T.P. Gabb, J. Telesman, and L. Ghosn

Glenn Research Center, Cleveland, Ohio

A. Garg

University of Toledo, Toledo, Ohio 

J. Gayda

Glenn Research Center, Cleveland, Ohio

Factors Infl uencing Dwell Fatigue Cracking in 

Notches of Powder Metallurgy Superalloys

NASA/TM—2011-217118

June 2011
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T.P. Gabb, J. Telesman, and L. Ghosn

Glenn Research Center, Cleveland, Ohio

A. Garg

University of Toledo, Toledo, Ohio 

J. Gayda

Glenn Research Center, Cleveland, Ohio

Factors Infl uencing Dwell Fatigue Cracking in 

Notches of Powder Metallurgy Superalloys

NASA/TM—2011-217118

June 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors wish to acknowledge the support of the NASA Propulsion 21 and Aviation Safety programs. The assistance of 

Ron Jaworsky on fractography is also appreciated. LSHR powder atomization was performed at PCC Special Metals Corporation 

under the direction of Tony Banik, now at Allvac ATI. LSHR disk forging and heat treatments were performed at PCC WymanGordon Forgings under the direction of Ian Dempster. David Mourer of GE Aviation and Mark Hardy of Rolls-Royce are 

acknowledged for very helpful technical discussions.

Available from

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
Factors Influencing Dwell Fatigue Cracking in Notches of 

Powder Metallurgy Superalloys 

T.P. Gabb, J. Telesman, and L. Ghosn 

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

A. Garg

University of Toledo

Toledo, Ohio 43606 

J. Gayda 

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

Abstract

The influences of heat treatment and cyclic dwells on the notch fatigue resistance of powder 

metallurgy disk superalloys were investigated for low solvus high refractory (LSHR) and ME3 disk 

alloys. Disks were processed to produce material conditions with varied microstructures and associated 

mechanical properties. Notched specimens were first subjected to baseline dwell fatigue cycles having a 

dwell at maximum load, as well as tensile, stress relaxation, creep rupture, and dwell fatigue crack growth 

tests at 704 °C. Several material heat treatments displayed a bimodal distribution of fatigue life with the 

lives varying by two orders-of-magnitude, while others had more consistent fatigue lives. This response 

was compared to other mechanical properties, in search of correlations. The wide scatter in baseline dwell

fatigue life was observed only for material conditions resistant to stress relaxation. For selected materials 

and conditions, additional tests were then performed with the dwells shifted in part or in total to minimum 

tensile load. The tests performed with dwells at minimum load exhibited lower fatigue lives than max 

dwell tests, and also exhibited early crack initiation and a substantial increase in the number of initiation 

sites. These results could be explained in part by modeling evolution of peak stresses in the notch with 

continued dwell fatigue cycling. Fatigue-environment interactions were determined to limit life for the 

fatigue cycles with dwells. 

Introduction

A new generation of powder metallurgy (PM) disk superalloys has been designed for higher engine 

operating temperatures by improvement of their strength and creep resistance. New PM disk superalloys 

have been designed with higher refractory element contents than cast and wrought superalloys, while 

avoiding segregation of chemistry and microstructure that are produced by cast and wrought processing 

(Refs. 1 and 2). Compared to cast and wrought superalloys, PM disk superalloy processing produces more 

uniform microstructures, with more consistent grain and precipitate size, and finer carbide sizes (Ref. 3). 

This processing also enables composition modifications designed to improve tensile, strength, and creep 

resistance, which allow disk applications at higher temperatures (Refs. 4, 5, and 6) in aerospace gas 

turbine engines. 

However, the increased strength and creep resistance of these alloys may also increase the alloys 

notch sensitivity to fatigue loading and have a profound effect on the notched low cycle fatigue (NLCF) 

life (Ref. 7). Recent work has indicated that NLCF life for PM disk alloys is highly affected by sustained 

dwells at the higher disk operating temperatures (Ref. 8). It is therefore important to understand what 

NASA/TM—2011-217118 1
factors influence the resistance of PM superalloys to cracking at notches during fatigue cycling with 

load dwells. 

The effect of prolonged high temperature dwells on low cycle fatigue and notched low cycle fatigue 

(LCF) behavior in this class of alloys is thought to be a product of complex interactions of cyclic loading 

damage, surface environmental degradation and embrittlement, as well as the visco-plastic material 

response to temperature and load history. Further, in superalloy gas turbine components, fatigue failures 

can often initiate at holes and other notched features. This is largely driven by the concentration of 

applied stresses and resulting damage due to the geometry of such features. But it can also be influenced 

by numerous other factors, including the effects of microstructure and associated mechanical properties of 

the superalloy. Additionally, the machining processes used to produce the notches could produce altered 

surface roughness, microstructural damage, and residual stresses that also could affect fatigue resistance

(Ref. 7). Therefore, the effects of dwells on fatigue cracking at notches needs to be understood for these 

high temperature alloys. 

The effect of dwells is known also to significantly affect the hold time crack growth behavior of the 

PM disk alloys. It has been shown recently (Ref. 9) that control of microstructural parameters by varying 

the cooling rates from the solution temperature, and also varying subsequent aging treatment has a 

profound effect on dwell crack growth behavior. The relationship between microstructural parameters and 

hold time crack growth has been linked to the effect of these parameters on stress relaxation behavior. 

The relationship between stress relaxation and notch dwell LCF behavior has not been explored and needs 

to be investigated due to the possibility of exhibiting similar relationship as has been identified in case of 

dwell crack growth. 

The objective of this study was to investigate the effect of microstructure, environmental exposure 

and load history on notched LCF behavior of two advanced PM disk superalloys. The investigated 

microstructural parameters were grain size and precipitate size. Notched specimens were machined and 

fatigue tested with cycles incorporating dwells. Fatigue life and failure modes were characterized and 

related to microstructures and associated mechanical properties. 

Materials and Procedure

P/M Processing

The two PM superalloys tested in the study were low solvus high refractory (LSHR) (Ref. 10) and 

ME3 (Refs. 11 to 14) alloys. Two versions of the LSHR alloy were tested. A “subscale LSHR” version

was produced using a subscale atomizer by PCC Special Metals Corp., and passed through screens of 

–270 mesh (53 μm) mesh width to give powder particle diameters of no more than this size. The powder 

was then sealed in a stainless steel container, hot isostatic pressed, and extruded at a reduction ratio of 

6:1. The extrusion billet was machined to segments about 9 cm in diameter and 20 cm long, which were 

isothermally forged into flat LSHR disks about 15 cm in diameter and 4 cm thick by PCC WymanGordon Forgings. 

An additional “production scale LSHR” version (Ref. 15) was also produced, using different powder 

processing procedures. This powder was produced in argon using a production-scale atomizer and 

handling procedures designed to minimize non-metallic inclusion content, by PCC Special Metals Corp. 

This powder was passed through –270 mesh screens, sealed in a container, hot compacted, and extruded 

at a reduction ratio of 6:1. A segment of the extrusion billet was machined to 15 cm diameter and 20 cm 

long, then isothermally forged into a larger flat disk than for the subscale material, about 30 cm in 

diameter and 5 cm thick. The measured compositions of both LSHR heats are listed in Table 1. 

Specimens of another powder metallurgy disk superalloy, ME3 were also obtained. This powder was 

also produced in argon using production-scale atomization and handling procedures designed to minimize 

non-metallic inclusion content. This powder was passed through –270 mesh screens, then sealed in 

containers, hot compacted, and extruded at reduction ratios near 6:1. Extrusion segments were forged into 

NASA/TM—2011-217118 2
larger, contoured disks having diameters of about 61 cm and maximum thicknesses of about 5 cm. The 

average measured composition of ME3 alloy is also listed in Table 1. 

