DOT/FAA/AR-05/31 

Office of Aviation Research 

Washington, D.C. 20591

Feasibility Study of ThermalElastographic Detection of 

Nonvoided Hard Alpha Inclusions 

in Titanium Alloys 

September 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
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 Technical Report Documentation Page

1. Report No.

DOT/FAA/AR-05/31

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

5. Report Date 

September 2005

 4. Title and Subtitle 

FEASIBILITY STUDY OF THERMAL-ELASTOGRAPHIC DETECTION OF 

NONVOIDED HARD ALPHA INCLUSIONS IN TITANIUM ALLOYS 6. Performing Organization Code

7. Author(s)

Waled Hassan*, Jim Hall*, Fred Vensel*, Peter Nagy**, and Kevin Kramer** 

8. Performing Organization Report No. 

10. Work Unit No. (TRAIS) 

 

9. Performing Organization Name and Address

*Honeywell Engines, Systems & Services 

 Phoenix, Arizona 

**University of Cincinnati 

 Cincinnati, Ohio

11. Contract or Grant No.

 DTFA03-020C-00045 

13. Type of Report and Period Covered

 Final Report

12. Sponsoring Agency Name and Address

U.S. Department of Transportation 

Federal Aviation Administration

Office of Aviation Research 

Washington, DC 20591

14. Sponsoring Agency Code

 

 ANE-110

15. Supplementary Notes 

The FAA William J. Hughes Technical Center Technical Monitors were Paul Swindell and Cu Nguyen. 16. Abstract 

Hard alpha or high interstitial defects (HID), often and erroneously referred to as low-density inclusions (type I) in titanium

alloys, have been the focus of attention off and on since the application of titanium to rotating components in gas turbine 

aeroengines. There have been a number of failures due to hard HIDs that were not voided in critical engine rotating components; 

they escaped the quality assurance net of testing centered on some variant of ultrasonic inspection with focus on detection of 

internal interfaces such as voids, or microcracks. The objective of this research was to investigate the feasibility of a new 

ultrasonic nondestructive test method aimed at detecting hard alpha inclusions in titanium billets. The method is based on the

basic principle of elastography, a recently developed imaging technique that can detect both soft and hard inclusions in a highly 

heterogeneous medium inclusions, which are highly inhomogeneous themselves. According to the proposed modified 

elastographic approach, heating the specimen produces the local strain. The observed shift in the grain speckle is, therefore, a 

combination of the increasing propagation distance caused by thermal expansion and the decreasing sound velocity caused by the 

temperature dependence of the sound velocity. In this case, the relevant physical contrast mechanism is mainly thermal rather 

than solely elastic. Based on the measurements conducted during the research, it was established that the acoustothermal 

coefficient is the dominating factor and is an order of magnitude higher than the effect of the acoustoelastic and thermal 

expansion coefficients. Although the feasibility of such a technique was established based on the property measurements in 

titanium nitride specimens (synthetic hard alpha) and using simulation tools, the lack of appropriate naturally occurring, unvoided 

hard alpha inclusion specimens rendered a complete laboratory demonstration impossible under the program. Should such 

specimens become available, it would be straightforward to verify feasibility in future efforts. 

17. Key Words 

Hard alpha inclusions, Nonvoided hard alpha inclusions, Ultrasonic 

testing, Elastography, Thermal elastography, Billet inspection, 

Thermal expansion coefficient, Acoustothermal coefficient, 

Acoustoelastic coefficient, Longitudinal velocity, Shear velocity 

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 

 60 

22. Price

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

Page 

EXECUTIVE SUMMARY ix 

1. INTRODUCTION 1 

1.1 Purpose 2 

1.2 Background 2 

1.3 Technical Approach 4 

2. SPECIMENS 8 

2.1 Synthetic TiN Material 8 

2.2 Synthetic Inclusions (TiN) Embedded in Ti-6Al-4V Block 10 

2.3 Naturally Occurring Voided Hard Alpha Inclusions 11 

3. EXPERIMENTAL SETUP FOR MEASUREMENT OF THE THREE 

NONLINEAR PARAMETERS 17 

3.1 Thermal Expansion Coefficient Measurements 17 

3.2 Acoustoelastic Coefficient Measurements 18 

3.3 Acoustothermal Coefficient Measurements 20 

4. RESULTS FROM THE EXPERIMENTAL MEASUREMENTS OF THE 

THREE NONLINEAR PARAMETERS 21 

4.1 Thermal Expansion Coefficient 21 

4.2 Acoustoelastic Coefficient 24 

4.3 Acoustothermal Coefficient 28 

5. RELATIVE CONTRIBUTIONS TO THE OVERALL THERMAL EFFECT 32 

6. VELOCITY MEASUREMENTS 33 

7. NUMERICAL SIMULATION FOR PRELIMINARY FEASIBLITY 

ASSESSEMENT 34 

8. FURTHER ANALYSIS OF DEFECT X 39 

8.1 Eddy-Current and Acoustic Microscopy Imaging 39 

8.2 Metallurgical and Microprobe Analysis 41 

9. SUMMARY 46 

10. RECOMMENDATIONS 49

11. REFERENCES 50 

iii
LIST OF FIGURES 

Figure Page 

1 Forging and Billet Inspectability Comparison 3 

2 Micrograph of a Nonvoided HID 4 

3 Synthetic TiN Specimens as Received and Before Cutting 8 

4 Synthetic TiN Specimens After Cutting Into 0.22- by 0.38- by 1.0-inch 

Parallelepiped Specimens 9 

5 Billet and Forging Ti-6Al-4V Specimens After Cutting Into 0.22- by 0.38- by 

1.0-inch Parallelepiped Specimens 9 

6 Ti-6Al-4V Specimen With Three Cylindrical Synthetic TiN Inclusions 10 

7 Peak Reflected Amplitude C-Scan Images of the Ti-6Al-4V With Synthetic 

Cylindrical Inclusions With the UT Transducer Focused at (a) the Diffusion Bond 

Line and (b) the Tip of the Cylinders With Axes Perpendicular to the Diffusion 

Bond Line 11 

8 Locations of Defects X and D in the Billet B2W2C 12 

9 Two Blocks Containing Defects X and D 13 

10 C-Scan Images of Defect X 13 

11 Time of Flight Data for Defect X 14 

12 C-Scan Images of Defect D 14 

13 Time of Flight Data for Defect D 15 

14 C-Scan Images of the Block Containing Defect X 15 

15 Time of Flight Data for the Block Containing Defect X 16 

16 C-Scan Images of the Block Containing Defect D 16 

17 Time of Flight Data for the Block Containing Defect D 17 

18 Schematic Diagram of the TEC Measurements 18 

19 Experimental Setup for TEC Measurements 18 

20 Schematic Diagram of the AEC Measurements 19 

iv
21 Experimental Setup for AEC Measurements 19 

22 Schematic Diagram of the ATC Measurements 20 

23 Experimental Setup for ATC Measurements 20 

24 Percent Linear Change vs Temperature for Titanium With Varying Levels of 

Nitrogen Content 23 

25 The ACE vs Temperature for Titanium With Varying Levels of Nitrogen Content 23 

26 Relative Velocity Change vs Stress in Al-2024 Using a Longitudinal Transducer 26 

27 Relative Velocity Change vs Stress in Al-2024 Using a Shear Transducer 26 

28 Relative Velocity Change vs Stress in Ti-6Al-4V Using a Longitudinal 

Transducer 27 

29 Relative Velocity Change vs Stress in Ti-6Al-4V Using a Shear Transducer 27 

30 Apparent Velocity Change vs Temperature in Titanium With 3.1% N Using a 

Longitudinal Transducer (Acceptable Data) 29 

31 Apparent Velocity Change vs Temperature in Titanium With 3.1% N Using a 

Longitudinal Transducer (Unacceptable Data) 30 

32 Distribution of the Coefficient of Determination (R2) vs the Longitudinal ATC 

(β) 31 

33 Representation of the Longitudinal ATC Data Listed in Table 3 31 

34 Longitudinal Velocity in Titanium With Varying Levels of N Content 34 

35 Shear Velocity in Titanium With Varying Levels of N Content 34 

36 A Numerical Simulation Model Setup 35 

37 Simulation Results for a Cigar-Shaped Inclusion 15 mm Long and 4 mm Wide 36 

38 Analysis of the Time Shift in the Peak Due to Sound Speed Modulation in the 

Inclusion for the Scenarios Depicted in Figure 37 37 

39 The TOF and Reflected Peak Amplitude Scans for Defect X 37 

40 Simulation Results for a Cigar-Shaped Inclusion 15 mm Long and 1 mm Wide 38 

41 Slicing Process Performed on Defect X Perpendicular to its Axis to Produce 

Twenty 100-mil Slices With 15-mil Gaps 39 

v
42 Sample Results of the Eddy-Current Imaging of Some of the Slices From 

Inclusion X 40 

43 Sample Results of the Acoustic Microscopy Imaging of Some of the Slices From 

Inclusion X 41 

44 Microstructure for Slice 13 Showing the Nonuniform Thickness Along the Width 

of About 8 mm 42 

45 Higher Magnification View of Area Circled in Figure 44 42 

46 High Magnification, Backscattered Electron Image of Mottled Area Seen in 

Figure 45, Highlighting Areas Where Energy Dispersive X-Ray Analyses Were 

Made 43 

47 Nitrogen Composition Trace (From Microprobe Analysis) Across Defect of Slice 

13 Along Profile 1 44 

48 Microprobe Trace Also From Slice 13 Along Profile 2 Showing Much Higher N 

Content Than the First Trace 44 

49 Combined Traces of N and Al for Slice 13 Along Profile 2 45 

vi
LIST TABLES 

Table Page 

1 The PLC and ACE of Ti-6Al-4V Directional (Textured) Specimens 22 

2 The PLC and ACE of Titanium Specimens With Varying Levels of Nitrogen 23 

3 Measured Average ATCs Using a 5-MHz Longitudinal Transducer 29 

4 Measured Average ATCs Using a 5-MHz Shear Transducer 29 

5 Size of the Defect in Each Examined Slice Cross Section 41 

6 Qualitative Differences in Composition With Respect to the Base Alloy 

Composition 45 

vii
LIST OF ACRONYMS 

A/D Analog to digital 

ACE Average coefficient of expansion 

AEC Acoustoelastic coefficient 

Al Aluminum 

ATC Acoustothermal coefficient 

C Carbon 

CBS Contaminated Billet Study 

D/A Digital to analog 

DCE Differential coefficient of expansion 

EDX Energy dispersive x-ray analysis 

ETC Engine Titanium Consortium 

FAA Federal Aviation Administration 

FBH Flat-bottom hole 

GE GRC GE Global Research Center 

GE General Electric 

HID High interstitial defects 

HIP Hot isostatic press 

LVDT Linear variable differential transducer 

MTS Material test system 

MZI Multizone inspection 

N Nitrogen 

NDE Nondestructive evaluation 

NDT Nondestructive test 

Nsec Nanoseconds 

O Oxygen 

OEM Original equipment manufacturer 

PLC Percent linear change 

SNR Signal-to-noise ratio 

SRS Stanford Research Systems, Inc. 

TEC Thermal expansion coefficient 

TIC Time interval counter 

TiN titanium nitride 

TOF Time of Flight 

TRMD Turbine Rotor Material Design 

UT Ultrasonic 

VAR Vacuum arc remelting 

WPAFB Wright-Patterson Air Force Base 

viii
EXECUTIVE SUMMARY 

Hard alpha or high interstitial defects (HID) often, and erroneously, referred to as low-density 

inclusions (type I) in titanium alloys, have been the focus of attention off and on since the 

application of titanium to rotating components in gas turbine aeroengines. There have been 

failures of components and, unfortunately, some loss of life associated with the presence of such 

defects and a concomitant-reduced fatigue life. The early characterization of this class of defects 

resulted in identifying a few key characteristics that are responsible for the reduced fatigue life of 

titanium components that contain them. Chief amongst these characteristics is a higher hardness 

than the nominal alloy due to interstitial (nitrogen, carbon, and oxygen) alloying at levels much 

higher than the nominal alloy. Besides being hard and brittle, these nuggets of high interstitial 

stabilization are refractory, relative to the melting temperature for the host titanium alloy, and are 

slow to dissolve. This refractory nature, combined with the size of the initial seed, may permit 

the nugget to survive, with its hardness characteristics, all the vacuum arc remelting (VAR) 

process steps normally used to create a titanium ingot. This realization resulted in the institution 

of triple VAR to replace the standard double VAR for premium-grade applications, resulting in a 

significant reduction in occurrence of hard alpha defects. 

Recently, ultrasonic technology for the detection of internal discontinuities, such as voids and 

microcracks, has made significant improvements. However, there have been a small number of 

recognized failures due to HIDs that were not voided in critical engine rotating components. 

Obviously, they had escaped the quality assurance net of testing centered on some variant of 

ultrasonic inspection with focus on detection of internal interfaces such as voids, or microcracks; 

HIDs must be recognized as the most insidious of the hard alpha defects. 

The objective of this research was to investigate the feasibility of a new ultrasonic nondestructive 

test method aimed at detecting hard alpha inclusions in titanium billets. The method was based 

on the basic principle of elastography, a recently developed imaging technique that can detect 

both soft and hard inclusions in a highly heterogeneous medium, inclusions that are highly 

inhomogeneous themselves. According to the proposed modified elastographic approach, 

heating the specimen produces the local strain. The observed shift in the grain speckle is then a 

combination of the increasing propagation distance caused by thermal expansion and the 

decreasing sound velocity caused by the temperature dependence of the sound velocity. In this 

case, the relevant physical contrast mechanism is mainly thermal rather than solely elastic. 

