image resolution: All are of 0.005” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right for sides 1~4; side 2 on left and side 4 on right for sides 5~6

0.5” X 0.5”

Gain=34 dB

Side 5Side 6

0.7” X 0.7”

Gain=32 dB

0.7” X 0.7”

Gain=34 dB

0.5” X 0.5”

Gain=45 dB

0.7” X 0.7”

Gain=37 dB

0.7” X 0.7”

Gain=22 dB 0.7” X 0.7”

Gain=26 dB

Multizone Color Code

0 128 255

C-scan Images

Ni Billet Indication # 3 – Ultrasonic Measurements

A-27
Ni Billet Indication # 3 – 100X composite micrograph 

Sectioning occurred at 0.002” increments

#17

#18

#19

#20

#21

#22

#23

#27

#28

#25

#26

#24

A-28
Ni Billet Indication # 3 – 100X composite micrograph 

Sectioning occurred at 0.002” increments

#29

#30

#31

#32

#33

#34

#35

#36

#37

#38

#39

#34

#35

#36

#37

#38

#39

A-29
Ni Billet Indication # 3 – 100X composite micrograph 

Sectioning occurred at 0.002” increments

#40

#41

#42

#43

#44

#45

#46

#47

A-30
Ni Billet Indication # 3 – 100X composite micrograph 

Sectioning occurred at 0.002” increments

#48

#49

#50

#51

#52

#53

#54

#48

#49

#50

#51

#52

#53

#54

#55

#56

#57

SEM completed on this slice

A-31
Ni Billet Indication # 3 – 100X composite micrograph 

Sectioning occurred at 0.002” increments

#58

#59

#60

#61

#62

#63

#64

A-32
Ni Billet Indication # 3 – 100X composite micrograph 

Sectioning occurred at 0.002” increments

#65

#66

#67

#68

#69

#70

#71

A-33
Ni Billet Indication # 3 – SEM analysis performed at slice #55

A-34
Ni Billet Indication # 3 – SEM analysis performed at slice #55

Comparison of area 1 and area 3

Base alloy away from contaminant – red

Overlay of contaminant - blue

Ni Billet Indication # 3 – SEM analysis performed at slice #55

A-35
Base alloy away from contaminant – red

Ni Billet Indication # 3 – SEM analysis performed at slice #55

Ni Billet Indication # 3 – SEM analysis performed at slice #55

A-36
Base alloy away from contaminant – red

Ni Billet Indication # 3 – SEM analysis performed at slice #55

Ni Billet Indication # 3 – SEM analysis performed at slice #55

A-37
Ni Billet Indication # 3 – SEM analysis performed at slice #55

A-38
Ni Billet Indication #4 – General Information

• Alloy = Waspaloy

• Melt method = VIM-VAR (Vacuum Induction Melting followed by Vacuum Arc 

Remelting)

• Billet Diameter = 11”.

• Calibration was in accordance industry standards. The required flat bottom hole 

(FBH) size is a 5/64” diameter. Calibration reflectors were positioned at 

approximately 0.92” and 6.6” below the sound entry surface. These reflectors were 

gated and electronically DAC was used.

• Calibration amplitude = 80% of full screen height (FSH).

• Reject level = 100% of the required FBH (i.e. 80% FSH). 

• Alarm threshold = 60% FSH (75% of the calibration height).

• Maximized indication amplitude = 98% FSH (122% of a #5 FBH) 

• Indication length ~ 0.420” ; depth ~ 5.6”

• Transducer Information:

a. Frequency = 3.5 MHZ

b. Transducer Size = 0.75” by 1.0” rectangular

c. Cylindrically Focussed at 7.00”

d. Beam Diameter = 0.090 in cylindrical direction (scanning direction)

e. Longitudinal Wave

f. Water path set at 4.50”

• Indication: nonmetallic inclusion typical of Waspaloy

DIRECTION OF POLISH

Approximate fiducial

hole locations

3”

145.5”-148.5” 181.2”

(Ingot 

Bottom)

0”

(Ingot 

Top)

3”

145.5”-148.5” 181.2”

(Ingot 

Bottom)

0”

(Ingot 

Top)

Axial

Radial

1”

1”

UT beam direction

1”

1”

UT beam direction

Ni Billet Indication #4 – Naming convention for sectioning

C=1

Billet axis A=6

D=4

F=2 UT beam direction

E=3

B=5

C=1

Billet axis A=6

D=4

F=2 UT beam direction

E=3

B=5

A-39
50 MHz

Side 1 Side 3

15 MHz

Side 1 Side 2 Side 3 Side 4 Side 5 Side 6

0.5” X 0.4”

Gain=26 dB

Image resolution: All are of 0.005” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right for sides 1~4; side 2 on left and side 4 on right for sides 5~6

0.5” X 0.4”

Gain=39 dB

Side 5Side 6

0.4” X 0.3”

Gain=22 dB

(missed left-most defect part)

0.4” X 0.3”

Gain=27 dB

0.3” X 0.3”

Gain=41 dB

0.3” X 0.3”

Gain=42 dB

0.5” X 0.5”

Gain=17 dB

0.5” X 0.5”

Gain=19 dB

Multizone Color Code

0 128 255

Side 2

0.5” X 0.4”

Gain=40 dB

(missed left-most defect part)

Image center

w.r.t. cube face X

X

X

X X X X X X

Ni Billet Indication #4 – Ultrasonic Measurements

5 MHz

Side 1 Side 2 Side 3 Side 4 Side 5 Side 6

Image resolution: All are 1.1” by 1.1” of 0.01” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right for sides 1~4; side 2 on left and side 4 on right for sides 5~6