Heat Treatments

To evaluate the influence of microstructural parameters on the mechanical properties, six heat 

treatments of the subscale atomized LSHR disks of 15 cm in diameter and 4 cm thick were performed to 

vary microstructure in this material, as described in Table 2. Four of the conditions used a subsolvus 

solution heat treatment to produce fine grain size, followed by either oil quenching (FGOQ) or fan 

cooling (FGFC) of the disks. Two of the conditions used a supersolvus heat treatment of 1171 °C for 1 h, 

then fan cooling of the disk (CGFC). The disks were then sliced in half, and halves were given either a 

one step aging heat treatment of 775 °C for 8 h or a two step aging heat treatment of 855 °C for 4 h + 775 

°C for 8 h, to vary γ ′ size. The resulting six material conditions of subscale LSHR were therefore 

designated FGOQ1, FGOQ2, FGFC1, FGFC2, CGFC1, and CGFC2. The larger “production scale” 

LSHR disk was given a supersolvus heat treatment of 1171 °C for 1 h and fan cooled, then a single-step 

aging heat treatment of 855 °C for 8 h. 

The ME3 disks were given a supersolvus heat treatment at 1171 °C for 1 h, fan plus oil quench, a 

stabilization heat treatment at 843 °C for 4 h, and an aging heat treatment of 760 °C for 8 h. The 

processing and properties of this material have been previously described and published elsewhere 

(Refs. 13 and 14).

Mechanical Testing

Tensile-stress relaxation, creep rupture, dwell fatigue crack growth, strain-controlled fatigue, and 

dwell notched fatigue tests were performed at 704 °C. Tensile-stress relaxation tests were performed on 

specimens having a gage diameter of 4 mm and gage length of 20 mm in a uniaxial test machine 

employing a resistance-heating furnace and axial extensometer. The tests were performed in general 

accordance with the tensile test specification ASTM E21, using an initial test segment with strain 

increased at a uniform rate of 0.5 percent per min. However, the tests were stopped at 1 percent strain and 

held for 100 h, to measure relaxation of stress as a function of time. The tensile tests were then continued 

to failure, at a faster uniform displacement rate of 1 mm per min., giving an approximate strain rate of 

5 percent per min. across the gage length. 

Combination uniform gage-notched creep rupture specimens (Fig. 1) having a notch elastic stress 

concentration factor Kt = 2 were machined for each LSHR condition. The notch was consistently lowstress ground, then polished parallel to the loading direction to meet or exceed a 0.21 μm (8 μin.) rms 

finish requirement. Six specimens were tested for each material condition at Metcut Research Associates 

in general accordance with creep test specification ASTM E139, using uniaxial lever arm creep testing 

machines with resistance-heating furnaces and extensometers attached to the uniform gage section. These 

creep-rupture tests were run at a constant load giving an initial applied stress of 793 MPa, interrupted at 

0.5 percent strain to verify creep strain and elongation, then continued to failure at NASA GRC. 

Fatigue crack growth specimens had a rectangular gage section 1 cm wide and 0.46 cm thick, with a 

surface flaw on one side of the gage section about 0.356 mm wide and 0.178 mm deep, produced by 

electro-discharge machining (Ref. 16). The fatigue crack growth specimens were then tested at NASA. 

Tests were performed in a closed-loop servo-hydraulic test machine using resistance heating, with 

potential drop measurement of crack growth. Pre-cracking was performed at room temperature, then tests 

were performed at elevated temperatures using a maximum stress of 620 MPa, stress ratio of 0.5, and 

dwell time of 90 s at maximum stress in each cycle. 

Notched fatigue tests of LSHR were performed on cylindrical notched specimens (Fig. 1) having a 

geometric stress concentration factor Kt = 2.0. The notch was consistently low-stress ground, then 

polished parallel to the loading direction to a 0.21 μm (8 μin.) finish. Notched fatigue tests of ME3 were 

performed on slightly different cylindrical notched specimens (Fig. 1) having a geometric stress 

NASA/TM—2011-217118 3
concentration factor Kt of 2.0. Notch radius, diameter, and roughness were measured on all specimens 

before testing to insure consistency. All specimens were tested using uniaxial closed-loop servo-hydraulic 

testing machines with resistance heating furnaces at NASA GRC, in accordance with load-controlled 

fatigue test specification ASTM E466. 

Six tests were performed for each material condition of subscale LSHR, with a baseline dwell fatigue 

cycle having 1 s loading ramps, dwell of 90 s at maximum net section stress (load/notch diameter) of 

793 MPa, and minimum/maximum stress ratio (Rσ = σmin/σmax) of 0.05 in each cycle. Each test was

continued to at least 2,000 cycles. At least two surviving specimens were continued to failure for each 

condition. Additional tests were performed on selected samples of subscale LSHR, full-scale LSHR, and 

ME3 using different cycle profiles (Fig. 2) with dwells at maximum (“maximum dwell”) and minimum

(“minimum dwell”) tensile load, or combined dwells at both maximum and minimum load (“maximumminimum dwells”), with maximum net section stresses of 793 or 855 MPa and Rσ = 0.05. 

Several strain-controlled tests of ME3 were also performed on conventional fatigue specimens having 

a uniform cylindrical gage of 6.4 mm diameter and 19 mm length. These low cycle fatigue (LCF) 

specimens were tested using a uniaxial closed-loop servo-hydraulic testing machine with a resistance 

heating furnace and axial extensometer. The tests were performed according to strain-controlled fatigue 

test specification ASTM E606, with strain controlled to fixed limits. A triangular waveform was 

employed to vary strain at a frequency of 0.5 Hz over a total strain range of 1.0 percent at a strain ratio 

(Rε = εmin/εmax) of 0. Dwells of 90 s were superimposed in each cycle at maximum or minimum strain.

Fractographic and Microstructural Evaluations 

Fracture surfaces of fatigue specimens were evaluated by scanning electron microscopy (SEM). 

Crack initiation origins and crack propagation failure modes were identified and characterized.

Metallographically-prepared sections from selected specimens were used to determine the grain sizes 

according to ASTM E112 linear intercept procedures using circular grid overlays, and as-large-as (ALA) 

grain sizes were determined using E930. Precipitate microstructures were compared using transmission 

electron microscopy (TEM) in dark field imaging conditions, of grains oriented with an <001> zone axis 

nearly perpendicular to the electrochemically-thinned foil. Statistical analyses of variance were performed 

using JMP (SAS Institute, Inc.) software, with significance assessed at a probability p = 0.05, 

representing 95 percent confidence. 

Results and Discussion

Material Microstructures

The various heat treatments performed on the LSHR alloy did vary both grain size and the precipitate 

size, as listed in Table 2. Typical grain microstructures are shown in optical images from etched 

metallographic sections of test specimens in Figure 3. Subscale LSHR samples had mean intercept grain 

sizes near ASTM 12.4 (4.4 μm) for subsolvus heat treatment conditions, and ASTM 8.9 (14.5 μm) for 

supersolvus heat treatment conditions. Production-scale LSHR samples had a mean intercept grain size of 

29 μm (ASTM 6.9). The ME3 production-scale samples had mean intercept grain sizes of 28 μm 

(ASTM 6.9). 

TEM dark field images of subscale LSHR sections showing γ ′ precipitates are compared in Figure 4 

and detailed in Table 3. As shown, mean equivalent radius of mean secondary γ ′ precipitates was larger 

for fan cooled than oil quenched fine grain disks, and coarse grain fan cooled disks had the largest mean 

size. Mean radius of tertiary γ ′ precipitates increased in going from the single to two step aging heat 

treatments, while their numbers appeared to decrease. Images for production-scale LSHR and ME3 are 

also shown in Figure 4. Mean tertiary γ ′ precipitate radius of production scale LSHR was similar to that

of the subscale conditions having two-step aging heat treatments. Secondary γ ′ precipitate size was larger 

NASA/TM—2011-217118 4
for the production scale LSHR, due to the larger disk size and slower corresponding cooling rate during 

fan cooling. ME3 had intermediate secondary and tertiary γ ′ precipitate sizes. 

Baseline Dwell Fatigue Resistance as a Function of 

Material Condition in Subscale LSHR

Fatigue Life

Fatigue life for the baseline dwell fatigue cycle is compared for the six different subscale LSHR 

material conditions studied, in Figure 5 (Ref. 10). Notch dwell fatigue testing of the first twelve 

specimens, two from each condition, produced interesting divergent results. Three of twelve failed in less 

than 500 cycles, while all others failed well over 10,000 cycles. Based on these results, the remaining 24 

tests were run to 2,000 cycles to screen for more of the short life failures, having lives less than a 500 

cycles. 