Based on the measurements conducted during the research using different samples, it was 

established that the acoustothermal coefficient was the dominating factor and was an order of 

magnitude higher than the effect of the acoustoelastic and thermal expansion coefficients. 

Although the feasibility of this inspection technique was established based on the properties 

measurements in titanium nitride specimens (synthetic hard alpha) and using simulation tools, 

the lack of appropriate naturally occurring, unvoided hard alpha inclusion specimens rendered a 

complete laboratory demonstration impossible under the program. Should such specimens 

become available, it would be straightforward to verify feasibility in future efforts. 

ix/x
1. INTRODUCTION.

Hard alpha or high interstitial defects (HID), often and erroneously referred to as low-density 

inclusions (type I) in titanium alloys, have been the focus of attention off and on since the 

application of titanium to rotating components in gas turbine aeroengines. There have been 

failures of components and, unfortunately, some loss of life associated with the presence of such 

defects and a concomitant-reduced fatigue life [1-3]. 

Early characterization of this class of defects resulted in identifying of a few key characteristics 

that were responsible for reduced fatigue life of titanium components that contain them [4-6]. 

Chief amongst these characteristics was a higher hardness than the nominal alloy due to 

interstitial (nitrogen (N), carbon (C), and oxygen (O)) alloying at levels much higher than the 

nominal alloy. Hence, the descriptive name, high interstitial defect. Besides being hard and 

brittle, these nuggets of high interstitial stabilization were refractory, relative to the melting 

temperature for the host titanium alloy, and are slow to dissolve. This refractory nature, 

combined with the size of the initial seed, may permit the nugget to survive, with its hardness 

characteristics, all the vacuum arc remelting (VAR) process steps normally used to create a 

titanium ingot. This realization resulted in the institution of triple VAR to replace the standard 

double VAR for premium-grade applications, resulting in a significant reduction in occurrence of 

hard alpha defects. 

Most of the finds of HIDs were associated with internal voids or cracks that facilitated their 

detection by ultrasonic (UT) test methods. This led to the refinement of UT techniques in 

attempts to detect smaller and smaller voids or internal flaws associated with most HIDs. The 

industry responded with improved UT technology, including multizone inspection (MZI), and 

with improved titanium manufacturing and quality controls to reduce, significantly, the 

occurrence of hard alpha defects [7]. While the incidence of HIDs has become quite small, it 

remains finite. Furthermore, the probability of detection, or more to the point, the probability of 

missing a finite hard alpha defect in the combination of billet and forging inspections is still high 

enough to cause concern. 

The detection technology for finding (not missing) defects with associated voids or cracks has 

improved and has been the focus of a great deal of attention in recent years. Currently, there is 

redundant UT inspection in rotor material. First, there is the billet inspection followed by an 

inspection of the finished forging that has been made oversized and machined to a shape (sonic 

shape) that lends itself to maximum inspectability. In the period of time that UT technology for 

the detection of internal discontinuities, such as voids and microcracks has made significant 

improvements, there have been a small number of recognized failures due to hard alphas (HIDs) 

that were not voided. Obviously, they had escaped the quality assurance net of testing centered 

on some variant of UT inspection with focus on detection of internal interfaces such as voids or 

microcracks, which must be recognized as the most insidious of the hard alpha defects. 

This report outlines the results of a feasibility study that was aimed at investigating the 

possibility of using a rather new approach for the detection of nonvoided, hard alpha inclusions 

in titanium alloys. The technique is based on the basic principle of elastography, a recently 

developed imaging technique that can detect both soft and hard inclusions in a highly 

1
heterogeneous medium, which are highly inhomogeneous themselves. This will lead to means of 

improved detectability and characterization of such defects using UT technology. 

1.1 PURPOSE.

The objective of this research was to investigate the feasibility of a new UT nondestructive test 

(NDT) method aimed at detecting nonvoided hard alpha inclusions in titanium billets. The 

method is based on the basic principle of elastography, a recently developed imaging technique 

that can detect both soft and hard inclusions in a highly heterogeneous medium, which are highly 

inhomogeneous themselves. 

1.2 BACKGROUND.

The quality level in the manufacture of rotor-grade titanium materials for propulsion turbine 

engine applications has significantly improved over the past decade and a half, partially due to 

the efforts sponsored by the Federal Aviation Administration (FAA) following the Sioux City, 

Iowa, crash. While the implementation of strict foundry controls and improved processing 

techniques such as cold hearth and triple melting have significantly reduced the rate of defects 

present in titanium products, there is still room for error in these processes, which may manifest 

itself in terms of latent defects in the volume of the final product. The engine manufacturers 

guard against these latent defects with a redundant set of UT inspections, one applied to the billet 

and another applied to each forging manufactured from the billet. 

There remains a disagreement in the engine community on the best practices for the 

implementation of billet and forging inspections on rotor-grade titanium. Some believe that a 

screen of the billet to a #2 to #5 flat-bottom hole (FBH) sensitivity is adequate provided the 

inspection on the final forging is more stringent in order to safeguard against such latent defects 

that may have been missed in the billet. Others in the industry believe that latent defects, which 

remain undetected in the billet, may be stretched or pushed into areas within the forging, which 

may result in a lower probability of detection. 

A joint Engine Titanium Consortium (ETC) and turbine rotor material design (TRMD) effort 

investigated the impact of processing naturally occurring and synthetic defects on UT inspection 

detectability in the recent past [8]. For the efforts involving naturally occurring defects, the team 

selected three billet segments from the Contaminated Billet Study (CBS) material, ultrasonically 

characterized the defects in the billet stage, subsequently forged the billet segments, and again, 

ultrasonically characterized the forging shapes. While the data was quite sparse and all the 

forging processing was done isothermally, the results of this exercise clearly demonstrated that 

the detectability of a given defect changes dramatically as a result of the forging processing. 

Figure 1 illustrates a forging and billet inspectability comparison. The solid line on the graph 

represents a 1:1 correlation in defect detectability. This figure was published by TRMD and 

includes data from billet, pancake forging, semifinished forgings, dogbone, and backflow 

forgings. It clearly indicates that nearly all of the HID defects are much more detectable in the 

billet than in the subsequent forging shapes. 

2
FIGURE 1. FORGING AND BILLET INSPECTABILITY COMPARISON 

Another CBS effort investigated the impact of healing a defect on the UT detectability [6]. This 

effort was undertaken for two reasons: (1) to investigate the feasibility of manufacturing a 

nonvoided hard alpha inclusion using this process and (2) to investigate the impact of the void 

closure on the UT inspection process. For this investigation, the team used a billet segment with 

an indication approximately equivalent to a #3 FBH, as detected by conventional billet 

inspection (100% amplitude with a signal-to-noise of 2.9), this billet segment was then hot 

isostatic press (HIP)ped at a subtransus temperature, time, and pressure consistent with industry 

convention for this alloy. The HIPped segment was then re-inspected to investigate the resultant 

impact. No indications were observed in the post-HIPped sample, demonstrating that the process 

effectively closed the voids and that detection of nonvoided hard alphas requires additional 

investigation. 

The importance of understanding and maintaining strict process control and the need for robust, 

highly accurate inspection techniques became evident on several occasions over the past couple 

of years. Nearly 6 years ago, a titanium melter received silica material extensively contaminated 

with pure tungsten wire pieces. These contaminates were discovered during the processing of 

rotor-grade Ti-6-2-4-2. The result of this quality escape was the contamination of 22 heats of 

rotor-grade material (approximately 190,000 lb or > 2,500 engine parts) allocated for engine 

builds. Fortunately, this material did not make it to engines, as some of the material was 

multizone inspected and all 22 heats of the associated material were downgraded. In excess of 

100 individual tungsten inclusions were discovered in the billet using MZI, a technique that is 

more sensitive than the conventional billet inspection. Furthermore, upon subsequent evaluation, 

3
it was discovered that none of these defects would be rejectable using industry standard, 

conventional billet UTs. Two questions immediately come to mind when formulating risk 

assessment for components manufactured from associated material: (1) Would have any of these 

defects (and any other defects that were missed by MZI) been machined away or detected in the 

forging? (2) If any of these defects made it to the final product in a critical location, would they 

be detrimental to component operation? 

More recently, another titanium melter reported a higher than typical rate of hard alpha finds in 

the manufacture of its rotor-grade titanium. Four hard alpha inclusions were discovered over a 

3-month period, which amounts to approximately a 6 times increase in the typical rate of hard 

alpha detection. This resulted in a cascading effect through the industry. When supplies are 

short, this places an additional strain on the availability of rotor-grade alloy. This occurrence, 

and the associated reactions by original equipment manufacturers (OEM), pointed out that while 

engine manufacturers have high confidence in the UT inspection technique, the presence of 

nonpreferentially oriented defects and nonvoided hard alpha inclusions make detection more 

difficult. In fact, there have been some failures associated with nonvoided hard alpha defects. 

Figure 2 is an actual photomicrograph of one such defect. It remained nondetected in both the 

billet and the forging inspection. Current technology allows these sort of defects to escape and 

here-in lies a most serious challenge to UT technology. There is a keen awareness of the 

potential consequences of a hard alpha defect (HID) escaping into a fielded part, therefore, very 

costly precautionary measures are taken. 

A nonvoided hard alpha defect that 

was responsible for a field failure 

FIGURE 2. MICROGRAPH OF A NONVOIDED HID 

1.3 TECHNICAL APPROACH.

Hard alpha inclusions are low-density, hard, brittle regions of spuriously high nitrogen and 

oxygen content that occur in titanium alloys. These inclusions are often cracked or include 

voids, which makes it much easier to detect them by conventional NDT methods, including UT 

backscattering. However, the detection of uncracked or unvoided hard alpha inclusions 

4
represents a very significant challenge for UT NDT. Although every weight percent of nitrogen 

was found to increase the sound velocity by roughly 3% [9], the absolute acoustic impedance 

contrast is expected to be substantial (10%-15%), the actual signal-to-noise ratio (SNR) is badly 

limited by three adverse effects: (1) the nitrogen content varies gradually towards the center of 

the inclusion; therefore, the reflected signal remains rather weak, (2) the shape of the inclusion is 

typically very irregular; therefore, the flaw signal is further reduced by random geometrical 

scattering effects, and (3) the competing grain noise is frequently very high because of the often 

coarse microstructure of the surrounding material. As a result, the backscattered UT signal from 

the unvoided hard alpha inclusion is usually indistinguishable from the incoherent material noise 

based on its strength alone and the probability of its detection remains inherently low. 

The objective of this program was to investigate the feasibility of a new UT NDT method aimed 

at detecting nonvoided hard alpha inclusions in titanium billets. As a working hypothesis, it was 

postulated that, in general, nonlinear elastic properties are more sensitive to the presence of 

interstitial (N, C, and O) elements that led to higher hardness than the corresponding linear 

elastic properties. Although this assumption can be supported by a great variety of observations 

in similar cases, there is no evidence at this point that the nonlinear contrast between a typical 

hard alpha inclusion with respect to the surrounding titanium alloy is actually higher than the 

better-known linear contrast. One of the primary goals of the program was to determine how 

different nonlinear material properties are affected by increasing nitrogen content. 

There are a great variety of nonlinear elastic parameters that could be exploited for 

nondestructive evaluation (NDE) of hard alpha inclusions. The three nonlinear parameters that 

were investigated in detail were chosen with special regard to the particular nonlinear NDE 

method to be used in this study, namely, thermally activated elastography. These nonlinear 

parameters include the thermal expansion coefficient (TEC), the thermal coefficient of sound 

velocity acoustothermal coefficient (ATC), and the stress coefficient of sound velocity 

acoustoelastic coefficient (AEC). Of course, these nonlinear parameters are mutually 

interrelated and are all-dependent on the higher-order elastic moduli of the material. 

One of the most obvious manifestations of elastic nonlinearity in solids is the existence of 

thermal expansion due to the nonparabolic nature of the atomic potential. Since the potential 

well is tilted outward, the nearest neighbors will separate from each other more at higher 

temperatures when they oscillate more around their equilibrium position. Another well-known 

manifestation of the same elastic nonlinearity is the effect of material deformation on the 

propagation velocity of acoustic waves. Regardless whether the strain is caused by stress (elastic 

strain) or temperature variation (thermal strain), the changing sound velocity is not only a 

sensitive indication but also a reliable, quantitative measure of material nonlinearity. All these 

parameters are related to the third- and higher-order elastic constants of the material and can be 

readily measured as functions of the fractional density of contaminating interstitial elements. It 

should be noted that other nonlinear phenomena, such as the generation of second and higher 

harmonics by finite amplitude acoustic waves propagating through the material, could also be 

readily used for studying the nonlinear contrast produced by variations in interstitial 

contamination, but the three parameters described above bear direct relevance to the NDE 

procedure of interest in this program. 

5
Three important basic tasks within the current feasibility study were aimed at experimentally 

determining the (1) TEC, (2) the AEC, and (3) the ATC of titanium alloy coupons of varying 

interstitial (N) content. These measurements were conducted in the NDE Laboratory of the 

University of Cincinnati and provided the basic information for subsequent determination of the 

dominating factor of the previously mentioned effects in the development of a special thermally 

activated elastographic inspection method for the detection of nonvoided hard alpha inclusions. 

All three parameters will play a role in the formation of a nonlinear contrast between the 

inclusion and the surrounding host material. Due to the presence of substantial differences in 

nonlinear material properties, when the material is heated to elevated temperatures, (1) the sound 

velocity in the inclusion changes with respect to the host, (2) the inclusion expands with respect 

to the host, and (3) it also develops thermal stresses that again affect the sound velocity. It was 

expected that these three nonlinear effects combine with each other to produce an even more 

substantial overall nonlinear contrast. In particular, the effective propagation delays between 

scattering grains increase not only because of thermal expansion, but also because the sound 

velocity decreases—partly because of the direct effect of the increased temperature and partly 

because of the development of thermal stresses. However, the actual measurements revealed that 

the ATC is the dominating effect and the other two factors, for all practical purposes, can be 

ignored as will be detailed in a later section. 