Multizone Color Code

0 128 255

Gain=30 dB Gain=33 dB Gain=30 dB Gain=34 dB Gain=38 dB Gain=40 dB

Ni Billet Indication #4 – Ultrasonic Measurements

A-40
Ni Billet Indication #4 – 100X composite micrograph 

Sectioning at 0.002” increments

#57

#58 – SEM completed on this slice

#59

#60

#61

#62

#63

A-41
#64

#65

#66

Ni Billet Indication #4 – 100X composite micrograph 

Sectioning at 0.002” increments

#68

#69

#67

A-42
#71

#72

#70

Ni Billet Indication #4 – 100X composite micrograph 

Sectioning at 0.002” increments

#74

#75

#73

A-43
#77

#78

#76

Ni Billet Indication #4 – 100X composite micrograph 

Sectioning at 0.002” increments

#80

#81

#79

A-44
#83 – SEM completed on this slice

#84

#82

Ni Billet Indication #4 – 100X composite micrograph 

Sectioning at 0.002” increments

#86

#87

#85

#88

A-45
#90

#91

#89

#92

Ni Billet Indication #4 – 100X composite micrograph 

Sectioning at 0.002” increments

#94

#95

#93

#96

A-46
#98

#99

#97

Ni Billet Indication #4 – 100X composite micrograph 

Sectioning at 0.002” increments

#100

A-47
Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

10X

3000X

1200X

A-48
Polish #58

Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

Polish #58

Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

A-49
Polish #58

Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

Polish #58

Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

A-50
Polish #58

Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

Polish #58

Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

A-51
Polish #58

Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

Polish #58

Ni Billet Indication #4 – SEM analysis performed at slice #58

0.18834” polished in from side D

A-52
Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

Polish #83

Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

A-53
Polish #83

Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

Polish #83

Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

A-54
Polish #83

Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

Polish #83

Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

A-55
Polish #83

Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

Polish #83

Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

A-56
Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

Ni Billet Indication #4 – SEM analysis performed at slice #83

0.23734” polished in from side D

A-57
Ni Billet Indication #5 – General Information

• Alloy = Waspaloy

• Melt method = VIM-VAR (Vacuum Induction Melting followed by Vacuum 

Arc Remelting)

• Billet Diameter = 11”

• Calibration was in accordance industry standards. The required flat bottom 

hole (FBH) size is a 5/64” diameter. Calibration reflectors were positioned 

at approximately 0.92” and 6.6” below the sound entry surface. These 

reflectors were gated and electronic DAC applied.

• Calibration amplitude = 80% of full screen height (FSH)

• Reject level = 100% of the required FBH (i.e. 80% FSH) 

• Alarm threshold = 60% FSH (75% of the calibration height).

• Maximized indication amplitude = 80% FSH (100% of a #5 FBH) 

• Indication length ~ 0.670” ; depth ~ 4.0”

• Transducer Information:

a. Frequency = 3.5 MHZ

b. Transducer Size = 0.75” by 1.0” rectangular

c. Cylindrically Focused at 7.00”

d. Beam Diameter = 0.090 in cylindrical direction (scanning direction)

e. Longitudinal Wave

f. Water path set at 4.50”

• Indication: Nonmetallic inclusion

Ni Billet Indication #5 – Sample sectioning and naming convention

50.0”-53.0” 176.4”

(Ingot 

Bottom)

0”

(Ingot 

Top)

3”

Axial

Radial

1”

1”

UT beam direction

1”

1”

UT beam direction

C=1

Billet axis A=6

D=4

F=2 UT beam direction

E=3

B=5

C=1

Billet axis A=6

D=4

F=2 UT beam direction

E=3

B=5

A-58
15 MHz

Side 1 Side 2 Side 3

Side 5

Side 4

Side 6

Image resolution: All are of 0.005” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right for sides 1~4; side 2 on left and side 4 on right for sides 5~6

1.0” X 0.7”

Gain=31 dB

1.0” X 1.0”

Gain=45 dB

0.9” (horizontal) X 0.7” (vertical)

Gain=20 dB

Multizone Color Code

0 128 255

X X X

X

X X

Experimental C-scan Images

Image center

w.r.t. cube face

1.0” X 0.7”

Gain=33 dB

1.0” X 0.7”

Gain=23 dB

1.0” X 1.0”

Gain=45 dB

air bubble 

on surface

Ni Billet Indication #5 – Ultrasonic Measurements

15 MHz

Side 1 Side 2 Side 3

Side 5

Side 4

Side 6

Image resolution: All are of 0.005” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right for sides 1~4; side 2 on left and side 4 on right for sides 5~6

1.0” X 0.7”

Gain=31 dB

1.0” X 1.0”

Gain=45 dB

0.9” (horizontal) X 0.7” (vertical)

Gain=20 dB

Multizone Color Code

0 128 255

X X X

X

X X

Experimental C-scan Images

Image center

w.r.t. cube face

1.0” X 0.7”

Gain=33 dB

1.0” X 0.7”

Gain=23 dB

1.0” X 1.0”

Gain=45 dB

air bubble 

on surface

Ni Billet Indication #5 – Ultrasonic Measurements

A-59
5 MHz

Side 1 Side 2 Side 3 Side 4 Side 5 Side 6

Image resolution: All are 1.1” by 1.1” centered; of 0.01” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right for sides 1~4; side 2 on left and side 4 on right for sides 5~6

Multizone Color Code

0 128 255

Gain=25 dB

Experimental C-scan Images (conti.)