All the short life failures were confined to two of the single-step aging heat treatments investigated, 

FGOQ1 and CGFC1. All specimens of these two material conditions were continued to failure, as well as 

those for the FGFC2 condition, to compare their distributions in lives. As seen in Table 4 and Figure 5, 

the population of failures was bimodal for the FGOQ1 and CGFC1 material conditions, with 7 of 12 

specimens failing in less than 500 cycles and the remaining specimens reaching lives up to 100,000 

cycles. Meanwhile, all failures for the FGFC2 condition were near 35,000 cycles. 

Comparing the respective material conditions, it was evident that the wide scatter in life did not show 

a consistent dependence on solution heat treatment temperature and resulting grain size. The wide scatter 

was observed in conditions having fast and slow quench cooling rates. It was present for some, though not 

all conditions having a one step aging heat treatment, and absent for conditions having a two step aging 

heat treatment. Thus, conditions with wide scatter in fatigue life did not possess consistent corresponding 

heat treatment, grain size, or secondary γ ' size. Therefore, other factors were considered, focusing on 

what differences segregated the FGOQ1 and CGFC1 conditions, and could help explain their wide 

variations in notched dwell fatigue life. 

Failure Initiation Modes

Typical failure initiation sites are shown in Figure 6. FESEM images from longitudinal sections are 

shown in Figure 7. Surface cracks initiated from grain boundaries or non-metallic inclusions along the 

notch root to cause failures for both short and long life tests for specimens of both FGOQ1 and CGFC1 

conditions, as well as for the other material conditions. Examination of the notch surfaces adjacent to 

fatigue cracks and of metallographically-prepared longitudinal sections of failed specimens indicated 

multiple oxidized secondary cracks occurred along grain boundaries (Fig. 7), extending in from the notch 

surface for all material conditions. However, specimens having short lives did not display as many 

secondary cracks. The inclusions were usually alumina, either singular (Type 1) or granulated (Type 2) 

inclusions (Ref. 3). Their occurrence as crack initiation sites in some specimens having both long and 

short lives indicated they were not the singular cause of short lives, but were a complicating factor. After 

initiation, cracks predominantly grew along grain boundaries. Overall, it was clear that the wide scatter in 

life did not consistently correlate with a change of the predominant failure modes. 

Examinations of the notch surfaces were conducted to determine whether the variability in the notch 

radius or surface roughness correlated with NLCF behavior. Specimen machining specifications for the 

notch permitted a tolerance and thereby variations in notch roughness as well as radius and associated 

stress concentration factor. However, notch radius and roughness had been measured, and found to be 

comparable for all material conditions. Radius and valley roughness (Rv) of the notches did not 

significantly correlate with fatigue life among specimens for the FGOQ1 and CGFC1 conditions, Figure

8. Hence, the wide scatter in lives could not be attributed to variations in notch roughness or radius. Cold 

work could also be qualitatively compared, by viewing distortion of the near-surface microstructure in 

longitudinal sections of several failed specimens. As shown in Figure 7, distortion of the near surface 

NASA/TM—2011-217118 5
microstructure was comparably small for both short and long life specimens of FGOQ1 and CGFC1 

material conditions. However, these notches were too small and constricted for measuring residual stress 

in the loading direction by x-ray diffraction. 

Correlations with Other Mechanical Properties

Fatigue crack growth tests with 90 s dwells at maximum stress were performed to assess the dwell 

fatigue crack growth behavior. Since the notch LCF life consists of both crack initiation and propagation 

portions, it was important to quantify the crack propagation to ascertain whether the differences in dwell 

crack growth could possibly explain the variation in NLCF lives. Fatigue crack growth rates are compared 

versus stress intensity factor in Figure 9. The six material conditions produced a range of 10x in crack 

growth rate response. The FGOQ1 condition exhibited the highest crack growth rates, while the CGFC2 

condition had the slowest. However, the CGFC1 scatter condition had intermediate crack growth rates.

Crack growth occurred by predominantly intergranular failure for all material conditions, Figure 10. So 

the FGOQ1 and CGFC1 conditions having wide scatter in life did not consistently have the highest crack 

growth rates, and had the same general intergranular failure mode as other conditions. Therefore, crack 

initiation rather than crack growth characteristics played a more significant role in explaining the wide 

scatter of notch dwell fatigue life. 

Lives for constant load tests of combination uniform gage-notched creep rupture specimens, tested at 

the same stress as for the fatigue tests, are compared in Table 5 and Figure 11. The FGOQ1 and CGFC1 

conditions did have highest 0.2 percent creep and rupture lives. However, only one specimen of the 36 

tested failed at the notch, a CGFC1 specimen with a very short life of only 5.8 h. All other specimens 

failed in the uniform gage section at over 100 h. Single tests of notched specimens at the same notch 

section stress indicated notched rupture lives were at least usually twice those of the uniform gage tests.

However, notch lives could be near or lower for the FGOQ1 and CGFC1 conditions. Associated rupture 

ductilities are also compared in Figure 11. The FGOQ1 and CGFC1 conditions had lowest rupture 

ductilities, while FGFC2 and CGFC2 conditions had highest values. Failure modes are compared in 

Figure 12. Intergranular surface cracks caused failures for all conditions. Hence, these rupture tests 

indicated the wide scatter in dwell fatigue life occurred for material conditions consistently having high 

monotonic creep resistance, and low rupture ductility. 

Tensile yield and ultimate strengths are compared along with tensile ductility for the six LSHR

conditions in Table 6 and Figure 13. The FGOQ1 and CGFC1 conditions had highest ultimate strengths

of the six conditions. The FGOQ1 condition also had highest yield strength, while the CGFC1 condition 

intermediate yield strength. The CGFC2 condition had the lowest strength, but highest ductility. The 

FGOQ1 condition had lowest ductility of the six conditions, while the CGFC1 condition had intermediate 

ductility. As shown in Figure 14, small intergranular surface cracks initiated for all conditions, while 

microvoid coalescence occurred over much of the cross-section. Therefore, conditions giving the wide 

scatter in fatigue life did not consistently have the highest yield strength or lowest ductility, but did have

consistently higher ultimate tensile strength. 

These tensile tests had been interrupted at 1 percent strain, to measure stress relaxation as a function 

of time, shown in Figure 15. The FGOQ1 and CGFC1 conditions producing the bimodal life distribution 

in max dwell fatigue life were more resistant to stress relaxation than all other conditions, and maintained 

higher stresses with increased time. The FGFC2 and FGOQ2 conditions had the most stress relaxation.

Prior work on LSHR has shown that stress relaxation resistance was correlated with tertiary γ ′ size

(Ref. 9). Comparisons for the FGOQ1 and CGFC1 conditions suggest the tertiary γ ′ do have finer radius

values of 0.0065 to 0.011 μm and higher contents of several percent, compared to those values for other 

tested conditions at the same grain size, Figure 4. Variations in this microstructural feature could 

therefore be responsible in part for the bimodal life response. However, Figure 4 shows the FGFC1 

condition had fine tertiary γ ′ size comparable to that of FGOQ1 and CGFC1, due to their common single 

step aging heat treatment, yet it had no evidence of the bimodal life response in the present tests. 

Therefore, it may be more likely that the stress relaxation response itself is linked to this bimodal life 

NASA/TM—2011-217118 6
behavior, which can be influenced by various combinations of secondary and tertiary γ ′ size and area 

fraction distributions within the material. 

Summarizing the correlations with other mechanical properties, the FGOQ1 and CGFC1 conditions 

having wide scatter in notched max dwell fatigue lives had highest ultimate tensile strengths, highest 

creep and rupture lives, and highest stress relaxation resistances, yet lowest rupture ductilities. The 

combination of high stress relaxation resistance with low rupture ductility appeared important in 

explaining the bimodal fatigue life response.

Dwell Fatigue Resistance as a Function of Dwell Cycle Profile 

Subscale, Fine Grain LSHR

To further examine how dwells influence fatigue life response, additional tests were performed on 

selected samples of LSHR to examine the effects of different cycle profiles on fatigue life. Samples of the 

FGFC2 condition, which displayed low scatter in baseline dwell fatigue life, were tested at the same 

maximum stress of 795 MPa using cycle profiles that varied the location of the dwells. Three additional 

NLCF load profiles were investigated: 1) tests performed with 90 s dwells occurring at the minimum 

load; 2) 90 s maximum hold followed by a 90 s minimum hold; and 3) 45 s maximum hold followed by a 

45 s minimum hold. 