To effectively conduct this program, the need for samples and specimens that represent a 

naturally occurring unvoided hard alpha could not be overemphasized. The following represents 

a summary of the samples that were made available to the program: 

• Synthetic material of titanium nitride (TiN) with various nitrogen content which is the 

widely accepted simulation of hard alpha inclusions from an UT properties point of view. 

These specimens were used for properties measurements (obtained from the NDE branch 

at Wright-Patterson Air Force Base (WPAFB) and made by GE Global Research Center 

(GE GRC)). 

• Synthetic inclusions (TiN) with various nitrogen content embedded in Ti-6Al-4V block 

for inspection validation (obtained from the NDE Branch WPAFB and made GE GRC). 

These specimens were tested to evaluate how representative they are to a naturally 

occurring hard alpha inclusion in Ti-6Al-4V. 

• Naturally occurring, voided hard alpha inclusions from the CBS billet material (obtained 

from the ETC). These specimens were HIPped to heal and evaluated to assess how 

representative they would be to a naturally occurring, nonvoided hard alpha inclusion in 

Ti-6Al-4V. These specimens were intended for validating the proposed thermalelastographic technique. 

More details on these specimens will be presented. 

The main deliverable result of this feasibility study was the above described experimental study 

of the nonlinear contrast mechanism between a hard alpha inclusion and a titanium alloy host. 

However, a series of preliminary experiments were also proposed to illustrate the feasibility of 

one particular nonlinear inspection method that has the potential of being sufficiently sensitive to 

detect the nonlinear contrast of nonvoided hard alpha inclusions. The method was based on the 

6
basic principle of elastography, a recently developed imaging technique that can detect both soft 

and hard inclusions in a highly heterogeneous medium, which are highly inhomogeneous 

themselves. Under these conditions, conventional sonography, based on the strength of UT 

backscattering alone, cannot be used to detect the inclusion with the required high level of 

confidence. In relatively compliant soft tissue, sufficient local displacements can be produced by 

a quasi-static external stress field so that local strains and, thereby, the local elastic stiffness, can 

be estimated from differential UT speckle displacements. The processed strain image is called 

an elastogram that has been shown to offer dramatically better detectability of scattering 

inclusions than the conventional sonogram itself. This technique has been primarily aimed at 

biomedical diagnostic applications where the elastic deformation is both substantial and easy to 

produce. 

A similar elastographic approach, based on external deformation of large pieces of structural 

metals like a Ti-6Al-4V billet, would require tremendous external forces and would still result in 

minuscule contrast because, as indicated by the mere 10%-15% variation in sound velocity, the 

elastic stiffness of the brittle inclusion was only about 20%-30% higher than the surrounding 

intact material. According to the proposed modified elastographic approach, the local strain was 

produced by heating the specimen. The observed shift in the grain speckle was, therefore, a 

combination of the increasing propagation distance caused by thermal expansion and the 

decreasing sound velocity caused by the temperature dependence of the sound velocity (these 

two effects are actually related through the elastic nonlinearity of the material). Although the 

absolute change in propagation time due to the thermal strain and the temperature dependence of 

the sound velocity is probably comparable to what one could achieve by external mechanical 

loading as a result of the corresponding elastic strain and the stress dependence of the sound 

velocity, the thermal loading method is inherently simpler. More importantly, the relevant 

physical contrast mechanism is thermal rather than elastic. This contrast mechanism has never 

been investigated before in the scientific literature. However, indirect evidence available in the 

published literature as well as these measurements suggest that the variation in thermal velocity 

coefficient are substantially, probably as much as one order of magnitude, stronger than in the 

corresponding elastic coefficients. This is not surprising based on the fact that the nitrogen and 

oxygen contamination induced hardening and embrittlement is a nonlinear rather than linear 

effect on the mechanical properties of the material. In summary, the proposed technical 

approach included: 

• acquiring Ti-6Al-4V specimens containing nonvoided hard alpha inclusions (for 

technique validation) and TiN specimens with various nitrogen content (for hard alpha 

properties measurements). 

• characterizing the three nonlinear coefficients that most likely will play a role in this 

phenomenon, namely, TEC, ATC, and AEC. 

• experimentally evaluating the relative effect of heating on the deformation of hard alpha 

inclusions and titanium alloys. 

• developing the algorithm needed to extract the thermal-elastogram from the 

corresponding sonogram and setup experiments for this purpose. 

7
• demonstrating the technique on the acquired Ti-6Al-4V specimens with hard alpha 

inclusions. 

• preparing and reporting the results of the feasibility study. 

The following sections detail the results, limitations, and conclusions based on the activities that 

occurred under this program. 

2. SPECIMENS. 

As mentioned in section 1.3, three different types of samples and specimens were obtained to 

facilitate the performance of the program. The following is a detailed description of these 

samples. 

2.1 SYNTHETIC TiN MATERIAL. 

These specimens were obtained from the NDE branch at WPAFB and were originally made by 

GE GRC. The TiN samples were prepared by three techniques: (1) HIP consolidation of 

blended titanium and TiN powders, (2) arc melting of titanium sponge in an N-containing 

atmosphere, and (3) arc melting of sponge of titanium plus TiN powder. Each technique 

produced TiN alloys containing certain amounts of oxygen carried with the constituent raw 

materials. Reference 9 contains more details on the processes used to manufacture these 

specimens. Figure 3 shows the specimens as they were received and before any further 

processing. When first manufactured, these specimens were cut into nominally 3-inch-diameter 

disks. These TiN synthetic specimens were considered as a good simulation of a hard alpha 

inclusion from a properties point of view and were intended to be used for measuring the three 

nonlinear coefficients of interest in this program, namely, TEC, ATC, and AEC. The specimens 

were cut into uniform parallelepiped specimens of dimensions 0.22 by 0.38 by 1.0 inch and had 

2.5%, 3.1%, 3.8%, 4.8%, and 5.9% nitrogen percentage by weight. Figure 4 shows these 

specimens after cutting, along with the proper percent nitrogen content by weight. Note the 

evident porosity of the specimens at higher levels of nitrogen content (4.8% and 5.9%). 

FIGURE 3. SYNTHETIC TiN SPECIMENS AS RECEIVED AND BEFORE CUTTING 

8
FIGURE 4. SYNTHETIC TiN SPECIMENS AFTER CUTTING INTO 0.22- BY 0.38- BY 

1.0-inch PARALLELEPIPED SPECIMENS 

In addition to the above specimens and to allow comparative measurements of the three 

nonlinear coefficient of the synthetic TiN specimens to those measured on the host materials 

Ti-6Al-4V for contrast assessment, similar 0.22- by 0.38- by 1.0-inch parallelepiped specimens 

were prepared from billet and forging Ti-6Al-4V specimens. These specimens are shown in 

figure 5. 

FIGURE 5. BILLET AND FORGING Ti-6Al-4V SPECIMENS AFTER CUTTING INTO 

0.22- BY 0.38- BY 1.0-inch PARALLELEPIPED SPECIMENS 

9
2.2 SYNTHETIC INCLUSIONS (TiN) EMBEDDED IN Ti-6Al-4V BLOCK. 

A Ti-6Al-4V specimen with three synthetic cylindrical inclusions, manufactured using one of the 

methods indicated in section 2.1, was obtained from the NDE branch at WPAFB in Dayton, OH. 

This block was originally manufactured by GE GRC by cutting out TiN cylindrical pieces from 

disks similar to those shown in figure 3. Two blocks of Ti-6Al-4V with approximate dimensions 

of 1.0 by 2.0 by 3.0 inches and 0.5 by 2.0 by 3.0 inches were used. In the two blocks, three 

different but matching cylindrical cavities of the same size as the cylindrical inclusions were 

machined. The inclusions were inserted in them in such a way that two of the cylindrical 

inclusions were inserted with their axes perpendicular to the surface of the block, and the third 

was inserted with its axis parallel to the surface. Note that the inclusions were inserted in such a 

way that half of the inclusion lies in each of the two blocks. The second block was then placed 

on top of the first one, and they were diffusion-bonded together. Figure 6 shows the final 

specimen, along with the overall dimensions. 

FIGURE 6. Ti-6Al-4V SPECIMEN WITH THREE CYLINDRICAL 

SYNTHETIC TiN INCLUSIONS 

This block was intended for use in the technique validation experiments with the assumption that 

the inserted inclusions are representative of the condition of an unvoided hard alpha inclusion in 

Ti-6Al-4V. After the initial inspection of the block, using backscattered UT inspection, it 

became very evident that this artificial inclusion specimen was not representative of the case of 

unvoided hard alpha inclusions in titanium alloys. Figure 7 shows the peak reflected amplitude 

C-scan images of the Ti-6Al-4V specimen with synthetic cylindrical inclusions for the cases of 

the UT transducer focused at the (a) the diffusion bond line and (b) at the tip of the cylinders 

with axes perpendicular to the diffusion bond line. The measurements were done using a Sonix 

Acoustic Microscope Quantum 350 with a 15-MHz, 0.5-inch-diameter Panametrics V319 

(serial number 326433) spherically focused UT transducer. It is very clear from the figure that 

the inclusions are very much detectable. As a matter of fact, the response from the inclusions 

perpendicular to the bond line saturated the instrumentation. This clearly shows that this 

specimen was not representative of the case of unvoided hard alpha inclusion in titanium alloys, 

as such defects are extremely difficult to detect using backscattered UT inspection. 

10
 

(b) (a) 

FIGURE 7. PEAK REFLECTED AMPLITUDE C-SCAN IMAGES OF THE Ti-6Al-4V WITH 

SYNTHETIC CYLINDRICAL INCLUSIONS WITH THE UT TRANSDUCER FOCUSED AT 

(a) THE DIFFUSION BOND LINE AND (b) THE TIP OF THE CYLINDERS WITH AXES 

PERPENDICULAR TO THE DIFFUSION BOND LINE 

Several factors can account for the very strong response received from these inclusions. First, 

the interface between the inclusions and the titanium host materials is sharp in terms of the 

change in nitrogen content, which is not the case for a naturally occurring hard alpha inclusion. 

Second, the porosity associated with such inclusions (specially the ones with the high nitrogen 

content) may contribute to a higher that expected UT signal. And finally, the questionable 

diffusion interface between the inclusion and the host material may result in such a high signal if 

the inclusion is not completely bonded to the host material. For all these reasons, this specimen 

was deemed to be not representative of naturally occurring hard alpha inclusions in titanium 

alloys, and therefore, was not used in any further activities in this program. 

2.3 NATURALLY OCCURRING VOIDED HARD ALPHA INCLUSIONS. 

In February 1995, the FAA-funded programs, the Engine Titanium Consortium and Turbine 

Rotor Materials Design program, were made aware of the existence of a contaminated heat of 

Ti-6Al-4V. The contaminated heat, number 883588-02, occurred as the result of loss of power 

during melting. Following completion of the double VAR processing, the ingot was split in two 

and converted to the final billet diameter of 6 inches. Note that this is not the standard practice 

for rotor-grade materials, which are typically either triple VAR or cold hearth melting/VAR 

processed and rotary forged from ingot to final billet geometry. However, the original customer 

did not intend to use the material for aerospace applications, hence, the difference in processing. 

The supplier indicated that this heat was formulated from 72% revert, 28% sponge, and was 

chamber welded. The primary ingot was piggy-back melted; the first electrode was melted 

uneventfully. Partway into the second melt, a total melt shop power outage occurred. The 

power outage shut down the vacuum pumps, and a pressure spike occurred before the furnace 

was back filled with argon. After power returned, melting was resumed to complete the primary 

ingot. The primary ingot was inverted and remelted to complete the processing. The initial 

inspection of the heat was completed using conventional 5-MHz transducers in two wave modes, 

longitudinal and refracted shear, which led to the detection of multiple indications. Based on the 

UT results, the heat was dispositioned for further commercial use. Further inspection of the heat 

using an MZI system revealed even more indications that were not detected by conventional 

inspection. To ensure that the defect types were typical of hard alpha defects found in 

11
rotor-quality materials, an assessment was made with input from the ETC and TRMD teams 

along with representatives from the metallurgy staff at the OEMs. Based on the chemical 

analysis and metallographic sectioning of the initial defects, it was determined that these defects 

were representative and, therefore, appropriate for further study. To ensure the appropriateness 

of the material for further study in that program, chemical and metallographic analyses were 

performed on two of the indications. The bulk composition consisted of C-0.02, N-0.014, 

Fe-0.18, Al-5.56, V-4.10, O-0.18, and Y-<50 parts per million, within the range of typical 

Ti-6Al-4V. The purpose of the study was to characterize the inclusion as a typical hard alpha 

inclusion, compared to inclusions previously removed from billet products or from forged 

components. As reported by Bogan [5], the inclusion was a hard, brittle, cracked, and voided 

hard alpha, containing up to 12.6% nitrogen and surrounded by diffusion zone, and could be 

considered typical of those encountered in billet and forged products. Microprobe data showed 

that the highest nitrogen areas contained approximately 1.5%-2.0% vanadium with 

approximately 0.1%-0.9% aluminum (Al). For more details on the CBS, see reference 6. 