Gain=34 dB Gain=28 dB Gain=33 dB Gain=38 dB Gain=40 dB

Ni Billet Indication #5 – Ultrasonic Measurements

50 MHz

Side 5

Side 6

Side 2

0.5” X 0.4”

Gain=40 dB

(missed left-most defect part)

Image center

w.r.t. cube face

X

1.0” X 0.7”

Gain=42 dB

Image resolution: 0.005” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right

Multizone Color Code

0 128 255

5 MHz

Side 1 Side 2 Side 3 Side 4 Side 5 Side 6

Image resolution: All are 1.1” by 1.1” centered; of 0.01” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right for sides 1~4; side 2 on left and side 4 on right for sides 5~6

Multizone Color Code

0 128 255

Gain=25 dB

Experimental C-scan Images (conti.)

Gain=34 dB Gain=28 dB Gain=33 dB Gain=38 dB Gain=40 dB

Ni Billet Indication #5 – Ultrasonic Measurements

50 MHz

Side 5

Side 6

Side 2

0.5” X 0.4”

Gain=40 dB

(missed left-most defect part)

Image center

w.r.t. cube face

X

1.0” X 0.7”

Gain=42 dB

Image resolution: 0.005” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right

Multizone Color Code

0 128 255

50 MHz

Side 5

Side 6

Side 2

0.5” X 0.4”

Gain=40 dB

(missed left-most defect part)

Image center

w.r.t. cube face

X

1.0” X 0.7”

Gain=42 dB

Image resolution: 0.005” scan increment (in-between pixels)

Orientation: side 5 on left and side 6 on right

Multizone Color Code

0 128 255

A-60
Ni Billet Indication #5 -100X composite micrograph 

Sectioning occurred at 0.002” increments, not all slices shown 

Slice 1

Slice 5

Slice 11

Slice 13

Slice 16

Slice 18

Slice 21

Slice 23

A-61
Ni Billet Indication #5 - 100X composite micrograph 

Sectioning occurred at 0.002” increments, not all slices shown

Slice 25

Slice 27

Slice 29

Slice 31

Slice 33

Slice 34

Slice 35

A-62
Ni Billet Indication #5 - 100X composite micrograph 

Sectioning occurred at 0.002” increments, not all slices shown 

Slice 37

Slice 39 

Slice 40

Slice 41

Slice 42 

Slice 43

Slice 36

A-63
Ni Billet Indication #5 - 100X composite micrograph 

Sectioning occurred at 0.002” increments, not all slices shown 

Slice 46

Slice 47

Slice 49 

Slice 50

Slice 51

Slice 44

A-64
NI Billet Indication #5 – 100X composite micrograph 

Sectioning occurred at 0.002” increments, not all slices shown 

Slice 52 

Slice 53 

Slice 55 

Slice 56 

Slice 57 

A-65/A-66
APPENDIX B—ULTRASONIC DATA ACQUISITION PROCEDURE FOR 

ETC REAL DEFECTS 

B-1/B-2
Using a Sonix Immersion System 

by C. Thomas Chiou, Mike Keller, Jeff Umbach and Andrei Degtyar 

Last update: 04-17-01 

1. Introduction 

This procedure is written for internal use in ETC-II Task 1.1.1 Fundamental Studies and other 

related tasks. The objective is to acquire UT a-scan, c-scan and multiple b-scan (volumetric ascan) data of naturally occurring defects embedded in a small Ti- or Ni-based cube block. The 

target readers are NDE staff with sufficient experience in use of a Sonix immersion system. 

Some technical details has been included here in an attempt to make this document selfcontained. Section 2 lists the basic requirements of system components and accessories. Section 

3 lays out the experimental setups of the reference and defect data acquisition, for which the 

step-by-step procedures are described in Sections 6 and 7. Other supporting materials are 

provided in the remaining sections. For clarification of several key terminologies, the readers 

are referred to Section 9. 

2. System Requirements 

The designated scan hardware is a computerized Sonix immersion scan system produced in the 

1990’s (or later) with three-axis scan capability, controlled by Sonix FlexSCAN-C software 

version 4.0 (or higher). Sonix default pulser/receiver such as DPR-35+ should be used if 

equipped, but other typical pulser/receivers such as Panametrics 505x series can be substituted. 

A standard transducer search tube (mounting pole) and a mechanical transducer manipulator 

capable of two angular position controls are assumed to be parts of the scan system. The defect 

specimen should have been sectioned down to a minimum size (typically around 1” cube) with 

the defect centered in the block. Knowledge of defect depth on each of the six sides is expected. 

The front and back surfaces of the defect cube block in the three orthogonal directions should 

also be made as parallel as possible. The accessories include a sample stand, a thermometer, 

transducer sets and fused-quartz reference sets. The specifications and usage of each accessory 

component are as follows. 

2.1 Sample stand 

A three- or four-legged flat stand of circular (or rectangular) shape with minimal size of 4” 

diameter (or side) is needed to support the cube sample and the reference plate in the scan tank. 

This stand should be made of rust-free materials such as stainless steel, titanium or epoxy glass, 

and the stand legs must be adjustable with good precision for leveling. 

2.2 Thermometer 

A thermometer is needed to measure the water temperature of the scan tank from which the 

water attenuation can be estimated. The precision of the thermometer should be within 1 degree 

Celsius or Fahrenheit. 

B-3
2.3 Transducer Set 

Minimally, two focused transducers are needed to acquire the UT data. Both should 

generally be spherically focused with central frequencies around 5MHz for one and at least 

15 MHz for the other. Assuming a maximum 0.5” defect depth below all six sides of the 

defect cube block, all transducers must have at least a 2” true focal length (see Section 9.1 

for definition) for good focusing. The size and focal length combination of the transducer 

should be carefully selected so that the beam entry area will not be larger than the 1” square 

surface of the defect cube block. The typical transducer set used consisted of a 5MHz, 1” 

diameter, 8” true focal length transducer and a 25 MHz, 0.25” diameter, 2” true focal length 

transducer. 

