J. Telesman and T.P. Gabb

Glenn Research Center, Cleveland, Ohio

Y. Yamada

Ohio Aerospace Institute, Brook Park, Ohio

L.J. Ghosn

Glenn Research Center, Cleveland, Ohio

D. Hornbach and N. Jayaraman

Lambda Research, Inc., Cincinnati, Ohio

Dwell Notch Low Cycle Fatigue Behavior of a 

Powder Metallurgy Nickel Disk Alloy

NASA/TM—2012-217691

December 2012
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J. Telesman and T.P. Gabb

Glenn Research Center, Cleveland, Ohio

Y. Yamada

Ohio Aerospace Institute, Brook Park, Ohio

L.J. Ghosn

Glenn Research Center, Cleveland, Ohio

D. Hornbach and N. Jayaraman

Lambda Research, Inc., Cincinnati, Ohio

Dwell Notch Low Cycle Fatigue Behavior of a 

Powder Metallurgy Nickel Disk Alloy

NASA/TM—2012-217691

December 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Superalloys 2012: The 12th International Symposium on Superalloys

sponsored by the The Minerals, Metals & Materials Society (TMS)

Champion, Pennsylvania, September 9–13, 2012
Acknowledgments

The authors wish to acknowledge the support of the NASA Aviation Safety program. The assistance of Rick Rogers and 

David Hull in performing x-ray diffraction analysis and the FIB-SEM microscopy work is gratefully acknowledged. The authors 

are particularly grateful for advice and assistance of Sue Draper, Chantal Sudbrack, and Pete Kantzos. 

Available from

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217691 1 

Dwell Notch Low Cycle Fatigue Behavior of a Powder 

Metallurgy Nickel Disk Alloy 

J. Telesman and T.P. Gabb 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Y. Yamada 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

L.J. Ghosn 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

D. Hornbach and N. Jayaraman 

Lambda Research, Inc. 

Cincinnati, Ohio 45227 

Abstract 

A study was conducted to determine the processes which 

govern dwell notch low cycle fatigue (NLCF) behavior of a 

powder metallurgy (P/M) ME3 disk superalloy. The emphasis 

was placed on the environmentally driven mechanisms which 

may embrittle the highly stressed notch surface regions and 

reduce NLCF life. In conjunction with the environmentally 

driven notch surface degradation processes, the visco-plastic 

driven mechanisms which can significantly change the notch 

root stresses were also considered. 

Dwell notch low cycle fatigue testing was performed in air 

and vacuum on a ME3 P/M disk alloy specimens heat treated 

using either a fast or a slow cooling rate from the solutioning 

treatment. It was shown that dwells at the minimum stress 

typically produced a greater life debit than the dwells applied 

at the maximum stress, especially for the slow cooled heat 

treatment. Two different environmentally driven failure 

mechanisms were identified as the root cause of early crack 

initiation in the min dwell tests. Both of these failure 

mechanisms produced mostly a transgranular crack initiation 

failure mode and yet still resulted in low NLCF fatigue lives. 

The lack of stress relaxation during the min dwell tests 

produced higher notch root stresses which caused early crack 

initiation and premature failure when combined with the 

environmentally driven surface degradation mechanisms. 

The importance of environmental degradation mechanisms 

was further highlighted by vacuum dwell NLCF tests which 

resulted in considerably longer NLCF lives, especially for the 

min dwell tests. 

Introduction 

A new generation of powder metallurgy (P/M) disk 

superalloys has been designed for higher engine operating 

temperatures by improvement of their strength and creep 

resistance. However, the increased strength and creep 

resistance of these alloys may also increase their notch 

sensitivity to fatigue loading and thus have a profound effect 

on the notched low cycle fatigue life. Notch low cycle fatigue 

is a key mechanical property of the highly stressed gas turbine 

rotating components since it simulates behavior at notches and 

bolt holes which are the regions from where the majority of 

failures of rotating components tend to originate. 

With the increase in the disk operating temperatures, 

sustained dwells (or hold times) have been shown to 

significantly influence NLCF fatigue life (Refs. 1 and 2). In 

our earlier study (Ref. 1) performed on a subsolvus P/M 

LSHR alloy, imposition of a 90 sec dwell at the maximum 

stress reduced the NLCF life by an order of magnitude 

compared to cyclically loaded specimens tested under 

identical test conditions. However a more surprising finding in 

that study as well as in an earlier study performed by 

Bache et al. (2) was that an imposition of a 90 sec dwell at the 

minimum stress produced an even greater reduction in NLCF 

life. In our study the imposition of the 90 sec dwell at the 

minimum stress produced an almost two orders of magnitude 

reduction in NLCF in comparison to the cyclic tests. The 

earlier work by Bache et al. (Ref. 2) done on a subsolvus P/M 

RR1000, showed that introduction of even a 10 sec dwell at 

the minimum stress produced an order of magnitude shorter 

NLCF lives when compared to the application of the dwell at 

the maximum stress. 