For the purposes of this program, several billet pieces having about 15 indications left over from 

the CBS were obtained from the ETC team. A number of these indications were verified again 

in these billets using the MZI procedure, and their locations in the respective billets were 

pinpointed. In particular, a 25.75-inch-long billet section (B2W2C) containing two indications, 

X and D, was extensively studied. After precisely locating the two defects in the billet, they 

were cut out from the billet material to form two blocks with dimensions of 2 by 2 by 2.82 inches 

and 2 by 2.73 by 2.82 inches, respectively. Figure 8 illustrates the location of the two indications 

in the original billet. Figure 9 shows the two blocks, each containing one indication after they 

were cut out from the billet. 

0.55′′

FIGURE 8. LOCATIONS OF DEFECTS X AND D IN BILLET B2W2C 

25.75′′

11.00′′

8.84′′

Back

Defect X 

MZ: 94% 

5.05 SNR 

6′′

Front

Billet B2W2C 

0.6′′

Defect D 

MZ: 134% 

5.46 SNR 

6′′

25.75′′

8.84′′

6.00′′

Front

Billet B2W2C 

Top

Left

Top

Back Left

12
FIGURE 9. TWO BLOCKS CONTAINING DEFECTS X AND D 

To verify that the defects are still in the blocks, the two blocks were ultrasonically inspected 

using a 5.5-MHz, 0.75-inch-diameter, spherically focused transducer with a 6-inch focal length. 

A 6-inch water path was used so that the transducer could be focused on the front wall of the 

block facing the transducer. Figure 10 shows C-scan images of defect X as obtained from 

characterizing the defect in the block from six different directions. The defect dimensions are 

estimated from the C-scans to be 0.625 inch in length, 0.230 inch in height, and 0.100 inch in 

thickness. Time-of-flight (TOF) measurements verifying the presence of the defect are shown in 

figure 11. Similar data is shown in figures 12 and 13 for defect D. In this case, the inclusion is 

estimated to be 1.19 inches in length, 0.270 inch in height, and 0.100 inch in thickness, based on 

the C-scan images. It is worth mentioning here that the UT response obtained from these two 

inclusions is largely attributed to the voids associated with the inclusions and the acoustic 

impedance contrast between these voids and the titanium host material. 

FIGURE 10. C-SCAN IMAGES OF DEFECT X 

13
FIGURE 11. TIME OF FLIGHT DATA FOR DEFECT X 

FIGURE 12. C-SCAN IMAGES OF DEFECT D 

14
FIGURE 13. TIME OF FLIGHT DATA FOR DEFECT D 

These two defects were HIPped to heal at 1650oF and 30 ksi pressure for 5 hours with the 

intention of creating samples representative of unvoided hard alpha inclusions to be used in the 

technique validation experiments. After the HIPping process was completed, the two blocks 

were re-inspected using the same procedure described above and no indication of the two defects 

were observed in both the backscattered C-scan images or the TOF images. Figures 14 and 15 

show the results for defect X after HIPping. Figures 16 and 17 show similar results for defect D. 

FIGURE 14. C-SCAN IMAGES OF THE BLOCK CONTAINING DEFECT X 

15
FIGURE 15. TIME OF FLIGHT DATA FOR THE BLOCK CONTAINING DEFECT X 

FIGURE 16. C-SCAN IMAGES OF THE BLOCK CONTAINING DEFECT D 

16
FIGURE 17. TIME OF FLIGHT DATA FOR THE BLOCK CONTAINING DEFECT D 

3. EXPERIMENTAL SETUP FOR MEASUREMENT OF THE THREE NONLINEAR 

PARAMETERS. 

3.1 THERMAL EXPANSION COEFFICIENT MEASUREMENTS. 

A schematic diagram and the experimental setup of the thermal expansion measurement are 

shown in figures 18 and 19, respectively. The thermal expansion tests were done on an Orton 

Dilatometer Model 1600D. A dilatometer measures the percent linear change (PLC) in a 

specimen as it is heated by means of a linear variable differential transducer (LVDT). Put 

simply, the LVDT is an alternating current device with a magnet and two coils that can very 

accurately measure displacement. The thermal extension of the specimen can be measured since 

the LVDT is connected to a spring-loaded pushrod that rests against the specimen. The spring 

compresses as the specimen expands, and this displacement is monitored by the LVDT. A 

furnace encircles the specimen and can be heated up to 1600°C in this particular model. A 

reference specimen made of alumina and a reference file was provided with the dilatometer 

equipment. A calibration run with the alumina specimen was made initially and compared with 

the reference values given by Orton so that the accuracy of the dilatometer could be ensured. 

The calibration runs closely matched the reference plot provided by the vendor, but it was not 

exactly the same; therefore, it was corrected so that future runs could be more accurate. Once 

the calibration run was completed, a calibration file was created with the necessary corrections 

and was used for all subsequent runs with other materials. 

The software accompanying the dilatometer made the experimental measurements relatively 

easy. The program included a simple sheet with boxes clearly labeled to enter the specimen 

length, create a heating program, enter a file to save data, and specify which calibration file will 

be used. All runs started at room temperature and were heated at 3°C/minute to 500°C. 

Nominally, the specimen length was 1 inch. Once an experimental run was started, a graph 

showed the PLC versus temperature and the data was written to a user-created file. The 

information could then be imported into an spreadsheet for further analysis, namely, to calculate 

17
the differential coefficient of expansion (DCE) and the average coefficient of expansion (ACE), 

which is the value given in most textbooks. 

rolling

heat

transverse

thickness

FIGURE 18. SCHEMATIC DIAGRAM OF THE TEC MEASUREMENTS 

LVDT 

Push Rod

Thermocouple 

Dilatometer

Electronics 

Sample Holder

Sample 

Computer 

Furnace 

FIGURE 19. EXPERIMENTAL SETUP FOR TEC MEASUREMENTS 

3.2 ACOUSTOELASTIC COEFFICIENT MEASUREMENTS.

The goal of acoustoelastic measurements is to measure the relative velocity versus stress in a 

specimen. Figure 20 shows a schematic diagram of the direction of loading from the material 

test system (MTS) and the position of the transducer for longitudinal and shear measurements, 

with the experimental setup of the acoustoelastic measurement shown in figure 21. This was a 

very difficult measurement and required some sophisticated instrumentation, namely, a Stanford 

Research Systems, Inc. (SRS) Universal Time Interval Counter and an SRS Four-Channel 

Digital Delay/Pulse Generator, a Panametrics Pulser/Receiver, and a LeCroy Waverunner digital 

oscilloscope. The basic concept of this measurement is straightforward. First, a signal is created 

on the oscilloscope by means of the Pulser/Receiver and an appropriate transducer and couplant. 

For longitudinal measurements, a 0.25 inch-diameter, 5-MHz transducer was used with either 

grease or Vaseline as the couplant, and for shear measurements, a 0.25 inch-diameter, 5-MHz 

transducer was used with honey as the couplant. Then, with the Digital Delay/Pulse Generator, 

18
an arming gate is placed around the first two echoes from the specimen. This arm specifies the 

region in which the time interval counter (TIC) measures the time delay. For accurate time 

measurement, one must choose to measure the corresponding positive or negative zero crossings 

on the first and second back-wall echoes of the specimen. It is important to choose common 

points on each pulse to obtain an accurate reading. The MTS program, through input channel 1, 

reads the time delay from the digital-to-analog (D/A) output marked MEAN on the TIC 

instrument. The apparent velocity can be determined directly from the measured propagation 

time delay and the nominal thickness of the specimen. To determine the actual sound velocity, 

the thickness of the specimen must be corrected for changes due to the Poisson effect. 

load

longitudinal

shear parallel

FIGURE 20. SCHEMATIC DIAGRAM OF THE AEC MEASUREMENTS 

shear normal

wave direction

MEAN 

MTS 

CH 1 CH 2 CH 3 CH 4 TRIGGER A B EXT A B 

Oscilloscope 

Universal 

Time Interval 

Counter

Digital 

Delay/Pulse 

Generator

FIGURE 21. EXPERIMENTAL SETUP FOR AEC MEASUREMENTS 

Once a good signal was established, the specimen was placed in the MTS and set in place. This 

meant gripping both ends of the specimen while the transducer was clamped somewhere in the 

middle of the specimen. A simple program was created on the MTS software for the 

acoustoelastic measurements. This program dwells for 2 seconds, then cycles at 0.05 Hz for 30 

cycles at a stress level of 1/3 the nominal yield stress of the material. During the experiment, the 

axial force and the two-way propagation time delay from the TIC was plotted against time. 

Interface 

TO TRANSDUCER 

SYNC SIGNAL 

OUTPUT

Pulser/ 

Receiver Computer 

19
When the cycles were completed, the force was ramped to zero, and the program was complete. 

The complete data set, namely, a time stamp, the axial force from the MTS, and the two-way 

propagation time from the TIC, was written to a file, which could be imported into a spreadsheet 

for further analysis. 

3.3 ACOUSTOTHERMAL COEFFICIENT MEASUREMENTS.

The goal of acoustothermal measurements is to measure the relative velocity versus temperature 

in a specimen. A schematic diagram and the experimental setup of the acoustothermal 

measurement are shown in figures 22 and 23, respectively. This is also a very complicated 

measurement like the acoustoelastic measurement. As can be expected, the same equipment was 

used to measure the time delay in the specimen, which in turn was used to calculate the velocity. 

However, instead of stressing the specimen in the MTS, the specimen was placed in a water bath 

in a Pyrex bowl and subjected to temperature variations. Five-MHz longitudinal and shear 

transducers were used in the acoustothermal measurements. The transducer was placed on one 

side of the specimen and a bracket was placed on the other side with everything clamped 

together and placed in the water. The bracket is U-shaped and provides a gap on the underside 

of the specimen where the transducer signal acts. 

heat

longitudinal

shear

FIGURE 22. SCHEMATIC DIAGRAM OF THE ATC MEASUREMENTS 

wave direction

MEAN 

CH 4 A B A B A/D Board CH 1 CH 2 CH 3 TRIGGER EXT 

Oscilloscope 

Universal 

Time Interval 

Counter

Digital 

Delay/Pulse 

Generator

FIGURE 23. EXPERIMENTAL SETUP FOR ATC MEASUREMENTS 

TO TRANSDUCER 

SYNC SIGNAL 

OUTPUT

Pulser/ 

Receiver Computer 

20
An identical reference specimen with a type K thermocouple mounted on it was also placed in 

the bath to measure the temperature. For the longitudinal measurements, the glass dish was 

placed on a Dataplate Digital Hot Plate to be heated. The temperature range used in these 

experiments was from room temperature to 80°C. The water bath was allowed to cool back to 

room temperature after the initial heating was finished. The cooling part of the temperature 

profile was slower and more reproducible; therefore, the ATC was measured in that stage. The 

propagation time delay increased as the temperature increased and decreased as the temperature 

decreased. This was recorded by a Virtual Bench Data Logger that is part of the Labview 

routine. A connection was made to the D/A output marked MEAN on the TIC and passed to an 

analog-to-digital (A/D) board. The thermocouple was also connected via a preamplifier to the 

A/D board to be relayed to the computer and displayed and written to a file by the Data Logger. 

The Data Logger displayed the temperature of the reference specimen and the two-way 

propagation time delay versus time. The only difference with the shear measurements was in the 

method of heating and cooling. The longitudinal wave couplant used was grease, and although it 

probably lost some of its viscosity when heated, it did not affect the measurements. Shear wave 

couplants lose their needed high viscosity at higher temperatures, and therefore, the signal slowly 

decays on the oscilloscope. Eventually, the signal disappears completely, typically around 55°-

60°C. In addition to decreased viscosity, honey dissolves in water and cannot be used for the 

thermal measurements; therefore, a special Panametrics shear wave couplant was used in these 

experiments. Instead of heating on the hot plate, a Neslab chiller, which can heat and cool water, 

was used to extend the temperature range. The temperature was first increased from room 

temperature to 45°C, then cooled to 5°C. This range was broader by approximately 15°C than 

what would be achievable using the hot plate, since the hot plate would only allow the specimen 

to be heated to 45°C, then cooled to room temperature. 

The effect of temperature on the strength of the UT signal in the case of shear wave UT 

measuremets was further investigated in a study that used different couplants. In the following 

sections, the results from the above measurements are presented and discussed in detail. 

4. RESULTS FROM THE EXPERIMENTAL MEASUREMENTS OF THE THREE 

NONLINEAR PARAMETERS. 

4.1 THERMAL EXPANSION COEFFICIENT. 

Materials can be characterized by a TEC that measures how much the material expands over a 

certain temperature range. This parameter was mostly used to compare materials, referred to as 

ACE, but other parameters can be calculated as well, including the PLC and DCE. The ACE 

was of the most interest in the experiments conducted in this study. The PLC is the percentage 

that the specimen expanded from its original length, or 

0

0

100 T

l l PLC

l

− = × (1) 

where l0 is the original length of the specimen, and lT is the length of the specimen at the final 

temperature. The DCE is the amount the specimen expanded from its orginal length over a 20°

temperature range 

21
20 0 ( )

20

T T ll l DCE − − / = (2) 

where lT-20 is the length at a temperature that is 20° less than the final temperature. Finally, the 

ACE is the amount the specimen expanded from its original length over the entire temperature 

range of the experiment 

0 0

0

( )/ Tlll ACE

T T

− = − (3) 

where T is the final temperature, and T0 is the initial temperature. The experiments performed in 

this study compared pure titanium and Ti-6Al-4V titanium alloy with no nitrogen content to 

titanium with varying levels of nitrogen content from 2.5% to 5.9%. 

Several practice runs were carried out with the dilatometer using Ti-6Al-4V specimens. The 

objective was to obtain results similar to the accepted TEC for this common titanium alloy, 

namely, αTi-6Al-4V ≈ 8.6×10-6/°C to 9.7×10-6/°C between 20°C and 500°C [10]. An extensive 

study was done on textured Ti-6Al-4V and the results were comparable to the above values. Six 

specimens were machined from a large piece of forged Ti-6Al-4V. Two pieces were machined 

in each of the axial, transverse, and vertical directions, namely, the typical x, y, and z coordinate 

directions. Experimental runs were performed on these specimens, and the ACE for each 

orientation was calculated. The results listed in table 1 are comparable to the accepted expansion 

coefficient of Ti-6AL-4V, and it was determined that the dilatometer was giving accurate results. 