In order to determine the key transducer parameters such as the true focal lengths, geometric 

focal lengths and equivalent diameters (Section 9.1), all transducers must also be 

characterized. The transducer characterization task may be performed at ISU before a 

procedure is specified and tested at other sites in the future. 

2.4 Reference Set 

Minimally one (two recommended) fused-quartz plate is needed for the above-mentioned 

transducers to take reference waveforms (a-scans). These reference waveforms will be used 

by UT models to estimate the “system efficiency factor” of the scan system (Section 3.1). 

The plate thickness should be 1” (used with 5MHz transducer) and 0.5” (used with 15 MHz 

transducer and above) for the primary and the second optional plates, respectively. The size 

of the plate(s), normally 2”-4” in diameter (or per side), should fit with the service area of the 

sample stand. Like the defect cube block, the front and back surfaces of the plate(s) should 

also be parallel. 

3. Experimental Setups 

3.1 Basic Setups 

One of the objectives of acquiring UT data with this procedure is to support UT modeling 

activities. The UT models used in ETC are based on ISU’s Thompson-Gray measurement 

model framework1

. In order to predict real defect signals at absolute levels as seen on the 

oscilloscope, these UT models require a separate reference experiment from which the scan 

system response or “system efficiency factor” can be estimated. Thus, the data acquisition 

actually consists of two parts: the main task is to acquire the defect data from the cube block, 

and the other is to obtain the reference waveforms from, as mentioned in Section 2.4 above, 

the fused-quartz set(s). The setup of these two experiments, both in immersion mode, are 

illustrated below. 

In Figure 3.1, a focused transducer (specified in Section 2.3) is normalized (Section 3.4) to 

the front surface of a fused-quartz plate of thickness Yr,s. Given the UT speeds in water and 

fused-quartz (Sections 9.2 and 9.3), plate thickness, and the transducer focal length, the 

waterpath is set (see Section 9.4 for formula) to Yr,w such that the geometric focal length 

B-4
(section 9.1) is on the back surface of the fused-quartz plate. Echoes from the back surface, 

i.e. the reference waveforms (a-scans), are to be acquired in this experiment. Note that 

additional spacers need to be placed between the fused-quartz plate and the stand, which 

ensures water to be the backing medium. This is important for UT models to set up correct 

interface parameters without ambiguity. 

In Figure 3.2, the same transducer is similarly normalized to the front surface of the defect 

cube block. However, a different waterpath Yd,w is used so that true focal length is on the 

nearest (front) surface of the defect, for which the defect depths Yd,s in all six sides should 

have been measured (Section 1). Prior to transducer normalization, the stand needs to be 

leveled with respect to the paths of the scan bridge (Section 3.2). It is also assumed that both 

front and back surfaces of the defect block are in parallel to the leveled stand surface 

(Section 1). In this experiment, single-shot a-scans, two-dimensional c-scans, and threedimensional b-scans (see Section 9.1 for definition) will be acquired. 

fused

quartz

focused

transducer

stand

Figure 3.1 Reference Experiment

Yr,w

defect

cube

block

focused

transducer

stand

Figure 3.2 Defect Experiment

Yr,s

Yd,s

Yd,w

It should be pointed out that all hardware connections (such as the co-axial cables and 

transducer search tube) and settings (such as damping, pulse rate, energy level on 

pulser/receiver unit) must be kept the same for both the reference and defect experiments. 

The only adjustable setting is the gain (or attenuation on Panametrics models) of the 

pulser/receiver unit. This is to ensure the accuracy of the system efficiency factor as stated 

above. 

In order to warrant the best possible data obtained in an optimal condition of a given scan 

system, extra care should also be taken on the specimen placement issues. Specimen placement 

involves the leveling of the specimen and orienting the specimen with respect to the scan system 

coordinates. These are the subjects of the following subsections. 

B-5
3.2 Specimen Leveling 

For a typical linear scan, the rule of thumb is to maintain a constant distance between the beam 

entry surface and the transducer face. This constant distance can in turn be related to the planar 

path that the scan bridges move about, assuming there is no bending of the scan bridges (see 

Section 4.2). This ensures the alignment of the specimen and the scan bridges to avoid the 

fluctuations of beam focusing and signal time-of-flight. This is referred as “specimen leveling 

with constant water path”, and the surface to be leveled is often the front surface of the 

specimen. A common technique of leveling a front surface is to perform a lengthy line scan on 

that surface. During the scanning, the amount of change in time-of-flight of an echo from the 

front surface, as seen on the oscilloscope, determines the degree of tilting of the surface. One 

can then adjust, for instance, the screws of the sample stand to correct the tilt. However, given 

the relatively large size of the transducer (1” diameter for the 5MHz transducer) and small 

dimension of the defect block (~1” cube), this technique does not work well in this case. The 

solution here is to level the surface of the sample stand instead. The leveling of the sample stand 

ensures the same for the front and back surfaces of the defect cube block, since they are parallel 

to each other as was required (Section 1). For the reference fused-quartz plate, the leveling is 

not as crucial because only single-shot a-scans are acquired. 

3.3 Specimen Orientation 

Based on past experience with the Sonix system, issues related to specimen orientation have 

frequently been the source of error and confusion. Often the c-scan images were rotationally 

disoriented or mirror-inverted from the actual specimen layout. The problem arises largely from 

the inconsistency between the specimen orientation (in the water tank) and the Sonix scan plan. 