The resulting NLCF fatigue lives and the associated load histories are compared in Figure 16. All six

baseline dwell fatigue cycle tests were run to failure, and had shown to reduce life about 10x (90 percent) 

from cyclic tests run at the same maximum stress and stress ratio at a frequency of 10 Hz. Surprisingly, 

tests with 90 s dwells at minimum tensile stress had significantly lower lives, about 10x (90 percent) 

lower than that of the baseline dwell fatigue cycle. This significant reduction in life in comparison to the 

maximum dwell specimens also occurred for the two other wave forms in which minimum load holds 

were incorporated.

Failure initiation modes for these alternative cycles are compared for FGFC2 LSHR in Figures 17 and 

18. A 90 s dwell at minimum stress resulted in a significant increase in crack initiations occurring along 

the notch root, compared to maximum dwell and maximum-minimum dwell cycles, Figure 17. The modes 

of crack initiation are compared in Figure 18. The 90 s minimum dwell activated an enhanced, 

transgranular failure initiation mode. The cracks appeared associated with sites of local, concentrated

oxidation. These cracks initiated at numerous locations along the notch root, and transgranular cracks 

were also evident adjacent to the fracture surface, Figure 19. Tests with dwells at maximum stress or at 

both maximum and minimum stress often failed from cracks initiating at surface grain boundaries.

However, some of these tests also had failure initiations at inclusions and pores near the notch root. These 

defects were related to the pilot-scale atomization process used to produce powder for this material, and 

were a common fatigue failure initiation site due to this material’s fine grain size (Ref. 3). 

The crack propagation failure mechanism in these specimens was predominantly intergranular for 

maximum dwell and maximum-minimum dwell tests. Conversely, cyclic no dwell and minimum dwell 

tests had more transgranular crack growth near the initiation points. However, this trend was complicated 

by failures initiating from inclusions that sometimes were located at a small distance below the notch 

surface. In all cycle profiles, cracks at these inclusions would grow in a predominantly transgranular 

mode, until the resulting circular crack intersected the notch surface. The environment then appeared to 

alter the crack growth mode, activating intergranular cracking for tests with any dwell time at maximum 

stress. 

In general, creep-fatigue-environment interactions and damage are known to occur during dwell 

fatigue in superalloys (Refs. 17 and 18). Such damage could be expected in these disk superalloys tested 

in the notch fatigue test conditions evaluated. Imposition of a 90 s maximum dwell reduced the life by 

approximately an order-of-magnitude in comparison to the cyclic tests (Fig 16). This reduction of life 

could have been caused by either the creep, fatigue, environmental damage or an interaction of these 

mechanisms. 

NASA/TM—2011-217118 7
The use of both maximum and minimum dwell load histories allowed for separate evaluations of 

damage contributions for the test conditions studied. While the maximum dwell test could encourage 

creep, fatigue, and environmental interactions and damage, the minimum dwell test clearly eliminates 

creep damage at high applied stresses. Yet, cyclic life significantly decreased, not increased, by 

eliminating the creep damage. This indicated creep deformation was not limiting dwell cyclic life, and 

that fatigue-environment interactions and damage were predominantly limiting life in all of these dwell 

fatigue cycles for fine grain LSHR material condition. 

Similar results were recently published for notched dwell fatigue tests of another fine grain powder 

metallurgy disk superalloy, RR1000 (Ref. 8). While tests of rectangular double-edge notched specimens 

at 650 °C with shorter 10 s dwells at maximum stress did not reduce mean life from that of no dwell 

cyclic tests, 10 s dwells at minimum stress reduced life nearly 10x (90 percent). Enhanced crack 

initiations were also observed for this minimum dwell cycle. 

Production Scale, Coarse Grain LSHR

Additional LSHR material was then tested in a similar manner, to investigate the effects of altered 

powder processing, heat treatment, and resulting microstructure on this response. This material had been 

produced using production-scale powder atomization and handling procedures, carefully designed to 

minimize inclusion content (Ref. 19). The resulting forged disk was given a supersolvus solution heat 

treatment to produce a coarse grain size, which had been shown in prior work to be much less prone to 

inclusion-initiated fatigue failures than fine grain material (Ref. 20). This was intended to minimize the 

chances for any inclusion-initiated failures near the notch surface, which had complicated failure mode 

evaluations in subscale LSHR. This disk was fan cooled, and then given a single aging heat treatment at

the high aging temperature of 855 °C, but for a longer time of 8 h (Ref. 15). This aging heat treatment 

was intended to determine if the results obtained for the CGFC2 condition in the baseline dwell fatigue 

tests could be further improved by altered ageing. 

Testing again was performed with 90 s dwells at maximum stress, minimum tensile stress, or split 

between maximum and minimum stress. The NLCF fatigue results are compared in Figure 20. Again, 

cycles with partial or total dwell times at minimum stress had significantly lower lives, up to 10x 

(90 percent) below that of the baseline maximum dwell fatigue cycle. This indicated creep damage was 

not limiting maximum dwell cyclic life, and that fatigue-environment interactions and damage were 

apparently limiting life in all of these dwell fatigue cycles for both fine- and coarse-grain LSHR. 

Failure initiation modes for the alternative dwell cycles are compared for this production scale, coarse 

grain LSHR in Figures 21 and 22. Cracks again initiated at numerous locations along the notch root for 

the minimum dwell cycle, both at and below the fracture surface, Figure 21. Fewer cracks initiated failure 

for cycles with dwells only at maximum stress or at both maximum and minimum stress. For this coarse

grain LSHR material, minimum dwell tests again activated transgranular failure initiations at sites of 

local, concentrated oxidation, Figure 22. Subsequent crack propagation was predominantly transgranular. 

Maximum dwell tests again had intergranular surface crack initiations and crack growth, but without 

inclusion-initiated failures as observed in subscale LSHR. Maximum-minimum dwells had more 

transgranular surface crack initiations, which transitioned within 100 μm depth to intergranular crack 

growth. 

Therefore, both fine- and coarse-grain materials displayed lower fatigue lives when subjected to cycle 

profiles containing dwells at minimum applied stress. Minimum dwells promoted a higher frequency of 

cracks initiating along the notch surface than for maximum dwells. The crack propagation failure mode 

for minimum dwell cycles was transgranular in nature. From these results, it appeared that fatigueenvironment damage interactions that drove crack initiation to limit life for tests with dwells at minimum 

stress were not concentrated at grain boundaries. However, subsequent crack propagation was affected 

differently by cycle profile. Crack propagation became intergranular in the presence of dwells at 

maximum stress, as observed in numerous prior studies of dwell crack growth at high temperatures 

(Refs. 18, 19, 21, and 9). This intergranular crack propagation would be more rapid for maximum stress 

dwells than the transgranular crack propagation from minimum dwells, due to sustained times at high 

NASA/TM—2011-217118 8
dwell stresses which could drive crack growth. This could help explain the lower lives observed for 

maximum-minimum dwell tests than for minimum dwell tests, for both fine- and coarse-grain LSHR. 

Production Scale, Coarse Grain ME3

Notched fatigue specimens from another disk superalloy, ME3 (Refs. 11 and 12), were also obtained 

from larger, contoured disks. This material had been produced using production-scale powder 

atomization, handling, consolidation, forging, and supersolvus solution heat treatment procedures giving a 

coarse grain size (Ref. 13). 

The effects of cycle profiles having 90 s dwells on fatigue life of ME3 were again assessed at 704 °C, 

using load cycles with a ramp of 1.5 s and Rσ = 0.05, but at a higher maximum stress of 855 MPa. The 

effects of maximum and minimum stress dwell cycles on life are shown in Figure 23. Lives at this higher 

stress level were lower than for LSHR, and maximum stress dwells reduced mean fatigue life by about 4x 

(75 percent) from 0.5 Hz cyclic tests. However, minimum stress dwells again further reduced mean 

fatigue life, giving 10x (90 percent) reduction in mean life from 0.5 Hz cyclic tests. 