TABLE 1. THE PLC AND ACE OF Ti-6Al-4V 

DIRECTIONAL (TEXTURED) SPECIMENS 

At 500°C 

Axial 

Trial 1 

Axial 

Trial 2 

Transverse

Trial 1 

Transverse

Trial 2 

Vertical 

Trial 1 

Vertical 

Trial 2 

PLC (%) 0.439 0.449 0.453 0.435 0.450 0.412 

ACE (10-6/°C) 9.39 9.58 9.68 9.30 9.67 8.85 

The next step was to test the TiN specimens provided by MLLP/Air Force Research Laboratory. 

These were titanium specimens of varying nitrogen content, i.e., 2.5%, 3.1%, 3.8%, 4.8%, and 

5.9% nitrogen. Also provided by Honeywell Engines was a titanium billet specimen with no 

nitrogen content and canted and forged Ti-6Al-4V specimens. Four experimental runs were 

performed, and the averaged results are listed in table 1. 

Figures 24 and 25 show the PLC and ACE versus temperature curves of one of the four trials. 

Table 2 shows that the PLC and ACE both decrease with increasing levels of nitrogen. This is to 

be expected since nitrogen makes titanium more brittle and brittle materials do not expand as 

much as the more ductile material, such as pure titanium with no nitrogen content. However, as 

will be shown in a following section, the relative contribution of this observed dimensional 

change to the overall temperature effect is very small and practically negligible. 

22
0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

100 140 180 220 260 300 340 380 420 460 500

Temperature [°C]

PLC [%]...

Ti 2.5% N Ti 3.1% N

Ti 3.8% N Ti 4.8% N

Ti 5.9% N Ti Billet

Ti Canted Ti Forged

FIGURE 24. PERCENT LINEAR CHANGE VS TEMPERATURE FOR TITANIUM WITH 

VARYING LEVELS OF NITROGEN CONTENT 

0.0E+00

2.0E-06

4.0E-06

6.0E-06

8.0E-06

1.0E-05

1.2E-05

100 140 180 220 260 300 340 380 420 460 500

Temperature [°C]

ACE [in/in-°C .]

Ti 2.5% N Ti 3.1% N

Ti 3.8% N Ti 4.8% N

Ti 5.9% N Ti Billet

Ti Canted Ti Forged

FIGURE 25. THE ACE VS TEMPERATURE FOR TITANIUM WITH VARYING 

LEVELS OF NITROGEN CONTENT 

TABLE 2. THE PLC AND ACE OF TITANIUM SPECIMENS WITH VARYING 

LEVELS OF NITROGEN 

At 500°C 2.5% N 3.1% N 3.8% N 4.8% N 5.9% N Billet Canted Forged 

PLC (%) 0.386 0.381 0.365 0.331 0.321 0.439 0.438 0.439 

ACE (10-6/°C) 8.29 8.19 7.81 7.08 6.88 9.41 9.39 9.41 

23
4.2 ACOUSTOELASTIC COEFFICIENT.

The acoustoelastic measurements were the most difficult of the three measurements. Since the 

nitrogen-enriched specimens were very small and weakened by both embrittlement and 

microporosity, they could not be tested in the MTS machine for fear of breaking them. In 

addition, the grips of the 100-kN MTS machine could not be adapted to hold these small pieces 

in tension. Therefore, the specimens had to be tested in compression with a high risk of buckling 

failure. Also, since the variations in the acoustoelastic effect were expected to play a minimal 

role in the detectability of hard alpha inclusions, only baseline measurements were made in 

Ti-6Al-4V reference specimens without jeopardizing the small, embrittled TiN specimens. 

First, a piece of Al-2024 was tested since the AEC was previously measured for this material 

[11]. Then a rolled piece of Ti-6Al-4V was tested in the same manner. The goal was to gather 

some baseline data on the AECs and to estimate the order of magnitude of the maximum relative 

change in velocity that could be seen in the presence of thermal stresses. Both longitudinal and 

shear transducers were used in these measurements. Data acquisition was controlled by the MTS 

program. A shear transducer was also used at normal and parallel polarization, meaning the 

particle displacements were aligned normal and parallel to the direction of the unidirectional 

stress, respectively. The crucial part of the measurements was to measure the relative change of 

the instantaneous time delay by the TIC. The initial time delay was recorded before the MTS 

program was started, and then the Set Rel(ative) button on the TIC was pressed so that the 

measurements would be taken relative to this initial time delay. The MTS data acquisition 

system also recorded the voltages at the initial time delay and when it was zeroed at the start of 

the measurement for subsequent calibration of the analog output signal of the TIC. The time, 

applied force, and analog output from the TIC were written to a file as the program was 

executed. 

Also very important for the measurement was the scaling factor on the TIC. The scaling factor is 

a measure of how much the input signal to the computer, i.e., the analog output of the TIC, 

changes for a given change in time delay. The scaling factor had to be adjusted on a case-bycase basis to avoid saturating the output signal from the TIC. The available signal is from 0-8 V 

and the scaling factor was increased for shear measurements since the relative increase in the 

time delay was greater than in the longitudinal measurements. For Al-2024, the scaling factor 

was set to 5,000 for longitudinal measurements and 10,000 for shear measurements. For 

Ti-6Al-4V, the scaling factor was set to 2000 for longitudinal measurements and 5000 for shear 

measurements. 

Once the data was imported into Excel, the relative velocity was calculated. From the time 

delays that were recorded before the program started, relative change in the two-way propagation 

time Δt / t0 was calculated from 

0

0 0

t A(V V )

t t

Δ − = (4) 

where A [μs/V] is the measured slope associated with the scaling factor, V [V] is the analog 

output of the TIC recorded by the computer as the specimen is stressed and written to file, V0 [V] 

24
is the analog output of the TIC recorded once the Set Rel button was pressed, and t0 [μs] is the 

initial time delay recorded by the operator from the TIC. 

The measured slope value is dependent on the scaling factor. It was calculated by first obtaining 

a signal on the oscilloscope with a transducer using a generic specimen, then pressing the Set Rel 

button. The relative value on the TIC and the corresponding voltage on the MTS were then 

recorded several times. Next, the transducer was moved to a different spot on the specimen, 

where the thickness was, inevitably, slightly different, and the procedure was repeated. In this 

way, a well-controlled time delay could be simulated for calibration purposes. The averaged 

time delays at both transducer locations from the TIC were then subtracted. The same procedure 

was followed for the average values of the voltages recorded by the A/D converter. The results 

were finally divided to obtain a slope in μs/V. In this way; the entire data acquisition system 

could be accurately calibrated without calibrating each element of the system separately. 

The Poisson correction was then made to account for the changing cross-sectional area as the 

specimen was put in tension and compression. The lateral strain is given by 

0

d

d E

Δ σν = − (5) 

where σ is the applied stress, ν is Poisson’s ratio in the material, and E is the modulus of 

elasticity. For Al-2024, σ was varied from -15 to 15 ksi with a corresponding cross-sectional 

area of S = 0.5 in2, ν = 0.33, and E = 10,700 ksi. For Ti-6Al-4V, σ was varied from -30 to 30 ksi 

with a corresponding cross-sectional area of S = 0.203 in2, ν = 0.33, and E = 16,500 ksi. Without 

the Poisson correction, the slope of the relative velocity versus stress curve is larger than with the 

correction since some of the apparent velocity change is due to the Poisson effect. Using the 

correction yields a more accurate AEC. Finally, the relative velocity change is the Poisson 

correction minus Δt / t0, or 

0 0 AE

c

c E

Δ σν =− − t

t

Δ (6) 

Figures 26 and 27 show typical examples of the relative velocity versus stress in Al-2024 using a 

0.25-inch-diameter, 5-MHz longitudinal transducer and a 0.25-inch-diameter, 5-MHz shear 

transducer at both normal and parallel polarizations. The slope of the graph of the relative 

velocity versus stress is the AEC, K. The corresponding AECs, using a straight-line 

approximation, are 6.2×10-5, 6.4×10-5, and -15.3×10-5 ksi-1 for longitudinal and shear waves at 

normal and parallel polarizations, respectively. These values are in reasonable agreement with a 

previously published report [10]. 

25
-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

-20 -15 -10 -5 0 5 10 15 20

Axial Stress [ksi]

Relative Velocity Change [%]..

FIGURE 26. RELATIVE VELOCITY CHANGE VS STRESS IN Al-2024 USING A 

LONGITUDINAL TRANSDUCER 

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

-20 -15 -10 -5 0 5 10 15 20

Axial Stress [ksi]

Relative Velocity Change [%]..

parallel polarization

normal polarization

FIGURE 27. RELATIVE VELOCITY CHANGE VS STRESS IN Al-2024 USING A 

SHEAR TRANSDUCER 

Similarly, figures 28 and 29 show typical examples of the relative velocity versus stress in 

Ti-6Al-4V using a 0.25-inch-diameter, 5-MHz longitudinal transducer and a 0.25-inch-diameter, 

5-MHz shear transducer at normal and parallel polarizations. The corresponding AECs, using a 

straight-line approximation, are 1.4×10-5, 7×10-5, and -11.5×10-5 ksi-1 for longitudinal and shear 

waves at normal and parallel polarization, respectively. 

26
-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

-40 -30 -20 -10 0 10 20 30 40

Axial Stress [ksi]

Relative Velcity Change [%]..

FIGURE 28. RELATIVE VELOCITY CHANGE VS STRESS IN Ti-6Al-4V USING A 

LONGITUDINAL TRANSDUCER 

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

-40 -30 -20 -10 0 10 20 30 40

Axial Stress [ksi]

Relative Velocity Change [%]..

parallel polarization

normal polarization

FIGURE 29. RELATIVE VELOCITY CHANGE VS STRESS IN Ti-6Al-4V USING A 

SHEAR TRANSDUCER 

27
As can be seen, the AECs are very small because the relative velocity changes merely 1% at 

stress levels as high as the yield strength of the material. It would be possible to test the effect of 

nitrogen in titanium with either larger specimens or at lower levels of applied stress, but the 

evaluation of the data would be susceptible to large errors because of the small change in relative 

velocity. Using a larger stress range would be dangerous, too, because there would be a risk of 

breaking the specimens. 

4.3 ACOUSTOTHERMAL COEFFICIENT. 

The acoustothermal measurement was very similar to the acoustoelastic measurement in that the 

same equipment is used to measure the time delay. The calculation of the relative velocity was 

somewhat simpler than discussed above because, for simplicity, only the apparent velocity was 

analyzed without correcting for the changing propagation length due to thermal expansion, i.e., 

the apparent velocity change includes a small part due to changes in dimension. Once the data 

was imported into Excel from the Data Logger, the apparent relative velocity change was 

calculated from 

0 0

apparent c t A

c t

Δ

0

V

t

Δ Δ =− =− (7) 

where, as before, A [μs/V] is the measured slope associated with the scaling factor, ΔV [V] is 

the change in the analog output signal from the TIC, and t0 [μs] is the initial time delay 

recorded by the operator on the TIC. 

Among the specimens that were tested, titanium grade 2 and rolled Ti-6Al-4V were included. 

These specimens were only tested for a better comparison in addition to the specimens described 

previously. Each specimen had an identical counterpart to be used as the reference for 

temperature measurements so that electrical grounding problems could be avoided. The 

specimens provided by Honeywell Engines, namely, a forged Ti-6Al-4V specimen and a 

titanium billet specimen, as well as the 2.5% N, 3.1% N, and 3.8% N titanium specimens from 

AFRL, were also tested in the same way. The Ti-6Al-4V-canted specimen was used as the 

reference specimen when these specimens were tested. The goal was to obtain six acceptable 

velocity versus temperature plots for each specimen, using both longitudinal and shear wave 

transducers. To limit gross measurement errors, acceptable experimental data was defined as a 

curve with a linear regression of R2 = 0.95 or better. This acoustothermal test was a very 

complicated one, and it became excessively difficult with the smaller specimens. Many factors 

can easily hinder the experiment, such as the amount of couplant on the transducer, the water 

level in the bath, or even a degenerating (thermally aging) transducer. Therefore, the R2 criterion 

was imposed to accept only sufficiently linear velocity versus temperature curves. Tables 3 and 

4 list the averaged ATCs of the tested specimens and the corresponding standard deviations for 

longitudinal and shear waves, respectively. 

28
TABLE 3. MEASURED AVERAGED ATCs USING A 5-MHz LONGITUDINAL 

TRANSDUCER 

ATC (10-4/°C) 

Titanium Ti-6Al-4V Forged Billet 2.5% N 3.1% N 3.8% N 

Average -2.41 -2.35 -2.10 -1.92 -2.32 -1.86 -1.20 

Std. Dev. 0.10 0.28 0.19 0.47 0.25 0.42 0.16 

TABLE 4. MEASURED AVERAGED ATCs USING A 5-MHz SHEAR TRANSDUCER 

ATC (10-4/°C) 

Titanium Ti-6Al-4V Forged Billet 2.5% N 3.1% N 3.8% N 

Average -6.86 -8.94 -7.28 -7.65 -6.96 -8.01 -12.86 

Std. Dev. 0.44 0.61 0.28 0.33 0.57 0.48 0.48 

To illustrate the actual raw data used to obtain the results listed in tables 3 and 4, figures 30 and 

31 show two measured velocity versus temperature graphs for the 3.1% TiN specimen. 