To avoid such difficulty, it is recommended that the Sonix scan plan be set up to match exactly 

with the actual specimen orientation. As an example, Figure 3.3a illustrates a hypothetical 

specimen orientation and the matching Sonix c-scan line paths (see Section 9.5 for detailed 

scan/step direction settings under FlexSCAN-C’s Scan Setup menu). If the “bottom-up” image 

convention (with origin at lower-left corner) is followed, then the orientation of the c-scan 

image, as depicted in Figure 3.3b, should match correctly with that of the specimen. It is also 

recommended to keep a drawing of the specimen orientation with respect to the scan plan and a 

copy of the details of the scan plan settings for future reference. 

B-6
defect

scan lines

Step direction

defect

origin origin

Figure 3.3.a specimen and scan orientations Figure 3.3b resulting c-scan image

3.4 Transducer Normalization 

In addition to specimen leveling, the normalization of the transducer is also an important but 

often difficult task. By “normalization” we mean to orient the transducer with respect to the 

beam entry surface in such a way that the sound beam enters the specimen with minimum energy 

loss and achieves the optimal focusing. For spherically focused transducers, a popular “ring 

wave” method of normalization has proven reliable. The "ring wave" method takes advantage of 

the time-of-flight difference between the outer transducer “ring wave” and inner main beam. As 

depicted in Figure 3.4b, when the transducer is very close to a flat surface, the central main 

component of the beam takes a longer time to travel back and forth than the outer “ring” 

components do. This results in a separation of the time domain front surface echo into a small 

leading “ring” signal and a large trailing main signal as shown in Figure 3.4b. If the transducer 

is well fabricated, the “ring” signal will reach its maximum when the transducer is perpendicular 

to the surface such that all ring components travel using the same amount of time. This 

phenomenon thus provides a practical way to “normalize” a focused transducer of axial 

symmetry. The degree of time separation between the ring and main components depends on the 

size, focal length and frequency of the transducer. For the 5MHz, 1” diameter, spherically 

focused transducer with focal length 8” at about 0.5” above a flat surface, the time separation is 

substantial. For a higher frequency transducer with shorter focal length and smaller diameter, 

the separation becomes less clear. In this case, the inseparable main and ring signals should be 

viewed as a whole and maximized in the process of transducer normalization. 

B-7
focused transducer

ring 

waves ring 

waves

main 

beam

main 

beam ring 

waves

time

Figure 3.4a “ring-wave” setup Figure 3.4b corresponding a-scan 

4. System Calibration 

In addition to an optimal experimental setup, a thorough system calibration process is the key to 

data accuracy and should be conducted BEFORE any data acquisition. In this procedure, there 

are three elements in the system calibration process: pulser/receiver, mechanical scan bridges, 

and oscilloscope. It is also strongly recommended to follow all calibration procedures described 

in FlexSCAN-C user’s guide.

Pulser/Receiver 

Important electronic parts like the pulser/receiver are often overlooked in the calibration process. 

While the accuracy of other aspects of the pulser/receiver matters, the single most critical 

function of the pulser/receiver for this procedure is the gain (or attenuation) setting, the only 

adjustable electronic parameters in the system (recall Section 3.1). Extra care should be taken in 

operating this setting. For example, the receiver of Panametrics 5052 is rated maximum output 

voltage of +0.5 volt into 50 Ohms. Any signal that has peak-to-peak amplitude exceeding 1 volt 

is subjected to distortion and loss of data accuracy. Thus, it is vital to maintain a linear 

undistorted operation of the gain (attenuation) setting in the pulser/receiver unit and acquire 

measurements in this linear range. A simple test of the linearity is to measure the peak-to-peak 

amplitude of a stationary signal (within the maximum output limit) with a sequence of gain 

settings that lie in the expected range of operation. The amplitude change in measurement 

should be linearly proportional to that in the gain settings. Proper remedy needs to be made if 

significant deviation from the linearity is observed. 

Mechanical Scan Bridges 

Typical problems with mechanical scan parts are scan bridge bending due to aging and scan 

“backlash” effects. Severe scan bridge bending may cause data loss and/or incorrect data 

registration. For example, heavy weight over time can cause the center of a scan bridge to 

buckle downward, which leads to a shorter waterpath in the central portion of scan. For a fixed 

narrow time gate in which UT data are digitized, a portion of the target UT signal may then shift 

out in this central scan path depending on the amount of bending. The misalignment problem 

B-8
may also arise when the beam entry surface of specimen has variable curvature. Because of 

these shortcomings, at most times it is good idea to turn ON the “front surface follower” feature 

in Gate Control window of FlexSCAN-C. 

The “backlash” effect frequently originates from excessive “looseness” in the lead screws of the 

mechanical scan bridge. This looseness can cause misalignment problem in data when scanned 

in bi-directional mode (i.e. taking data in both scan and return pass of a scan line). One way to 

determine the amount of backlash is to scan an object of well-defined shape and examine the 

degree of distortion in the c-scan image, especially the outlines of the object image. In 

FlexSCAN-C, the backlash effect may be compensated by software operation (Scan -> Backlash 

Correction). In any event, it is recommended to scan only in unidirectional mode (scan -> Scan 

Setup -> Bidirectional OFF) to reduce this effect.

Oscilloscope 

The Sonix digital oscilloscope comes with default settings, as seen in the A/D Offset of A/D 

8100 window, for the vertical baseline position of signal trace. Normally, these default settings 

are sufficient to keep the trace baseline in the middle (zero) of the scope display. In situations 

that the signal baseline deviates substantially from this zero position, the A/D Offset setting must 

be adjusted accordingly. 