Unlike LSHR, mostly intergranular failure initiations were observed for ME3 under all dwell test

conditions, Figure 24. For 90 s minimum dwell tests, three out of the four specimens tested exhibited 

intergranular initiation, with the fourth specimen exhibiting transgranular initiation. Relatively few failure 

initiation sites were present for cyclic no-dwell and maximum dwell tests. However, many failure 

initiation sites were observed along the notch surface for minimum dwell tests. Transgranular propagation 

was apparent for cyclic and minimum dwell, but intergranular crack propagation was observed following 

maximum stress dwell tests. This suggested the tensile load ramp was responsible for crack propagation 

in the minimum stress dwell test. 

To determine the early stages of the crack initiation failure mode, an interrupted minimum dwell test 

was performed. The test was stopped periodically with the specimen placed in the SEM to determine the 

onset of cracking and the subsequent damage mechanisms. As shown in Figure 25, surface intergranular 

cracks initiated very early, and were present within the first 10 percent of fatigue life. Additional 

intergranular cracks initiated throughout the test. With continued cycling, cracks grew and were often 

linked. 

The difference in failure initiation modes observed for minimum dwells in coarse grain ME3 and 

LSHR could be related to the higher stress of 855 MPa applied to ME3, in comparison to 793 MPa 

maximum stress applied to LSHR specimens. ME3 and LSHR did have comparable crack propagation 

mode response for varied cycle profiles. For both alloys, cyclic no-dwell and minimum dwell tests 

exhibited transgranular crack propagation, while cycles with dwells at maximum stress propagated by an

intergranular crack propagation mechanism. 

Strain-Controlled Tests of ME3 

Cyclic dwell testing was conducted on uniform gage ME3 specimens to ascertain stress relaxation 

behavior under both maximum and minimum dwell conditions. The testing had total strain controlled 

using cyclic waveforms having no dwells, 90 s dwells at maximum tensile strain, or 90 s dwells at

minimum tensile strain. These tests were intended to aid understanding of how fatigue response is 

affected by dwells in a notch, where the plastic zone at the root of a notch is constrained by the mostly 

elastic surrounding material, to constrain notch strains (Ref. 22). 

Maximum and minimum stress for the strain-controlled tests as a function of cycles is compared for 

tests at 1.0 percent strain range in Figure 26. Tests with maximum strain dwells exhibited stress relaxation 

during each dwell, and both maximum and minimum stresses rapidly moved down in stress. Maximum 

stress was reduced by 20 percent after 200 cycles. Tests with minimum strain dwells had sustained 

maximum and minimum stresses, and remained relatively unchanged after 1,000 cycles. In similar tests of 

a prior study of ME3 at a lower 0.7 percent strain range (Ref. 23), which were continued to failure, the 

peak tensile cyclic stress in the maximum dwell test was reduced in a similar manner, which explained its 

very long life of about 100,000 cycles. In contrast, the corresponding min strain dwell cycle tests did not 

NASA/TM—2011-217118 9
show any significant stress relaxation, which resulted in considerably lower fatigue life of about 20,000 

cycles, Figure 27. These collective results indicated dwells at minimum strains maintained higher peak 

stresses, which produced lower fatigue lives than for dwells at maximum strains. Similar effects would be 

expected in notches. However, effects of the stress concentration in a notch still had to be considered, 

using finite element modeling. 

Finite Element Modeling of Stress Evolution in Notch of ME3

Finite element modeling was performed to model the notch dwell low cycle fatigue behavior of ME3 

in an attempt to explain the unexpected behavior of the minimum dwells producing larger LCF life debit 

than maximum dwell tests. Two-dimensional axisymmetric finite element elastic-visco-plastic models 

were generated for the notched specimen. Stress-strain tensile data from uniform gage monotonic tensilestress relaxation tests was used to model visco-plastic behavior in the notch specimens, and monotonic 

tensile stress-time relaxation data was used to model stress relaxation during the 90 s dwells for the notch. 

Estimated maximum axial stresses generated at the notch root for ME3 specimens are compared as a 

function of maximum and minimum dwell cycles in Figure 28. As shown, tests with max dwells exhibited 

substantial stress relaxation during the maximum dwells, with the highest rate of stress relaxation

occurring during the initial cycles. In comparison for the minimum dwell tests, the maximum stress 

remained almost constant throughout duration of the test. These analytical results were consistent with 

results of the strain-controlled tests of ME3, but now also estimated the effects of notches on local, 

concentrated stresses near the notch. These results indicated the minimum dwell cycle maintained higher 

peak stresses at the notch root than the maximum dwell cycle for which the high stresses only lasted in 

the first few cycles. Thus if the cracking did not initiate in the first few cycles of the maximum dwell 

NLCF test it is plausible that the subsequent reduction in the maximum stress during the dwells hindered 

the initiation and resulted in longer NLCF lives. In contrast, it is suspected that the high maximum

stresses of the minimum dwell tests caused earlier initiations and led to lower NLCF lives. 

Effect of Environment

The argument that the enhanced stress relaxation of the stresses during the maximum holds may 

explain the longer lives of the maximum dwell tests in comparison to the minimum dwells, but still does 

not account for the substantially larger number of cycles-to-failure observed in the no-dwell specimens in 

comparison to the minimum dwell specimens. The cyclic stresses estimated at the notch root for ME3 in 

minimum dwell and no-dwell fatigue tests at the same applied loads are compared in Figure 29. 

Maximum and minimum stresses at the notch root were sustained, and quite comparable for these two

cases with continued cycling. However, the minimum dwell cycle produced significantly lower cyclic life 

than the no-dwell cycle.

This illustrated the strong effect of environment on fatigue life under these conditions. This signifies

that fatigue-environment interactions and damage during dwells can severely limit notch fatigue life in the 

various dwell fatigue cycles performed here on fine grain LSHR, coarse grain LSHR, and coarse grain 

ME3, after accounting for stress evolution at the notch. 

However, tests at higher temperatures and stresses could also activate other failure modes such as netsection creep rupture, while tests at lower temperatures could activate failure modes such as grain facet 

failures. Therefore, additional dwell fatigue testing and characterization would be necessary before 

extrapolating the present results to other test conditions. 

Additional Work 

Additional finite element modeling could be performed to estimate the maximum axial stresses 

generated at the notch root as a function of maximum dwell cycles for the FGOQ1 and FGFC2 conditions 

of subscale LSHR. The difference in monotonic stress relaxation at the notch root for the FGFC2 and 

FGOQ1 heat treatments could be reflected in cyclic stress relaxation rates. This could help explain the 

NASA/TM—2011-217118 10
different notched LCF behaviors for these two heat treatments, where the FGOQ1 heat treatment showed 

a bimodal distribution of dwell NLCF lives with half of the specimens failing within few hundred cycles, 

and the other half exhibiting two orders of magnitude longer lives. The lives of the FGFC2 specimens did 

not show this type of behavior with all the tested specimens exhibiting lives greater than 10,000 cycles. 

Additional damage characterization work could help to increase understanding and enable modeling 

the effects of dwells on notched fatigue life. Careful longitudinal sectioning of secondary cracks in failed 

specimens, by metallographic preparation and other microscopy, could be used to examine multiple 

secondary fatigue cracks at various stages of formation and growth, to explain how the cracks initiate and 

grow. For the current materials and test conditions, additional interrupted testing could then be performed 

using various cycle profiles to quantify exactly how much crack initiation life is reduced by dwells, and 

what fatigue-environment damage feature is first to fail. With this knowledge a critical parameter of the 

damage feature could be derived, such as size necessary for failure, and how this parameter varies with 

time and cycles. Tests at other stresses and temperatures would provide the data to determine how this 

critical parameter varies with stress and temperature, and where other failure modes may emerge. Tests 

could also be performed using more complicated cycle profiles, which more closely simulate the dwell 

and cyclic segments of service cycles for aerospace gas turbine engines. These cycle profiles could 

eventually incorporate the temperature variations that occur in turbine engine cycles. Specimens with 

larger notches having open access could be designed and tested to allow measurements of residual stress 

and plasticity in the loading direction by x-ray diffraction both after machining and after interrupted 

fatigue testing. 