Figure 30 shows an acceptable curve of R2 = 0.996, whereas figure 31 shows a curve of 

R2 = 0.919 that was determined to be unacceptable because of its excessive nonlinearity. 

y = -0.0226x + 0.7031

R2

 = 0.996

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0 25 50 75

Temperature [°C]

Apparent Velocity Change [%]

100

...

.

FIGURE 30. APPARENT VELOCITY CHANGE VS TEMPERATURE IN TITANIUM WITH 

3.1% N USING A LONGITUDINAL TRANSDUCER (ACCEPTABLE DATA) 

29
y = -0.0097x + 0.2096

R2

 = 0.9193

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0 25 50 75

Temperature [°C]

Apparent Velocity Change [%]

100

...

FIGURE 31. APPARENT VELOCITY CHANGE VS TEMPERATURE IN TITANIUM WITH 

3.1% N USING A LONGITUDINAL TRANSDUCER (UNACCEPTABLE DATA) 

Even without positively identifying the underlying physical mechanism or mechanisms 

responsible for the observed variation in the ATC, the reliability and accuracy of the results 

could be increased by eliminating the obvious artifacts of excessively low coefficient of 

determination (R2 < 0.95). Figure 32 shows the distribution of the coefficient of determination 

versus the longitudinal ATC (β) after filtering. Even the filtered data exhibits an obvious trend 

of decreasing coefficient of determination, i.e., lower level of confidence in the accuracy and 

reliability of the data, with decreasing magnitude of the ATC. Unfortunately, increasing the 

threshold R2 value used for filtering would eliminate most of the data on the difficult to measure 

TiN specimens of high nitrogen content; therefore, further experiments will be needed to 

increase the accuracy of these results. Figure 33 shows a graphical representation of the crucial 

data listed in table 3. The error bars indicate the full range of the measured values. It should be 

noted that the apparent decrease in variation shown for the 3.8% TiN specimen is due to the 

small number of successful runs (only three) rather than to good reproducibility. Actually, the 

fairly strong, implicit correlation illustrated in figure 32 (between the increasing nitrogen content 

and decreasing measurement) repeatability suggests that the dominating uncertainty is somehow 

related to the reduced consistency (microporosity, microcracks, inhomogeneous grain structure, 

etc.) of the TiN specimens. Therefore, additional measurements should be taken on the TiN 

specimens to further investigate the possibility of increasing the measurement accuracy on these 

specimens by better controlling the transducer and couplant parameters and by improving data 

acquisition. 

30
0.90

0.92

0.94

0.96

0.98

1.00

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

Longitudinal Acousto-thermal Coefficient [10-4/°C]

Coefficient of Determinatio

n. FIGURE 32. DISTRIBUTION OF THE COEFFICIENT OF DETERMINATION (R2) 

VS THE LONGITUDINAL ATC (β) 

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Titanium Ti-6Al-4V Forged Billet 2.5% N 3.1% N 3.8% N

Acousto-Thermal Coefficient [10 -4/°C] . FIGURE 33. REPRESENTATION OF THE LONGITUDINAL ATC DATA 

LISTED IN TABLE 3 

31
5. RELATIVE CONTRIBUTIONS TO THE OVERALL THERMAL EFFECT.

Three factors contribute to the changing propagation time when a specimen is heated, i.e., the 

geometrical change due to thermal expansion (α), the acoustoelastic contribution (K), and the 

acoustothermal contribution (β). The relative velocity change caused by each contribution can 

be expressed by the following formula 

0 0

c d

c dt0

Δ Δ Δt = − (8) 

where t is the time of arrival of the sound wave, d is the distance that the wave travels, and c is 

the speed of the sound. By means of a simple example, it can be determined which mechanism 

produces the dominating effect that should be examined further. A TiN specimen of 4%-6% 

nitrogen content and a temperature change of ΔT = 50°C is assumed in the following example. 

The dimensional change due to unrestrained thermal expansion can be expressed as 

0 TE

d T

d

Δ = α Δ (9) 

where the TECs of Ti-6Al-4V and Ti-3.8%N are known from the experiments done previously, 

αTi-6Al-4V ≈ 10×10-6/°C and αTiN ≈ 7×10-6/°C (with a slight overestimation of the difference to be 

conservative). Therefore, the expected maximum differential strain due to thermal expansion 

can be calculated as follows: 

Ti-6Al-4V TiN 4 max Ti-6Al-4V TiN

0

( )1. d d T

d

Δ −Δ − ε = ≈ α −α Δ = ×5 10 (10) 

The corresponding maximum thermal stress can be expressed as 

σ =ε ≈ max max E 2.5ksi (11) 

where E = 1.65×104 ksi is the known modulus of elasticity in Ti-6Al-4V [10]. The 

acoustoelastic contribution can be expressed as follows: 

0 AE

c K

c

Δ = σ (12) 

where K ≈ 6×10-5 ksi-1 denotes the magnitude of the average expected AEC of Ti-6Al-4V so 

that the estimated upper limit for the resulting velocity change is 

4 max

0 AE

1.5 10 c K

c

Δ − ≈σ =× (13) 

which is the same as the value of the maximum differential thermal strain from equation 10. 

32
The acoustothermal contribution can be expressed as follows 

0 AT

c T

c

Δ = βΔ (14) 

Because of the relatively low level of thermal expansion, equation 14 can be approximated by 

0 0 AT AT

a 0.65% c t T

c t

Δ Δ ≈− =β Δ ≈ (15) 

where βa is the measured apparent ATC without correction for thermal expansion which is a 

much weaker effect (for Ti-6Al-4 βa ≈ -2.3×10-4/°C and -8.9×10-4/°C for longitudinal and shear 

waves, respectively, versus α ≈ 1×10-5/°C). 

Since the thermally induced acoustoelastic effect was found to cause only a minor relative 

velocity change of ≈3×10-6 for 1oC (33.8oF) change in temperature (see equation 13 for 50oC 

(122oF)), it was concluded that the dominating effect is that of the thermal modulation of sound 

velocity, i.e., the ATC, and that the other contributions, namely, the thermal expansion and the 

acoustoelastic effects, are essentially negligible. This is the reason why the ATC is emphasized 

in the rest of this report and the other two contributions are assumed to be negligible. 

6. VELOCITY MEASUREMENTS. 

The longitudinal and shear velocities were measured in the titanium grade 2, Ti-6Al-4V rolled, 

forged, and canted specimens, the titanium billet, as well as the 2.5% N, 3.1% N, and 3.8% N 

titanium specimens. The setup was used because it accurately measures the absolute propagation 

time delay in the specimen. From this, the unperturbed velocity c0 is easily calculated as 

0

0

0

2 d

c

t = (16) 

where d0 is the unperturbed thickness of the specimen measured by calipers, and t0 is the 

unperturbed two-way propagation delay measured by the Time Interval Counter. Figures 34 and 

35 show that, as expected, the sound velocity in TiN increases by roughly 3% with each weight 

percent of nitrogen, which is in good agreement with the findings from previous studies [12]. 

The specimens with 4.8% N and 5.9% N were not measured since no detectable signal could be 

obtained. These specimens were too porous and neither a longitudinal nor a shear wave could 

propagate all the way through them and back. Thus, they could not be used in the 

acoustothermal measurements either. 

33
5.8

5.9

6.0

6.1

6.2

6.3

6.4

6.5

6.6

6.7

6.8

6.9

Titanium Ti-6Al-4V Forged Canted Billet 2.5% N 3.1% N 3.8% N

Velocity [mm/μs] .

FIGURE 34. LONGITUDINAL VELOCITY IN TITANIUM WITH VARYING LEVELS 

OF N CONTENT 

2.9

3.0

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

Titanium Ti-6Al-4V Forged Canted Billet 2.5% N 3.1% N 3.8% N

Velocity [mm/μs] .

FIGURE 35. SHEAR VELOCITY IN TITANIUM WITH VARYING LEVELS 

OF N CONTENT 

7. NUMERICAL SIMULATION FOR PRELIMINARY FEASIBLITY ASSESSEMENT. 

From the results presented previously (see equation 15), it was concluded that the acoustothermal 

effect is the dominant effect amongst the three measured nonlinear parameters. For a 50o

C 

change in temperature, it was predicted that there would be approximately 0.65% change in 

velocity, which is less than the previously estimated upper limit of 1.0% velocity change. To 

assess the possibility of measuring any phase modulations of a propagating longitudinal wave 

due to such small velocity change, the UT finite difference-wave propagation simulation 

software, Wave2000Pro, was used in various simulations. Figure 36 shows the basic setup used 

in the simulations. A rectangular 60- x 50-mm piece of Ti-6Al-4V was used to simulate the host 

material. The acoustic properties of Ti-6Al-4V were taken as density , 

longitudinal sound speed , and shear sound speed 

3 ρ = 4500 kg/m

6070 m/sec Lc = 3310 m/sec s c = . Embedded 

34
in the titanium alloy was a cigar-shaped inclusion with certain dimensions. Based on previous 

studies (see reference 6) and earlier C-scan images (see figure 10), it was estimated that an 

inclusion representing defect X would be 15 mm long and 4 mm wide. These dimensions were 

used in the first model investigated. The green regions in figure 36 represent infinite boundary 

conditions to eliminate or minimize any reflections from these edges of the finite model. 

FIGURE 36. A NUMERICAL SIMULATION MODEL SETUP 

Figure 37 shows the results corresponding to this simulation model. Several scenarios were 

investigated with the longitudinal-wave velocity in the inclusion modulated up to 6% from that 

of the host alloy. Figure 37(a) shows the results corresponding to the case of a 5-MHz, 10-mmdiameter transmitter and a 10-mm-diameter receiver in pulse-echo configuration. Figure 37(b) 

shows similar results for the case of a 5-MHz, 10-mm-diameter transmitter and a 1-mm-diameter 

receiver in a pitch-catch configuration. Figure 37(c) shows similar results for the case of 5-MHz, 

10-mm-diameter transmitter, 10-mm-diameter receiver again in a pitch-catch configuration. 

Careful inspection of these figures show that the peak of the received signal starts to shift to the 

left (arrives earlier due to higher speed) up to about 4% velocity modulations. Beyond that, an 

interference phenomenon takes over and shifts the peak to the right, implying lower phase 

modulation compared to the host material. Figure 38 shows a plot of the absolute time shift in 

the peak of the signal in nanoseconds (nsec) versus the sound speed modulation in percent for the 

three scenarios depicted in figure 37. Note that in figure 38, the time shift for the pulse-echo 

case was divided by two to reflect the fact that the sound makes a two-way round trip before 

arriving at the receiver and to facilitate the comparison with the pitch catch case. It is also noted 

that for the case of the 1-mm-diameter receiver, the time shift in the peak is much higher as the 

sound speed modulation is increased compared to the other two cases due to the much higher 

spatial resolution of the wave front that reduces the averaging of the phase front compared to the 

10-mm-diameter receiver. Since the acoustothermal modulation of the sound speed, as measured 

experimentally, is approximately 0.65%. A peak shift of about 11 nsec was expected, which can 

be easily measured in a laboratory environment. This is, off course, subject to the validity of the 

assumption, which includes (1) the size of the defect used in the simulations is appropriate, 

(2) the sound velocity in TiN increases by roughly 3% with each weight percent of nitrogen, and 

(3) the shape of the inclusion used in the simulations, is reflective of the actual defects. 

50 mm

60 mm

Ti-6Al-4V Specimen

15 mm

1 mm

T/R

R

HA Inclusion

R

4 mm 

35
8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9 9.1 9.2 9.3 9.4

Time [μs]

Amplitude [a. u.]

 +0%-velocity inclusion

 +1%-velocity inclusion

 +2%-velocity inclusion

 +3%-velocity inclusion

 +4%-velocity inclusion

 +5%-velocity inclusion

 +6%-velocity inclusion

8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9 9.1 9.2 9.3 9.4

Time [μs]

Amplitude [a. u.]

 +0%-velocity inclusion

 +1%-velocity inclusion

 +2%-velocity inclusion

 +3%-velocity inclusion

 +4%-velocity inclusion

 +5%-velocity inclusion

 +6%-velocity inclusion

15.8 15.9 16 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 17

Time [μs]

Amplitude [a. u.]

 +0%-velocity inclusion

 +1%-velocity inclusion

 +2%-velocity inclusion

 +3%-velocity inclusion

 +4%-velocity inclusion

 +5%-velocity inclusion

 +6%-velocity inclusion

(a) 

(b) 

(c) 

FIGURE 37. SIMULATION RESULTS FOR A CIGAR-SHAPED INCLUSION 

15 mm LONG AND 4 mm WIDE 

36
0

10

20

30

40

50

60

70

80

90

100

012345

Sound Speed Modulation [%]

Time Shift in Peak [nsec].

10-mm-dia. T, 10-mm-dia. R, pulse-echo

10-mm-dia. T, 10-mm-dia. R, pitch-catch

10-mm-dia. T, 1-mm-dia. R, pitch-catch

FIGURE 38. ANALYSIS OF THE TIME SHIFT IN THE PEAK DUE TO SOUND SPEED 

MODULATION IN THE INCLUSION FOR THE SCENARIOS DEPICTED IN FIGURE 37 

The above results suggest that one should be able to detect the presence of defects X and D after 

HIPping at least in the scan representing the TOF data. Referring back to figures 15 and 16, one 

sees that there is no indication of the defects in any of the scans obtained. This implies that one 

of the assumptions used in the simulation models must be invalid. To verify previous results on 

defect X and to understand which of the assumptions could be questionable, the block with 

defect X was rescanned using the Sonix Quantum–350 acoustic microscope and a higher 

frequency 10-MHz, 0.5-inch-diameter Panametrics V311 UT transducer. The data acquired were 

the peak and TOF of the back-wall reflected signal. Figure 39 shows these results and again 

confirms that no indication of the presence of defect X was observed. Since the other two 

assumptions were fairly verified in this and other studies, it seems that the assumption on the size 

of the defect is the one most questionable. Figure 40 shows the results of similar simulations as 

those presented in figure 37 with the 15-mm-long and 4-mm-wide inclusion replaced by a 15-

mm-long by 1-mm-wide inclusion. A quick analysis of figure 40 reveals that the time shift of 

the signal peak for the later case is much smaller and suggests that the assumed defect size is 

probably inaccurate. This promoted further investigation of defect X by performing a 

metallurgical analysis and using other NDE techniques as detailed in the next section. 