Another problem with the Sonix digital scope is a constant time trace offset as seen in some 

systems using an external pulser/receiver unit. This is possibly due to inconsistent timing 

between the trigger signal and the initial transmitted pulse in the combination of pulser/receiver 

and Sonix A/D card. This offset, typically causing 1-2 s longer in a time-of-flight reading, 

should be pre-determined and compensated as much as possible. One way to determine this time 

trace offset is to take a time-of-flight measurement from a specimen of known thickness, e.g. the 

1” fused-quartz reference plate. This can be done as follows: 

First proceed with the procedures of leveling the sample stand, transducer normalization with 

respect to sample stand, and determining the water wave speed as described in Sections 3.2, 3.4 

and 9.2. Next, place the fused-quartz plate on the sample stand, manually lower the transducer 

to be in contact with the front surface of fused-quartz plate, and then carefully remove the fusedquartz plate from below. In this way, an exact 1” waterpath is readily set for a time-of-flight 

measurement of the front surface echo from the sample stand surface. Follow Section 9.4 to 

calculate the time-of-flight and compare that with the Sonix scope reading. The difference is the 

offset needed to be compensated in all future time-of-flight measurements. Note that this time 

trace offset depends on system loading (the combination of pulser/receiver, transducer, cables, 

etc.) and must be re-calibrated if any system component and/or major setting changes. 

5. Data Format 

Three types of UT data are acquired in this procedure: a-scan, multiple b-scan, and c-scans. 

By default, FlexSCAN-C generates a UT scan data file using the traditional DOS naming 

system: XXXXXXXX.YYY where X’s are the user supplied file name (8 characters 

maximum) and Y’s are the three-character file extension assigned by FlexSCAN-C. 

B-9
Files with CS1 extension are binary c-scans stored either in Sonix proprietary format or TIFF 

format. The former format can be read by any image processing software supporting RAW 

format. The actual data follow the first 9276-bytes header. Note that the image origin is at the 

lower left corner stored in row major. The TIFF format is one of the popular public-domain file 

formats and should be supported by most image processing software. From the programming 

point of view, the Sonix proprietary format is the preferred format because it is easier to work 

with. 

Files with RF extension are multiple-line b-scans stored in a Sonix proprietary format, which is a 

special version of multi-page TIFF format. Files of this type can be accessed by FlexSCAN-C 

directly. Some image processing software also support this format. 

Files with TXT extension are numeral “screen dump” of a-scan traces and can be opened by any 

text file editor. It contains a short header followed by waveform data points in 256-level integer, 

separated by commas. It is always a good practice to maintain a concise file naming convention 

and provide such convention along with the corresponding data structure in a “readme” file for 

future reference and use. 

6. Reference Waveform Acquisition 

To ensure best data quality and minimal errors, it is advised to follow the steps below when 

acquiring the reference waveform. The transducer should have been mounted on the angular 

manipulator, which is in turn attached to the search tube of the scan bridge. 

The following steps need only to be done once for each new fused-quartz plate: 

• Measure and record the dimensions and weight of the fused-quartz plate. Deduce the 

density accordingly and check with the tabulated generic value (Section 9.3) if 

applicable. Take necessary action if the difference is significant. 

• Measure and record the UT speeds of the fused-quartz plate and check with the 

tabulated generic values (Section 9.3) if applicable. Take necessary action if the 

difference is significant. 

The following steps need to be done for each transducer loaded with the system at the 

beginning of the scans: 

• Determine the time trace offset (Section 4.3). 

• Measure and record the UT speed in water (Section 9.2). 

• Measure and record water temperature within 1-degree precision. 

Steps below are to be done for both low-frequency (5 MHz) and high-frequency transducers 

with matching fused-quartz plate: 

• Measure and record the UT speed in water (Section 9.2) when water 

quality/temperature changes significantly during the scans. Be sure to update the 

new water speed in FlexSCAN-C (under Edit->Configuration->couplant velocity). 

B-10
• Measure and record water temperature within 1-degree precision when temperature 

changes significantly during the scans. 

• Normalize transducer with respect to front surface using "ring wave" method 

(Section 3.4). 

• Calculate the time-of-flight prediction of the front surface echo so that the 

geometric focus is set on the back surface (Section 9.2). Use the pre-determined 

plate thickness in the calculation. 

• Manipulate FlexSCAN-C’s digital scope to locate the front surface echo. Adjust 

waterpath until front surface echo appears at the predicted time instance as 

calculated in the last step. Be sure to account for the time trace offset if any 

(Sections 3.1 and 4.3, and Figure 3.1). 

• Manipulate FlexSCAN-C’s digital scope to display the back surface echo. 

• Measure the time-of-flight of the back surface echo. Check measurements against 

the predictions. Make correction if necessary. 

• Adjust both pulser/receiver’s gain setting (or attenuation setting on Panametrics 

505x series) and the A/D Gain setting in FlexSCAN-C’s A/D 8100 window so that 

the peak back surface waveform is about at 80% full screen height. Also set the 

time scale of FlexSCAN-C’s digital scope so that time resolution is sufficient (at 

least 20 time trace data points across the signal duration). 

• Check and adjust the baseline offset (A/D offset in A/D 8100 window) if necessary 

(Section 4.3). 

• Record all Sonix system settings. This can be done by properly arranging all 

FlexSCAN-C’s setting windows on the screen and then print screen from File 

menu. 

• Record all other experimental settings. 

• Take single-shot a-scan waveform of back surface echo. The time average feature 

should be turned ON to reduce noise. Use highest average number available. Ascan waveform data is obtained by entering File -> Export ... -> Scope to ASCII. 