This information could then be used as input for a fatigue life prediction model. The time and cycledependent growth of the life-limiting damage features could be modeled, as a function of notch geometry, 

surface residual stress and plasticity, cycle profile, applied stress and temperature. 

Using these tools, strategies for improving dwell notched fatigue life could then be designed for 

experimentation. Heat treatments, machining, and final surface treatments could be modified to lower the

stresses generated in notches that drive crack initiation and growth, for example by modifying yield 

strength and enhancing stress relaxation. Superalloy composition could possibly be modified to hinder

formation of the critical fatigue-environment damage features. Alternatively, surface composition 

modifications or fatigue-resistant environmental barrier coatings could be evaluated. 

However, potentially enhanced dwell notch fatigue cracking properties would need to be balanced

with many other mechanical properties of importance for mechanical design of disks. This would include 

monotonic strength and creep resistance, as well as cyclic fatigue crack initiation and growth properties, 

at both uniform and notched locations. 

Summary and Conclusions

The influences of heat treatment conditions and cycle profile on the notch fatigue resistance of 

powder metallurgy disk superalloys were investigated in both fine- and coarse-grain LSHR alloy and in a 

coarse-grain ME3 alloy. Several heat treatment conditions resulted in bimodal life distributions in 

baseline maximum dwell fatigue life, while others had more consistent fatigue lives. The bimodal 

distribution was characterized by some specimens failing within few hundred cycles, and others lasting 

two orders-of-magnitude longer. The bimodal failure distribution in baseline notch dwell fatigue life was 

present for only the heat treated material conditions that were resistant to stress relaxation and had high 

strength and low rupture ductility. 

Shifting of the dwells from maximum load to minimum load substantially lowered NLCF life for both 

alloys at the tested material conditions. Minimun dwells promoted both early crack initiation and 

exhibited much greater number of crack initiation sites. The results pointed to a strong fatigueenvironment interaction causing material damage which limited dwell fatigue life for all dwell test 

conditions studied. These results could be explained by considering the evolution of peak stresses in the 

notch with continued dwell fatigue cycling, and indicated dwell fatigue life in notches was limited by an 

interaction of fatigue and environmental damage. 

NASA/TM—2011-217118 11
It can be concluded from this work that: 

1) Dwells reduce fatigue life in notches largely through fatigue-environment damage interactions, for 

these fine- and coarse-grain superalloy materials and test conditions.

2) The effects of dwells in reducing fatigue life at high temperatures can be influenced by varying 

cycle profiles; profiles and service cycles having dwells near minimum load can more severely reduce 

life.

3) Testing and modeling indicate this can be due to different evolutions of cyclic stresses in notches 

during dwell cycles, dependent on the dwell cycle profiles as well as the materials’ stress-strain and stress 

relaxation responses. 

4) Wide scatter can be observed in dwell fatigue life for materials having high strength, creep, and 

stress relaxation resistance subjected to dwells at maximum load. This can be due to both the limited 

stress-relaxation of cyclic peak stresses which these creep resistant heat treatments exhibit as well as their 

low and variable rupture ductility. Combination of these two characteristics can result in pre-mature 

intergranular cracking or rupture leading to a low life NLCF test result. 

5) Dwell fatigue life is strongly influenced by the interaction of cyclic stresses in notches and 

environmental attack processes, so both aspects should be considered in modeling and prediction of 

fatigue life. 

References

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Minerals, Metals & Materials Society, Warrendale, PA, 2008, pp. 807–816. 

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2007-215046, Washington, D.C., October, 2007. 

NASA/TM—2011-217118 12
11. D.P. Mourer, J.L. Williams, “Dual Heat Treat Process Development for Advanced Disk 

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Alloy LSHR,” NASA/TM—2005-213645, Washington, D.C., June, 2005. 

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NASA/TM—2011-217118 13
TABLE 1.—ALLOY COMPOSITIONS IN WEIGHT PERCENT

Wt. % Al B C Co Cr Mn Mo Nb Ta Ti V W Zr Ni

Subscale LSHR 3.60 0.02 0.03 20.5 12.4 0.09 2.8 1.5 1.2 3.50 0.1 4.3 0.05 Bal.

Prod. scale LSHR 3.70 0.03 0.03 20.8 12.6 0 2.44 1.4 1.64 3.49 0 4.3 0.05 Bal.

ME3 3.31 0.03 0.06 20.6 12.8 0 3.85 0.9 2.3 3.54 0 2.01 0.05 Bal.

TABLE 2.—HEAT TREATMENTS OF THE DIFFERENT MATERIALS

Material condition Solution step Quench Age

LSHR FGOQ1 1135 °C / 1.5 h Oil 775 °C / 8 h 

LSHR FGOQ2 1135 °C / 1.5 h Oil 855°C / 4 h + 775 °C / 8 h 

LSHR FGFC1 1135 °C / 1.5 h Fan air 775 °C / 8 h 

LSHR FGFC2 1135 °C / 1.5 h Fan air 855 °C / 4 h + 775 °C / 8 h 

LSHR CGFC1 1171 °C / 1.5 h Fan air 775 °C / 8 h 

LSHR CGFC2 1171 °C / 1.5 h Fan air 855 °C / 4 h + 775 °C / 8 h 

Prod. scale LSHR 1171 °C / 1.5 h Fan air 855 °C / 8 h 

ME3 1171 °C / 1 h Fan air + oil 843 °C / 4 h + 760 °C / 8 h 

TABLE 3.—TYPICAL GRAIN SIZE AND γ ′ PRECIPITATE SIZES: 

MEAN VALUE ± ONE STANDARD DEVIATION. 

Material condition Mean linear 

intercept grain 

size no.

Equivalent

grain size,

μm 

Mean secondary γ ′

radius,

μm

Mean tertiary γ ′

radius,

μm

LSHR FGOQ1 12.5 ± 1.1 4.2 0.085 ± 0.016 0.0107 ± 0.0035 

LSHR FGOQ2 12.3 ± 1.1 4.6 0.085 ± 0.021 0.0169 ± 0.0049 

LSHR FGFC1 12.4 ± 1.1 4.3 0.089 ± 0.019 0.0118 ± 0.0048

LSHR FGFC2 12.5 ± 1.0 4.2 0.088 ± 0.024 0.0160 ± 0.0036 

LSHR CGFC1 8.9 ± 1.8 14.7 0.122 ± 0.025 0.0065 ± 0.0031 

LSHR CGFC2 8.9 ± 1.7 14.4 0.137 ± 0.031 0.0151 ± 0.0035 

Prod. scale LSHR 6.9 ± 1.9 29 0.172 ± 0.058 0.0156 ± 0.0043 

ME3 (Ref. 13) 6.9 ± 0.3 29 0.097 ± 0.021 0.0133 ± 0.0034

TABLE 4.—SUBSCALE LSHR CREEP RUPTURE RESULTS AT 704 °C/793 MPa: 

MEAN VALUE ± ONE STANDARD DEVIATION (NUMBER OF TESTS). 