FIGURE 39. THE TOF AND REFLECTED PEAK AMPLITUDE SCANS FOR DEFECT X 

37
15.8 16 16.2 16.4 16.6 16.8 17 17.2 17.4 17.6 17.8

Time [μs]

Amplitude [a. u.]

 +0%-velocity inclusion

 +1%-velocity inclusion

 +2%-velocity inclusion

 +3%-velocity inclusion

 +4%-velocity inclusion

 +5%-velocity inclusion

 +6%-velocity inclusion

15.8 15.9 16 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8

Time [μs]

Amplitude [a. u.]

 +0%-velocity inclusion

 +1%-velocity inclusion

 +2%-velocity inclusion

 +3%-velocity inclusion

 +4%-velocity inclusion

 +5%-velocity inclusion

 +6%-velocity inclusion

15.8 15.85 15.9 15.95 16 16.05 16.1 16.15 16.2 16.25 16.3

Time [μs]

Amplitude [a. u.]

 +0%-velocity inclusion

 +1%-velocity inclusion

 +2%-velocity inclusion

 +3%-velocity inclusion

 +4%-velocity inclusion

 +5%-velocity inclusion

 +6%-velocity inclusion

(a) 

(b) 

(c) 

FIGURE 40. SIMULATION RESULTS FOR A CIGAR-SHAPED INCLUSION 

15 mm LONG AND 1 mm WIDE 

38
8. FURTHER ANALYSIS OF DEFECT X. 

8.1 EDDY-CURRENT AND ACOUSTIC MICROSCOPY IMAGING.

To further characterize defect X and the effect the HIPping process had on the overall 

dimensions of the healed inclusion, a smaller block of 2 by 1 by 1 inches was cutout from the 

larger block. The location of the defect in the smaller block was then pinpointed and the 

inclusion sliced perpendicular to its axis. Twenty 100-mil slices were produced and each cutting 

step consumed about 15 mils of material. Figure 41 shows how the slicing process was 

performed. 

Defect "X"

100 Mil-Slices with

15 Mil-Gaps

1 in

1 in

2 in

FIGURE 41. SLICING PROCESS PERFORMED ON DEFECT X PERPENDICULAR TO ITS 

AXIS TO PRODUCE TWENTY 100-mil SLICES WITH 15-mil GAPS 

Once the slices were prepared, both eddy-current and acoustic microscopy imaging were 

performed on them. Figure 42 shows sample results from the eddy-current imaging, and figure 

43 shows similar sample results from acoustic microscopy imaging. The eddy-current imaging 

maps the variation in electrical conductivity that is affected by the presence of nitrogen and its 

distribution throughout the sample. The images shown in figure 42 were taken using a 6-MHz, 

eddy-current probe with 4-mils liftoff and a scanned image with dimensions of 1.2 by 1.2 inches. 

The presence of the defect in the slice is very clear from the indications in these images; 

however, the spatial resolution is not good enough to allow accurate sizing of the inclusion. 

According to these images, the estimated maximum defect width is 10 mm, and the estimated 

maximum thickness is as 4 mm, which are reasonably close to the dimensions assumed in the 

simulation models. 

39
Slice #9 Slice #10 

Slice #12 Slice #13 

FIGURE 42. SAMPLE RESULTS OF THE EDDY-CURRENT IMAGING OF SOME OF THE 

SLICES FROM INCLUSION X 

The acoustic microscopy images shown in figure 43 were collected using the Sonix acoustic 

microscope, using a Panametrics V324 (serial number 148627), 25 MHz, 0.25 inch in diameter, 

focused at the back wall of the specimen under consideration. The presence of the inclusion is 

clearly detected and some size information can be extracted. According to these images, the 

estimated maximum defect width is 7 mm, and the estimated maximum thickness is 1 mm. 

These estimates are significantly smaller than the dimensions assumed in the simulation models, 

which could explain why the defects where not observed in TOF data from the larger block. The 

overall length of the defect along its axis was calculated by counting the number of slices from 

when the first slice in which the defect appears to the last slice in which the defect is still 

detectable. There were a total of nine slices. Taking the slice thickness to be 100 mils and 

assuming only 15 mils in lost material in each cutting step, the estimated length of the inclusion 

is about 0.885 inch (about 22 mm). This value is larger than the length assumed in the 

simulations. It must be mentioned here that although the length of the defect along its axis is 

comparable between the experimental results and the simulation model, the other two 

dimensions of the inclusion seem to be much smaller than what was assumed and more reflective 

of the second simulation model that assumes a 15-mm-long and 1-mm-wide inclusion. The 

results from that simulation model clearly showed that no measurable effect would be detected 

for this case. Also, it should be emphasized that the actual inclusion size that can modulate the 

phase of the propagating UT wave is that of the region enriched by nitrogen (> 2% by weight). 

To more accurately determine this size, a metallurgical analysis was performed on some of these 

slices along with a chemical microprobe analysis. 

40
Slice #10 

Slice #12 Slice #13 

Slice #9 

FIGURE 43. SAMPLE RESULTS OF THE ACOUSTIC MICROSCOPY IMAGING OF 

SOME OF THE SLICES FROM INCLUSION X 

8.2 METALLURGICAL AND MICROPROBE ANALYSIS. 

Of the 20 slices making up the block that contained the inclusion X, only slices 9 through 13 

were characterized in the scanning electron microscope as to size, shape, and qualitative 

compositional differences. Size measurement results for the defect in each of the sampled cross 

sections is reported in table 5. Note that the thickness was not uniform, with nuggets at various 

positions along the width that were wider than most of the defect. The thickness of these nuggets 

defined the width dimension maxima in table 5. The general characteristics of all five defects 

were sufficiently similar, thus, it is necessary only to describe one. Slice 13 was selected for the 

characterization that follows. 

TABLE 5. SIZE OF THE DEFECT IN EACH EXAMINED SLICE CROSS SECTION 

Slice No. 9 10 11 12 13 

Length 8.5 mm 7.6 mm 7.5 mm 7.7 mm 8.1 mm 

Width ≤ 1.5 mm ≤ 1.4 mm ≤ 1.5 mm ≤ 1 mm ≤ 1 mm 

In figure 44, note that there are no cracks or pores of sufficient size to serve as reflectors of the 

sonic signal; the thickness dimension is quite small, so even if the properties are different, the 

effect on the sound signal will be difficult to detect. 

41
Profile 2

Profile 1

FIGURE 44. MICROSTRUCTURE FOR SLICE 13 SHOWING THE NONUNIFORM 

THICKNESS ALONG THE WIDTH OF ABOUT 8 mm 

Further analysis of the region indicated in figure 44 by a red circle was carried out, as this region 

is distinguishable from other parts of the defect. Figure 45 is a slightly higher magnification 

view of that area showing a very nonuniform microstructure. It also clearly illustrates a gradient 

of microstructures. It is assumed that the core of the defect is represented by the mottled area. 

The core is highlighted by a black arrow. Figure 46 is an even higher magnification view of this 

area and a multiphase zone of the inclusion can clearly be seen. 

Profile 2 

1.14 mm

FIGURE 45. HIGHER MAGNIFICATION VIEW OF AREA CIRCLED IN FIGURE 44 

42
D36 

D37

D32

D35 

D33

D34 

FIGURE 46. HIGH MAGNIFICATION, BACKSCATTERED ELECTRON IMAGE OF 

MOTTLED AREA SEEN IN FIGURE 45, HIGHLIGHTING AREAS WHERE ENERGY 

DISPERSIVE X-RAY ANALYSES WERE MADE 

From the energy dispersive x-ray analyses (EDX) spectra of these areas compared to the EDX 

scan of the base material, it is possible to make some qualitative observations regarding the 

composition of the selected areas. Note that there is a wide variety of compositions within a 

small area of the core of this defect. For the most part, nitrogen is the dominant interstitial alpha 

stabilizer and the nominal alloy constituent contents are variable. Separate, a spread beam 

analysis of the mottled area revealed that this area had lower Al content and higher N content 

than the baseline alloy. This is consistent with nitrogen-stabilized hard alpha defects but not 

likely one originating from splatter ring oxidation due to the lower Al content. The splatter ring 

is generally characterized as being higher than nominal with respect to Al content. It is clear that 

this hard alpha is far from homogeneous in composition or structure so models of its behavior 

must account for its inhomogeneity. Selected microprobe traces were done on samples to 

quantify N content across the defect. For those run on slice 13, trace marks are shown on figure 

44 with the results plotted in figures 47 and 48. From these figures, it is apparent that the N 

content in this defect is far from uniform, a situation not uncommon amongst other naturally 

occurring hard alpha defects. This nonuniformity is on a scale approaching the dimensions of 

the defect as well as on a smaller scale, as indicated in figure 46 and table 6. A quick review of 

hard alpha characteristics reported in the 1990 to 2001 time period shows N contents from 1.1% 

to 12%. (N = 8, avg. = 6.8%). 

Based on figure 47, the width of the inclusion with any appreciable N content (> 0.2% by 

weight) can be estimated as 0.3 mm. If one actually considers that appreciable modulation of the 

sound, speed will occur only if the N content is higher than 1.0% by weight. Figure 48 shows 

very clearly that the width of the inclusion is less than 0.2 mm. These dimensions are much 

smaller than those used in the simulation models and indicate very clearly that the HIPping 

process collapsed the cavities in the voided hard alpha inclusion and left a very thin narrow 

43
string running through the host material. Keeping these facts in mind, one can conclude that no 

phase modulation of the propagating longitudinal-wave front would be detectable in the original 

block that contained the defect, as the propagation width of the sound in the inclusion is a small 

fraction of the beam width and, hence, no indication of the defect would be observed in the TOF 

data as seen from the previous experimental results. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 100 200 300 400 500 600 700

Distance along profile (μm)

Nitrogen wt

%

FIGURE 47. NITROGEN COMPOSITION TRACE (FROM MICROPROBE ANALYSIS) 

ACROSS DEFECT OF SLICE 13 ALONG PROFILE 1 

(Microstructure is shown in figures 44 through 46.) 

0

2

4

6

8

10

12

14

0 200 400 600 800 1000

Distance along profile (μm)

Nitrogen wt

%

FIGURE 48. MICROPROBE TRACE ALSO FROM SLICE 13 ALONG PROFILE 2 

SHOWING MUCH HIGHER N CONTENT THAN THE FIRST TRACE 

44
TABLE 6. QUALITATIVE DIFFERENCES IN COMPOSITION WITH RESPECT TO THE 

BASE ALLOY COMPOSITION 

D32 D33 D34 D35 D36 D37 

Al 0 N/A + none + -- 

Fe 0 + + N/A N/A N/A 

V 0 + + 0 0 

N + 0 + ++ + ++ 

C 0 0 + + + + 

Figure 49 shows combined traces of N and Al for slice 13. Note the apparent exclusion of the 

two elements. Al and N are both alpha stabilizers so there is no phase partitioning to explain 

this. A possible explanation is as follows. In the Ti-6Al-4V alloy manufacturing process, Al 

diffuses slowly through nitrogen-enriched alloy. This analysis indicates that the N enriched the 

titanium alloy early in the melting process, and the Al did not have a chance to diffuse through 

these regions. It is also clear that the air leak affected the material prior to the point in the 

process where complete alloying had occurred. The principal suspect is that the air leak occurred 

during primary melting. It was incompletely homogenized melt or more likely, the unmelted 

electrode that was most affected by the air leak. 

0

2

4

6

8

10

12

14

0 200 400 600 800 1000

Distance along profile (μm)

Weight Percentage...

Nitrogen Aluminum

FIGURE 49. COMBINED TRACES OF N AND Al FOR SLICE 13 ALONG PROFILE 2 

(Note the apparent exclusion of the two elements.) 

45
9. SUMMARY.

Hard alpha inclusions are low-density, hard, brittle regions of spuriously high nitrogen and 

oxygen content that occur in titanium alloys. These inclusions are often cracked or include 

voids, which makes it much easier to detect by conventional NDT methods, including UT 

backscattering. However, the detection of uncracked/unvoided hard alpha inclusions represents 

a very significant challenge for UT NDT. Although every weight percent of N was found to 

increase the sound velocity by roughly 3%, therefore, the absolute acoustic impedance contrast is 

expected to be substantial (10%-15%), the actual SNR is badly limited by three adverse effects. 

First, the N content varies gradually towards the center of the inclusion; therefore, the reflected 

signal remains rather weak. Second, the shape of the inclusion is typically very irregular; 

therefore, the flaw signal is further reduced by random geometrical scattering effects. Third, the 

competing grain noise is frequently very high because of the often coarse microstructure of the 

surrounding material. As a result, the backscattered UT signal from the unvoided hard alpha 

inclusion is usually indistinguishable from the incoherent material noise based on its strength 

alone and the probability of its detection remains inherently low. 