Record peak-to-peak amplitude of the waveform. 

7. Defect Data Acquisition 

To ensure best data quality and minimal errors, it is advised to follow the steps below when 

acquiring the defect data. The defect depth in each side of the defect cube block should have 

been determined previously. The transducer should have been mounted on the angular 

manipulator, which is in turn attached to the search tube of the scan bridge. 

The following steps need only to be done once for each new defect cube block: 

• Measure and record the UT speeds of the defect cube block and check with the 

tabulated generic values (Section 9.3) if applicable. Take necessary action if the 

difference is significant. 

• Measure and record the dimensions and weight of the defect cube block. Deduce the 

density accordingly and check with the tabulated generic value (Section 9.3) if 

applicable. Take necessary action if the difference is significant. 

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The following steps need to be done for each transducer loaded with the system at the 

beginning of the scans. This step can be skipped if it has been done in reference data 

acquisition. 

• Measure and record the UT speed in water (Section 9.2). 

• Measure and record water temperature within 1-degree precision. 

• Determine the time trace offset (Section 4.3). 

• Go to Edit -> Configuration -> Image Format to set c-scan format to Sonix 

proprietary format. 

The steps below are to be done for both low-frequency (5 MHz) and high-frequency 

transducers. For each transducer, repeat the applicable steps on every accessible side of the 

cube block: 

• Measure and record the UT speed in water (Section 9.2) when water 

quality/temperature changes significantly during the scans. Be sure to update the 

new water speed in FlexSCAN-C (under Edit->Configuration->couplant velocity). 

• Measure and record water temperature within 1-degree precision when temperature 

changes significantly during the scans. 

• Level the sample stand and set the defect cube block on the stand (Section 3.2). 

• Maintain a consistent and precise orientation of the defect cube block with respect 

to the scan system coordinates (Section 3.3). 

• Normalize transducer with respect to front surface of the defect cube block using 

"ring wave" method (Section 3.4). 

• Calculate the time-of-flight prediction of the front surface echo so that the true 

focus is set on the nearest defect surface (Section 9.4). Calculate the prediction for 

the time-of-flight of defect signal from the pre-determined defect depth. Also 

calculate the prediction for the time-of-flight of back surface echo from the predetermined cube dimensions. 

• Manipulate FlexSCAN-C’s digital scope to locate the front surface echo. Adjust 

waterpath until front surface echo appears at the predicted time instance. Be sure to 

account for the time trace offset if any (Section 4.3). 

• Manipulate FlexSCAN-C’s digital scope to locate the defect signal and back 

surface echo. 

• Measure the time-of-flight of the defect signal and back surface echo. Check 

measurements against the predictions. Make correction if necessary. 

• Adjust both pulser/receiver’s gain (or attenuation on Panametrics 5052x series) 

setting and the A/D Gain setting in FlexSCAN-C’s A/D 8100 window so that the 

maximum defect waveform is about at 80% full screen height (Section 9.5). 

• Check and adjust the baseline offset (A/D offset in A/D 8100 window) if necessary 

(Section 4.3). 

• Set proper time gate location and width for defect signal in FlexSCAN-C’s Gate 

Control window. Make sure the sampling rate is at least 100 MHz (Section 9.5). 

Also turn on Front Follow gate (Sections 4.2 and 9.5). 

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• Record all Sonix system settings. This can be done by properly arranging all 

FlexSCAN-C’s setting windows on the screen and then print screen from File 

menu. 

• Record the cube block orientation drawing and all other experimental settings. 

• Take single-shot a-scan waveform of defect at peak position with time average 

feature ON. This is done by entering File -> Export ... -> Scope to ASCII. Record 

peak-to-peak amplitude of the defect waveform. 

• Turn OFF time average feature (time averaging requires time-consuming multiple 

data acquisitions at the same scan which is not possible in high-speed scans such as 

c- or b-scans. The scan speed will be greatly compromised if this feature is not 

turned off). 

• Go to Scan -> Scan Setup. Set all necessary parameters for c-scan. Make sure the 

scan and step increments are 5 mils for 5 MHz scan, and 2 mils for high-frequency 

scans, respectively (see example in Section 9.5). 

• Go to Scan -> Scan Go to dry run c-scan data acquisition. 

• Make changes such as scan sizes (pixels) in Scan Setup if necessary. 

• Repeat last three steps until data quality criterion is met. 

• Follow the last four steps to acquire b-scan data (see example in Section 9.5). 

8. Key Parameter Calculations/Settings 

In this section, the methods of calculation for all key parameters used in this procedure are given. 

In support of these calculations and clarification of the procedure contents, some key terms are 

first described. Examples of Sonix scan settings are also attached for future reference. 

8.1 Key Terminology 

Time-of-flight (TOF) in UT NDE is the round-trip time that ultrasound travels in a medium 

(or media) between transducer face and a point of interest, or between two points of interest. 

The horizontal time axis as seen on oscilloscope measures the time-of-flight of signals. 

Waterpath is the distance between the transducer face and the front surface of the specimen 

in an immersion test. 

Geometric and true focal lengths and equivalent diameter are the most important transducer 

parameters for a spherically focused transducer. Geometric focal length is the axial length 

(measured from the transducer face) as determined by geometric optics with respect to a given 

lens curvature. True focal length is the one most transducer specifications refer to, indicating the 

actual focal depth. Usually true focal length is shorter than geometric focal length. Equivalent 

diameter is the transducer size (diameter) produced by transducer characterization procedure, not 

necessarily the same as the manufacturer’s specification. 