Condition 0.2% creep, 

h 

Gage rupture, 

h 

Elongation, 

% 

Notch rupture, 

h 

FGOQ1 114.5 ± 20.5 (2) 333.7 ± 74.8 (6) 8.9 ± 2.0 (6) 400.7 (1) 

FGOQ2 16.7 ± 1.3 (2) 135.1 ± 16.7 (6) 11.6 ± 2.0 (6) 425.3 (1) 

FGFC1 79.2 ± 18.2 (2) 271.4 ± 37.4 (6) 12.6 ± 2.9 (6) 569.5 (1) 

FGFC2 8.8 ± 0.9 (2) 127.2 ± 10.3 (6) 14.5 ± 1.5 (6) 563.0 (1) 

CGFC1 104 (1) 387.4 ± 34.8 (5) 10.1 ± 5.4 (5) 5.8, 3687.9 (2) 

CGFC2 16.4 ± 1.0 (2) 327.1 ± 80.1 (6) 13.1 ± 6.3 (6) 2273.0 (1)

NASA/TM—2011-217118 14
TABLE 5.—TENSILE PROPERTIES AT 704 °C: MEAN VALUE ± ONE 

STANDARD DEVIATION (NUMBER OF TESTS)

Material condition 0.2% yield strength,

MPa

Ultimate strength,

MPa

Elongation, 

% 

Reduction in area,

% 

LSHR FGOQ1 1190 ± 26 (2) 1388 ± 23 (2) 7.3 ± 1.1 (2) 12.1 ± 3.6 (2) 

LSHR FGOQ2 1168 ± 4 (2) 1329 ± 3 (2) 9.3 ± 0.5 (2) 14.6 ± 0.5 (2) 

LSHR FGFC1 1129 ± 8 (2) 1327 ± 15 (2) 8.3 ± 1.8 (2) 12.7 ± 2.4 (2) 

LSHR FGFC2 1117 ± 10 (2) 1296 ± 19 (2) 10.7 ± 2.4 (2) 18.3 ± 4.6 (2) 

LSHR CGFC1 1073 ± 6 (2) 1367 ± 12 (2) 13.7 ± 0.9 (2) 18.4 ± 3.4 (2) 

LSHR CGFC2 1029 ± 3 (2) 1334 ± 24 (2) 18.0 ± 1.4 (2) 29.6 ± 4.8 (2) 

Prod. Scale LSHR 993 ± 59 (4) 1335 ± 48 (4) 17.2 ± 6.1 (4) 23.2 ± 9.5 (4) 

ME3 (Ref. 13) 1007 ± 49 (2) 1303 ± 39 (2) 18.5 ± 3.1 (2) 22.5 ± 8.8 (2) 

NASA/TM—2011-217118 15
30°

A

127

6.35

Notch

Dia

9.0

Gage

Dia

30

0.92 R

6.35 9 25.4R

12.7 19

19

A

a)

Dia

30°

A

4 52 A 4 52 6 35 3 4R

b)

4.52

Notch

Dia

6.35

Gage

Dia 0.66 R

4.52 A

18.5

4.52 6.35 3.4R

7.9

A

89

13 1 R

178

19 7 2

30°

A

Figure 1.—Notched test specimens, with dimensions in millimeters; a) LSHR notched fatigue specimen; b) LSHR notched creep76 R 12.7

5.1

Notch

Dia

7.2

Gage

Dia 0.81 R

c)

g p , ; ) g p ;) p

rupture specimen; and c) ME3 notched fatigue specimen.

NASA/TM—2011-217118 16
90 s 90 s 45 s

0.1, 3 s

Stress

Figure 2 Cycle profiles used in fatigue tests; a) No dwell 0 5 and 10 Hz; b) 90 s dwell at maximum load (“maximum dwell”); c) 90 s dwell at minimum load

90 s 90 s 45 s

Time

a) b) c) d) e)

Figure 2.—Cycle profiles used in fatigue tests; a) No dwell, 0.5 and 10 Hz; b) 90 s dwell at maximum load ( maximum dwell ); c) 90 s dwell at minimum load

(“minimum dwell”); d) 90 s dwell at maximum and minimum loads; and e) 45 s dwells at maximum and minimum loads.

NASA/TM—2011-217118 17
FGOQ1 FGFC1 CGFC1

FGOQ2 FCFC2 CGFC2

a)

Figure 3.—a) Grain structure for the six conditions of subscale LSHR; and b) Grain structure for production scale LSHR and ME3.

LSHR ME3

b)

NASA/TM—2011-217118 18
S

T

FGOQ1 FGFC1 CGFC1FGOQ2 FGFC2 CGFC2

200 nm a)

Figure 4.—a) Comparison of secondary (S) and tertiary (T) γ for the six conditions of subscale LSHR.

FGOQ2 FGFC2 CGFC2

NASA/TM—2011-217118 19
T

S

LSHR ME3

200 nm

Figure 4.—(Concluded) b) Comparison of secondary (S) and tertiary (T) γ for production-scale 

LSHR and ME3

b)

.

NASA/TM—2011-217118 20
FGOQ1 FGOQ2 FGFC1 FGFC2 CGFC1 CGFC2

Figure 5.—Comparison of fatigue lives for the six conditions of subscale LSHR, tested in baseline 90 s maximum dwell fatigue cycle at a 

maximum net section stress of 790 MPa. Bimodal distribution of dwell fatigue lives are apparent for boxed conditions.

NASA/TM—2011-217118 1
Nf=102 Nf=66,051 Nf=663

GB

IG

T1

GB

Nf=21,237 Nf=38,280 Nf=34,712

a) c) e)

GB

GB

T2

Figure 6.a.—Comparison of typical failure modes for the six LSHR conditions, tested in baseline maximum dwell fatigue cycle; a) FGOQ1; b) FGOQ2; 

c) FGFC1; d) FGFC2; e) CGFC1; and f) CGFC2. GB-Grain Boundary, T1-Type 1 Inclusion; and T2-Type 2 Inclusion.

b) d) f)

NASA/TM—2011-217118 22
Nf=102 Nf=290 Nf=67,083 Nf=52,863

GB T1

GB

GB

T1

a) FGOQ1

Nf=194 Nf=83,147 Nf=386

GB

Nf=663

T2

GB

T1

Figure 6.b.—Comparison of grain boundary and inclusion failure modes for a) FGOQ1 and b) CGFC1 conditions of subscale LSHR conditions, tested in baseline 

maximum dwell fatigue cycle. GB-Grain Boundary, T1-Type 1 Inclusion; and T2-Type 2 Inclusion.

b) CGFC1

NASA/TM—2011-217118 23
FGOQ1 N =67,083 f CGFC1 N =83,147 f

FGOQ1 N =68 f CGFC1 N =194 f

a)

Figure 7.—Comparison of failure initiation sites and notch surface 

microstructures for a) long and b) short life specimens for FGOQ1 and CGFC1 subscale LSHR conditions, tested in baseline maximum dwell fatigue cycle.

b)

NASA/TM—2011-217118 24
a)

Valley Roughness (Rv)- m

Figure 8 Comparison of a) notch radius and b) roughness versus fatigue

b)

Figure 8.—Comparison of a) notch radius and b) roughness versus fatigue

life, for FGOQ1 and CGFC1 conditions tested in baseline maximum dwell fatigue cycle.

NASA/TM—2011-217118 25
Figure 9.—Comparison of dwell crack growth rates measured for subscale LSHR

conditions. Bimodal dwell fatigue lives were observed for boxed conditions.

NASA/TM—2011-217118 26
FGOQ1 FGOQ2 FGFC1

Figure 10 —Comparison of dwell crack growth modes for subscale LSHR conditions Bimodal dwell fatigue lives were observed for boxed conditions

FGFC2 CGFC1 CGFC2

Figure 10. Comparison of dwell crack growth modes for subscale LSHR conditions. Bimodal dwell fatigue lives were observed for boxed conditions.

NASA/TM—2011-217118 27
1 Stan.Dev.

1 M & Mi

a)

1 Max. & Min.

b)

Figure 11.—Comparison of notch rupture properties for subscale LSHR

conditions; a) life and b) smooth gage elongation. Bimodal dwell fatigue 

lives were observed for boxed conditions.

NASA/TM—2011-217118 28
T1 GB

GB

GB

a) FGOQ1 b) FGOQ2 c) FGFC1

GB

GB GB

d) FGFC2 e) CGFC1 f) CGFC2

Figure 12.—Comparison of typical failure modes for rupture of subscale LSHR conditions.

d) FGFC2 e) CGFC1 f) CGFC2

NASA/TM—2011-217118 29
1 Stan.Dev.

1 Stan.Dev.

a)

Figure 13.—Comparison of tensile properties for subscale LSHR

conditions; a) strength and b) ductility. Bimodal dwell fatigue lives were observed for boxed conditions.

b)

NASA/TM—2011-217118 30
GB

GB GB

a) FGOQ1 b) FGOQ2 c) FGFC1

GB

TG GB

d) FGFC2 e) CGFC1 f) CGFC2

Figure 14.—Comparison of typical failure modes for tensile tests of subscale LSHR conditions; TG-Transgranular.

NASA/TM—2011-217118 31
Figure 15.—Comparison of stress relaxation versus time for all tested 

materials. Bimodal dwell fatigue lives were observed for boxed conditions.