The objective of this program was to investigate the feasibility of a new UT NDT method aimed 

at detecting nonvoided hard alpha inclusions in titanium billets. As a working hypothesis, it was 

postulated that, in general, nonlinear elastic properties are more sensitive to the presence of 

interstitial (N, C, and O) elements that lead to higher hardness than the corresponding linear 

elastic properties. There are a great variety of nonlinear elastic parameters that could be 

exploited for NDE of hard alpha inclusions. The three nonlinear parameters that were 

investigated in details were chosen with special regard to the particular nonlinear NDE method 

that was used in this study, namely, thermally activated elastography. These nonlinear 

parameters include the TEC, the thermal coefficient of sound velocity (ATC), and the stress 

coefficient of sound velocity (AEC). Of course, these nonlinear parameters are mutually 

interrelated and are all dependent on the higher-order elastic moduli of the material. 

All three nonlinear parameters were experimentally measured in order to assess the contrast in 

these properties between an unvoided hard alpha inclusion and the titanium host material that 

will ultimately determine the feasibility of such an inspection technique. Based on the 

measurements, the direct thermal expansion effect and the thermally induced acoustoelastic 

effect are all practically negligible relative to the acoustothermal velocity change. This last 

parameter was found to be rather sensitive to changes in N content (for simplicity, these 

coefficients were not corrected for thermal expansion, which represents a smaller effect than the 

other experimental uncertainties in the measurements). Both the longitudinal and shear ATCs 

are negative, i.e., the velocities decrease with increasing temperatures. Furthermore, the 

longitudinal coefficient decreases while the shear coefficient increases with increasing nitrogen 

content. The behavior of the shear coefficient is rather surprising in light of the fact that brittle 

materials usually exhibit reduced elastic nonlinearity, which should decrease the temperature 

dependence of the shear velocity too. One possible explanation is that shear wave propagation is 

more affected by the presence of microporosity and other microstructural imperfections, which 

accompany the increase in N content. This porosity is also believed to be responsible for the 

dramatically reduced mechanical strength of the nitrogen-enriched specimens and for the fact 

that specimens of the highest N content could not be tested by UT means at all. It is likely that 

this porosity has also adversely influenced the longitudinal acoustothermal measurements in 

46
specimens of higher than 3% N content; therefore, additional efforts to increase the accuracy and 

reliability of such experiments are needed in future efforts. 

From the results presented previously (see equation 15), it was concluded that the acoustothermal 

effect is the dominant effect amongst the three measured nonlinear parameters. For a 50o

C 

change in temperature, it was predicted that there would be approximately 0.65% change in 

velocity, which is less than the previously estimated upper limit of 1.0% velocity change. 

Simulation models using the finite difference wave propagation simulation software, 

Wave2000Pro, showed very clearly that a cigar-shaped, unvoided hard alpha inclusion, 15 mm in 

length and 4 mm in width, in titanium would produce a peak shift in the longitudinal UT wave 

signal of about 11 nsec, which can be readily measured in the laboratory. This is of course 

subject to the validity of the assumptions about the actual inclusion, which include (1) the size of 

the defect used in the simulations is appropriate and representative of real inclusions, (2) the 

sound velocity in TiN increases by roughly 3% with each weight percent of nitrogen, and (3) the 

shape of the inclusion used in the simulations is representative of the actual defects. 

Although preliminary feasibility (at least in a laboratory environment) has been predicted using 

the simulation models, the real test is to verify these predictions with experimental data from real 

unvoided hard alpha inclusions in titanium. The need for real unvoided hard alpha inclusion 

specimens in titanium alloys became the most important factor in the experimental verification 

process. To that end, two types of specimens were obtained from various sources during the 

course of the program. 

A Ti-6Al-4V specimen with three synthetic cylindrical inclusions, manufactured using one of the 

methods indicated, was obtained from the NDE branch at Wright-Patterson Air Force Base in 

Dayton, OH. This block was intended for use in the technique validation experiments with the 

assumption that the inserted inclusions are representative of the condition of an unvoided hard 

alpha inclusion in Ti-6Al-4V. After the initial inspection of the block using backscattered UT 

inspection, it became very evident that this artificial inclusion specimen is not representative of a 

case of unvoided hard alpha inclusions in titanium alloys, and the block was ruled out from any 

future investigations. Several factors can account for the very strong UT response received from 

these synthetic inclusions. First, the interface between the inclusions and the titanium host 

materials is sharp in terms of the change in N content, which is not the case for a naturally 

occurring unvoided hard alpha inclusion. Second, the porosity associated with such inclusions 

(specially the ones with the high N content) may contribute to a higher than expected UT signal. 

And finally, the questionable diffusion interface between the inclusion and the host material may 

result in such a high signal if the inclusion is not completely bonded to the host material. For all 

these reasons, this specimen was deemed to be not representative of naturally occurring hard 

alpha inclusions in titanium alloys and, therefore, was not used in any further activities in this 

program. 

To generate samples that might be more representative of real unvoided hard alpha inclusions in 

titanium, it was decided to acquire some voided hard alpha inclusion samples and put them 

through a HIPping process to close the voids, assuming that will preserve the overall dimensions 

of the inclusion. For the purposes of this program, several billet pieces having about 15 

indications left over from the CBS were obtained from the ETC team. A number of these 

47
indications were verified again in these billets using an MZI procedure, and their locations in the 

respective billets were pinpointed. In particular, a 25.75-inch-long billet section (B2W2C) 

containing two indications (X and D) was extensively studied. After precisely locating the two 

defects in the billet, they were cut out from the billet material to form two blocks with 

dimensions of 2 by 2 by 2.82 inches and 2 by 2.73 by 2.82 inches, respectively. The presence of 

the inclusions in the blocks were again verified and the two blocks were HIPped at 1650o

F and 

30 ksi pressure for 5 hours. UT inspection of these blocks after HIPping showed no indication of 

the inclusions, which was expected, and indicated that the HIPping did actually heal all the voids 

within these inclusions. What was surprising in the results obtained, despite very careful 

measurements, the fact that there were no indication of these inclusions in the TOF data in 

contradiction with the prediction from the simulations. This led to questions concerning the 

nature of the inclusions and how representative they were of real unvoided hard alpha inclusions. 

There were three assumptions about the inclusion type in the simulation models relating to the 

size, shape, and nature of velocity modulation it would produce. These assumptions were 

mentioned above. Assumptions two and three have been proven to be valid in this and other 

studies. The first assumption that relates to the size of the inclusion was very questionable and 

was proven (in this study) to be invalid for the unvoided hard alpha specimens generated from 

the CBS voided hard alpha specimens through the HIPping process. Although the length of the 

defect along its axis is comparable between the experimental results and the simulation model, 

the other two dimensions of the inclusion seem to be much smaller than what was assumed and 

more reflective of the second simulation model that assumes a 15-mm-long and 1-mm-wide 

inclusion. The results from that simulation model clearly showed that no measurable effect 

would be detected for this case. Also, it should be emphasized that the actual inclusion size that 

can modulate the phase of the propagating UT wave is that of the region enriched by N (> 2% by 

weight), which was shown through metallurgical and microprobe analyses to be much less than 1 

mm in width. Based on figure 47, the width of the inclusion with any appreciable N content 

(> 0.2% by weight) is estimated as 0.3 mm. With consideration that appreciable modulation of 

the sound speed will occur only if the nitrogen content is higher than 1.0% by weight, figure 48 

shows very clearly that the width of the inclusion is less than 0.2 mm. These dimensions are 

much smaller than those used in the simulation models and indicate very clearly that the HIPping 

process collapsed the cavities in the voided hard alpha inclusion and left a very thin narrow 

string of unvoided hard alpha material running through the host material. Keeping these facts in 

mind, one can conclude that no phase modulation of the propagating longitudinal wave front 

would be detectable in the original block that contained the defect, as the propagation distance of 

the sound in the inclusion is a small fraction of the overall propagation distance and, hence, no 

indication of the defect would be observed in the TOF data as seen from the experimental results. 

Although these results were rather disappointing, they do not constitute evidence that the 

proposed thermal elastography technique is not feasible. The predictions based on the results 

from the experimental measurements of the ATC show very clearly that the effect is measurable. 

However, the lack of specimens representing a real unvoided hard alpha inclusion in titanium of 

the type and size shown in figure 2 has made it impossible to demonstrate this feasibility 

practically in the laboratory or at an inspection facility. If such samples do exist, the information 

included in this report can be used for such a practical demonstration of the technique capability. 

48
10. RECOMMENDATIONS. 

The acoustoelastic measurements were not performed on the brittle TiN specimens because they 

were too small for the MTS and their mechanical strength was highly questionable. The 1-inchlong specimens could most likely be tested in compression, which would require an extensive 

setup preparation. Since it was shown earlier that the relative contributions of the thermal 

expansion and AECs were small compared to the ATC, it was not considered a serious limitation 

that the TiN specimens could not be tested in the MTS. 

In light of the research findings, further improvement of the acoustothermal measurements that 

were found to be crucial for the feasibility of the thermalelastographic detection of nonvoided 

hard alpha inclusions in titanium alloys is desired. As mentioned before, these tests are tedious 

and complicated, which can often lead to spurious artifacts and, ultimately, incorrect results. 

Many precautions have been taken to ensure that the tests are repeated in a similar manner. 

Intrinsic measurement uncertainties include electrical noise, mechanical vibrations, thermal 

instability, long-term drift, etc. Possibly, the lack of sufficient reproducibility is caused by aging 

of the UT transducers and imperfect coupling to the specimens, while instrument anomalies and 

imperfections appear to be under control. Many tests have been discarded because of spurious 

spikes in the data, noise from the MTS when in use by someone else, or a nonlinear velocity 

versus temperature curve. Explaining why a particular test went wrong is nearly impossible, but 

knowing that it did go wrong is easy. However, a study to understand the effect that different 

amounts of couplant, different water levels, and different transducers may have on the stability 

and sensitivity of the measurements would be quite beneficial. 

The longitudinal measurements seem to be easier than the shear measurements. The water is 

allowed to air-cool from 80°C to room temperature, whereas the water is cooled by the chiller to 

5°C in the shear measurements. This factor seems to produce a noticeable curvature in most of 

the shear plots at the same point in the temperature range. However, this method must be used 

because of the viscosity loss of shear couplants. Evaluating the possibility of using shear waves 

to perform the thermal elastographic detection of unvoided hard alpha inclusions in titanium 

should be considered in future activities. 

Finally, a future activity to try to secure unvoided hard alpha inclusion specimens of the type and 

size that is representative of real naturally occurring unvoided hard alpha inclusions in titanium 

should be considered. These specimens can be used to demonstrate (at least in the laboratory 

environment) the actual feasibility of the technique under consideration in this effort. 

Investigation of the possibility of producing such unvoided hard alpha samples if no such 

samples are currently in existence is also recommended. Understanding how such inclusions 

form without cracking and voiding is critically needed to produce such samples. If such samples 

do exist, the information included in this report can be used to practically demonstrate the 

thermal elastographic technique capability. 

49
11. REFERENCES. 

1. FAA Advisory Circular 33.15-1, “Manufacturing Process of Premium Quality Titanium 

Alloy Rotating Engine Components,” September 22, 1998. 

2. “United Airlines Flight 232 McDonnell Douglas DC-10-10 Sioux Gateway Airport, 

Sioux City, Iowa,” July 19, 1989, NTSB Report Number, AAR-90-06, November 1, 

1990. 

3. Brasche, L.J.H. and Buck, O., “Nondestructive Methods for Determination of Mechanical 

Properties of Aluminum and Titanium Alloys,” Review of Progress in Quantitative 

Nondestructive Evaluation, Vol. 10B, 1991. 

4. Chan, K.S., Perocchi, L., and Leverant, G.R., “Constitutive Properties of Hard-Alpha 

Titanium,” Metallurgical and Materials Transactions, Vol. 31A, 2000. 

5. Bogan W.W., “Metallurgical Evaluation: RMI 2XVAR Ti64 Heat 883588 Hard Alpha,” 

Internal ETC Letter Report, April 27, 1995. 

6. Brasche, L.J.H., Chiou, T., Thompson, B., Smith, K., Meeker, W., Margetan, F., Paul 

Panetta, P., Chenail, R., Galli, F., Umbach, J., Raulerson, D., Degtyar, A., Bartos, J., 

Copley, D., McElligott, B., Howard, P., and Bashyam, M., “Contaminated Billet Study,” 

FAA report DOT/FAA/AR-05/16, September 2005. 

7. Bartos, J.L., Copley, D.C., Gilmore, R.S., and Howard, P.J., “Advanced Ultrasonic 

Inspection Techniques for Titanium Billet Materials,” Titanium ’95: Science and 

Technology, Vol. 2, 1996. 

8. Leverant, G.R., McClung, R.C., Wu, Y.-T., Millwater, H.R., Riha, D.S., Enright, M.P., 

Chell, G.G., Chan, K.S., McKeighan, P.C., Gorelik, M., Fischer, C., Lehmann, D., Green, 

K., Bartos, J., Howard, N., and Srivatsa, S., “Turbine Rotor Material Design,” FAA 

Report DOT/FAA/AR-00/64, December 2000. 

9. Giglioti, M.F.X., Gilmore, R.S., and Perocchi, L.C., “Microstructure and Sound Velocity 

of Ti-N-O Synthetic Inclusions in Ti-6Al-4V,” Metallurgical and Materials 

Transactions, Vol. 25A, 1994. 

10. See www.matweb.com. 

11. Ruiz, A. and Nagy, P.B., “SAW Dispersion Measurements for Ultrasonic 

Characterization of Surface-Treated Metals,” Instrumentation, Measurement and 

Metrology, Vol. 3, 2003. 

12. Margetan, F.J., Gigliotti, M., Brashe, L., and Leach, W., “Fundamental Studies: 

Inspection Properties for Engine Titanium Alloys,” FAA report DOT/FAA/AR-02/114, 

December 2002. 

50