Multiple-line B-scan takes an a-scan (waveform) at each scan position in a two-dimensional cscan path. The results are three-dimensional a-scan data covering the volume being scanned. 

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8.2 UT speed in water 

A simple measurement of the longitudinal-wave speed in water can be done by using two 

reference signals from a flat surface. In most cases, the front surface echoes are conveniently 

used as the reference signals. First normalize the transducer with respect to the flat surface 

(Section 3.4). Measure the TOF, T1, of a reference point on the front surface echo, say, at the 

peak position. Secondly, move the transducer up or down a known distance, D, perpendicular to 

the flat surface. In a Sonix immersion system, this can be easily done by moving the motorized 

vertical scan bridge on which the transducer search tube attaches. The distance moved can be 

monitored in the Motor window in FlexSCAN-C. Next, measure the TOF, T2, of the same peak 

reference point on the front surface echo at the new position. Finally, by simple physics of 

motion, 

TT 12

2D speed water − =

where | | means the absolute difference. For example, let D=2 inch=5.08cm, T1=30 μs 

(microsecond; 1 μs =10-6 second) and T2=98.65 μs then water speed = 0.148 cm/μs (0.0583 

in/μs) which is typical for tap water at 70°

F. This water speed should also match with the 

“couplant velocity” in FlexSCAN-C (accessed from Edit menu -> Configuration -> Couplant 

Velocity). 

8.3 UT speeds in defect cube block/fused-quartz plate 

Longitudinal-wave speed in cube block/fused-quartz plate can be measured similar to that in 

water. The difference is that the back surface echo taken from the defect cube block or fusedquartz plate (without moving the transducer up or down) replaces the second front surface echo 

in the case of water. For example, let the TOF of front surface be 20μs and TOF of back surface 

echo be 28.2 μs. From the above formula, the longitudinal-wave speed in a 1” Ti-6Al-4V cube 

block is 0.62 cm/μs. Shear-wave speed, however, requires the use of a shear-wave transducer 

measured in contact mode. One can take the first and the second bounces of the back surface 

echoes as the reference signals. If no shear-wave transducer is available, the values in Table 8.1 

may be used. 

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Table 8.1 UT and material properties of several frequently used materials in ETC 

Material Longitudinalwave speed 

(cm/μs) 

Shear-wave speed 

(cm/μs) 

Density 

(g/cm3

) 

Water 0.148 NA 1.00 

Ti-64 0.620 0.319 4.43 

IN718 0.610 0.326 7.86 

Fused-quartz 0.597 0.309 2.20 

8.4 Waterpath 

Given transducer focal lengths, wave-speeds in water and in the specimen, the correct 

waterpath and the corresponding TOF can be easily computed. Let the geometric and true 

focal lengths be Fg and Ft, respectively. Also denote longitudinal wave-speeds in water, 

fused-quartz and defect cube block as Vw, Vf, and Vs. For the reference experiment, the 

following formula applies: 

sr, w

f

wr,g Y V

V YF +=

For the defect experiment, the parameters change slightly in the same formula: 

sd, w

s wd,t Y V

V YF +=

where the subscripted Y’s are the waterpaths and specimen thickness/depths described in 

Section 3.1 and Figures 3.1 and 3.2. The corresponding TOFr for the back surface echo 

(measured from beginning of the time trace) is the sum of TOF for front surface echo (also 

from beginning) and TOF for the back surface echo (measured after the front surface echo) in 

the reference experiment: 

s

sr,

w

wr,

r wr, sr, V

2Y

V

2Y

 TOF TOF TOF +=+=

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Likewise, TOFd for the defect in the defect experiment is 

d,w d,s

d d,w d,s

w s

2Y 2Y

TOF TOF TOF . V V = + =+

8.5 Examples of Sonix FlexSCAN-C scan settings 

Figures 8.1 through 8.5 are the actual scan settings used in the first Ni real defect. Most 

settings should be suitable for future use except those specifics on transducer characteristics 

and defect size. Figures 8.1 and 8.2 are examples of the SCAN SETUP menu in Sonix 

FlexSCAN-C settings for low-frequency (5 MHz) and high-frequency c-scans, respectively. 

Figure 8.3 shows the SCAN SETUP menu for high frequency b-scan. Figures 8.4 and 8.5 are 

examples of screen snap shot of Sonix FlexSCAN-C showing the various settings for lowfrequency (5 MHz) and high-frequency scans. 

9. Further Contact 

Questions/corrections/comments are welcome and should be forwarded to 

C. Thomas Chiou (515)294-0299 cchiou@cnde.iastate.edu

Mike Keller (513)552-4643 mike_keller@geae.com

Jeff Umbach (561)796-6047 umbachj@pwfl.com 

Andrei Degtyar (860)565-3987 degtyaa@pweh.com 

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Figure 8.1 The SCAN SETUP menu in Sonix FlexSCAN-C for low-frequency (5 MHz) c-scan settings 
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Figure 8.2 The SCAN SETUP menu in Sonix FlexSCAN-C for high frequency c-scan settings 
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Figure 8.3 The SCAN SETUP menu in Sonix FlexSCAN-C for high frequency b-scan settings 
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Figure 8.4 A screen snap shot of Sonix FlexSCAN-C shows the various settings for low-frequency (5 MHz) scan 
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Figure 8.5 A screen snap shot of Sonix FlexSCAN-C shows the various settings for high-frequency scan
 

1

 Gray, T. A., “Application of Measurement Models to Specification of Ultrasonic Inspections”, in Review of 

Progress in QNDE, Vol. 16B, edited by D. O. Thompson and D. E. Chimenti, Plenum, New York, 1997, pp. 

2061-2068. 

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