NASA/TM—2011-217118 32
10x

100x

90 s

90 s

90 s

90 s

45 s

45 s

0.1 s Stress

Time

a) b) d) c) e)

Figure 16.—Effects of cycle profile on fatigue life for subscale LSHR FGFC2 condition; a) no-dwell; b) 90 s maximum dwell; c) 90 s minimum dwell; 

d) 90 s maximum-minimum dwell; and e) 45 s maximum-minimum dwell.

NASA/TM—2011-217118 33
Nf=3,500 Nf=42,361 Nf=245,105

C

C C

C

C C

C

) b) ) a) b) c)

Nf=1,756 Nf=2,585

Pore

C

a) b) c) a) b) c)

C

Figure 17.—Effects of cycle profile on crack frequency for subscale LSHR FGFC2 condition; a) no-dwell; 

b) 90 s maximum dwell; c) 90 s minimum dwell; d) 90 s maximum-minimum dwell; and e) 45 s maximumminimum dwell. C-Crack initiation.

d) e) d) e)

NASA/TM—2011-217118 34
TG

GB

T2

T2

a) b) c)

Pore

d) e)

Figure 18.—Effects of cycle profile on typical crack initiation modes for LSHR FGFC2 condition; a) no-dwell; b) 90 s maximum dwell; 

c) 90 s minimum dwell; d) 90 s maximum-minimum dwell; and e) 45 s maximum-minimum dwell. GB-Grain Boundary, TGTransgranular; T1-T1 Inclusion; and T2-T2 Inclusion.

d) e)

NASA/TM—2011-217118 35
Nf=30 541 Nf=4 778 Nf=30,541 Nf=4,778

Figure 19.—Comparison of secondary cracks on the notch surface, adjacent 

to the primary failure initiation point for LSHR FGFC2 condition; a) 90 s maximum dwell and b) 90 s minimum dwell. Loading direction is horizontal.

a) b)

NASA/TM—2011-217118 36
Stress

90 s 45 s

90 s 45 s

Time

Figure 20.—Effects of cycle profile on fatigue life for production-scale coarse-grain LSHR; a) maximum dwell; b) minimum dwell; and 

c) 45 s maximum-minimum dwell.

a) b) c)

NASA/TM—2011-217118 37
Nf=123,358 Nf=4,786 TG Nf=25,453

C C

C

C

J‐N11

4,786

C

C

J‐N6

25 453 C

C

C

C

C

C

C

C

C

C 25,453

TG TG C C

C

C C

C

Figure 21.—Effects of cycle profile on crack frequency for coarse-grain LSHR; a) 90 s maximum dwell; b) 90 s minimum dwell; and c) 45 s maximum-minimum 

dwell. C-Crack initiation.

a) b) c)

NASA/TM—2011-217118 38
Nf=119,124 Nf=29,771 Nf=2,618

TG

IG

TG

Nf=123,358 Nf=25,453 Nf=4,786

IG TG TG

Nf=116,748 Nf=29,414 Nf=7,404

TG

IG

TG

a) b) c)

Figure 22.—Effects of cycle profile on crack initiation modes for all test specimens of production-scale coarse-grain LSHR; a) 90 s 

maximum dwell; b) 90 s minimum dwell; and c) 45 s maximum-minimum dwell. IG-Intergranular and TG-Transgranular.

NASA/TM—2011-217118 39
35000 10000

ilurea

300002500020000

Cyclic (20 cpm)90 Sec Hold

80006000

Max Hold- 90 secsMin Hold -90 secs

a Cycles To F20000 15000

10000

Cycles

40002000

Specimen

1 2 3 4 5 6 7 8 9 10 11 12

0

5000

Figure 23 Effects of cycle profile on fatigue life for production scale coarse grain ME3

Specimen

2 3 4 5 6 7 8 9 10

0

Figure 23.—Effects of cycle profile on fatigue life for production-scale coarse-grain ME3.

NASA/TM—2011-217118 40
GB Propagation

GB

TG Propagation GB

a) b)

TG Propagation GB

Figure 24.—Comparison of failure initiation modes versus cycle profile for coarse-grain ME3; a) no-dwell; b) 90 s maximum dwell; and c) 90 s minimum 

dwell. GB-Grain Boundary.

c)

NASA/TM—2011-217118 41
200 cycles 1,500 cycles

a) b)

3,000 cycles

Figure 25.—Notch surface cracks for interrupted 90 s minimum dwell test of coarse-grain ME3, which failed at 3,080 cycles; a) 200 cycles; 

b) 1,500 cycles; and c) 3,000 cycles. Loading direction is vertical.

c)

NASA/TM—2011-217118 42
Maximum Stress

Minimum Stress

Figure 26.—Comparison of maximum and minimum stress versus cycles for strain-controlled 

tests of coarse-grain ME3 with dwells at maximum or minimum strain.

NASA/TM—2011-217118 43
1.2e+5

90 sec Min Hold

lesc

1.0e+58.0e+46 0e+4

90 sec Min Hold90 sec Max Hold

c Cy6

4.0e+42.0e+4

.0e+4

Figure 27.—Comparison of cyclic lives for strain-controlled tests of coarse-grain ME3

with 90 s dwells at maximum or minimum strain

Specimen

5 6 7 8 9 10 11

0.0

with 90 s dwells at maximum or minimum strain.

NASA/TM—2011-217118 44
1300

12001100

tch root, MPao90 sec dwell @ max. load

90 sec dwell at min. load

1000

o ial stress at nx

90 sec dwell @ min. load

900800

xA

90 sec dwell at max. load

Figure 28.—Maximum axial stress at the notch root versus cycles predicted by visco-plastic finite element 

analysis for notch fatigue tests of coarse-grain ME3 with dwells at maximum or minimum stress.

0 5 10 15 20 25 30 35 40

Cycle Number

NASA/TM—2011-217118 45
35000

30000

90 sec Min DwellCyclic 20cpm

Failure

300002500020000

Cyclic - 20cpm

Cycles to 15000

10000

Test Type Notch Root 

Max Stress

Notch Root Min 

Stress

Cyclic (0 33 Hz) 1213 5 MPa 475 8 MPa

Specimen

7 8 9 10 11 12 13 14

0

5000

Cyclic (0.33 Hz) 1213.5 MPa -475.8 MPa

90 s Min Dwell 1214.2 MPa -475.1 MPa

Specimen

Figure 29.—Comparison of notch fatigue lives for cyclic no-dwell and 90 s minimum dwell tests of coarse-grain ME3, having comparable predicted maximum 

and minimum axial stresses at the notch root.

NASA/TM—2011-217118 46
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4. TITLE AND SUBTITLE

Factors Influencing Dwell Fatigue Cracking in Notches of Powder Metallurgy Superalloys 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

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6. AUTHOR(S)

Gabb, T., P.; Telesman, J.; Ghosn, L.; Garg, A.; Gayda, J. 

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WBS 284848.02.02.03.01.01 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The influences of heat treatment and cyclic dwells on the notch fatigue resistance of powder metallurgy disk superalloys were investigated 

for low solvus high refractory (LSHR) and ME3 disk alloys. Disks were processed to produce material conditions with varied 

microstructures and associated mechanical properties. Notched specimens were first subjected to baseline dwell fatigue cycles having a 

dwell at maximum load, as well as tensile, stress relaxation, creep rupture, and dwell fatigue crack growth tests at 704 °C. Several material 

heat treatments displayed a bimodal distribution of fatigue life with the lives varying by two orders-of-magnitude, while others had more 

consistent fatigue lives. This response was compared to other mechanical properties, in search of correlations. The wide scatter in baseline 

dwell fatigue life was observed only for material conditions resistant to stress relaxation. For selected materials and conditions, additional 

tests were then performed with the dwells shifted in part or in total to minimum tensile load. The tests performed with dwells at minimum 

load exhibited lower fatigue lives than max dwell tests, and also exhibited early crack initiation and a substantial increase in the number of 

initiation sites. These results could be explained in part by modeling evolution of peak stresses in the notch with continued dwell fatigue 

cycling. Fatigue-environment interactions were determined to limit life for the fatigue cycles with dwells. 

15. SUBJECT TERMS

Heat resistant alloys; Turbine wheels; Fatigue (materials) 

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