The effect of prolonged high temperature dwells on low 

cycle fatigue (LCF) and NLCF behavior in this class of alloys 

is thought to be a product of complex interactions of cyclic 

fatigue loading damage, time-dependent environmental 

degradation and embrittlement, as well as the complex viscoplastic material response in the notch root region to 

temperature and load history. 

Due to the above described complexities, currently 

available life prediction models are not able to accurately 

predict dwell notched low cycle fatigue life. Formulation of 

more accurate life prediction models requires an improved 
NASA/TM—2012-217691 2 

understanding of the damage mechanisms responsible for 

material degradation and accumulation of fatigue damage. 

The key to gaining this understanding is the identification of 

the active damage mechanisms for a given set of test 

conditions. As important as the identification of the prevalent 

material damage mechanisms, is an accurate assessment of the 

notch root stress and strain redistributions in which take place 

during the prolonged dwells through the visco-plastic notch 

root deformation response. 

The approach of the current study was to isolate as much as 

possible the major variables which affect the dwell NLCF 

behavior so that their individual mechanistic contributions to 

material durability could be studied, quantified and modeled. 

Specimens from a supersolvus heat treated ME3 P/M disk 

superalloy were subjected to three loading histories consisting 

of baseline cyclic loading, or 90 sec cyclic dwells applied at 

either the maximum stress or at the minimum stress. The 

cyclic NLCF testing was performed to obtain a baseline data 

set, while the two 90 sec dwell waveforms attempted to 

capture the environmental degradation which occurs as a 

function of high temperature exposures. Vacuum testing was 

performed to quantify the environmentally driven NLCF 

debit. The use of dwells at either the maximum stress or 

minimum stress was introduced so that the influence of notch 

root visco-plastic behavior could be evaluated. To further the 

evaluation of the notch root visco-plastic behavior, two 

different ME3 heat treatments were evaluated which are known 

to produce significantly different visco-plastic response. 

Materials and Procedure 

All specimens were blanked and machined from a powder 

metallurgy ME3 alloy pancake forging of the following 

composition in weight percent: 3.38 Al, 0.025 B, 0.053 C, 

20.7 Co, 12.8 Cr, 3.79 Mo, 0.90 Nb, 2.33 Ta, 3.67 Ti, 2.03 W, 

0.049 Zr and Bal. Ni. Each blank was individually heat treated 

to assure that the desired processing was achieved. Each blank 

was given the same supersolvus solutioning heat treatment at 

1171 °C. The blanks were then cooled from the solutioning 

temperature at either 72 or 202 °C/min cooling rate as 

measured by an embedded thermocouple. Subsequently the 

blanks were given a two step age treatment consisting of 

843 °C for 4 hr and 760 °C for 8 hr. The blanks were 

subsequently machined into a circumferential notch low cycle 

fatigue specimens with an elastic stress concentration factor 

Kt = 2.0. The notch was low-stress ground, then polished 

parallel to the loading direction to meet or exceed a 0.21μm 

(8 μin.) rms finish requirement. 

All testing was performed at NASA GRC using closed loop 

servo-hydraulic test machines equipped with resistance 

heating furnaces. Most of the testing was performed in a lab 

air environment however a selected number of specimens 

were tested in a test machine equipped with a vacuum 

chamber. The vacuum level measured in the chamber ranged 

mostly from 10–4 Pa to 10–5 Pa. All testing was done at 704 °C. 

Three waveforms were utilized, all with a minimum/ 

maximum applied stress ratio of Rσ = 0.05. The baseline 

cyclic testing was performed with no dwells at 0.333 Hz 

frequency. For the cyclic dwell tests, 90 sec holds were 

applied at either the maximum applied stress (max dwell 

waveform) or at the minimum applied stress (min dwell 

waveform). The testing for all three waveforms was 

performed with the maximum applied net section stress equal 

to either 855 or 896 MPa. 

Stress relaxation testing was conducted for both cooling 

rate heat treatments using a 4.06 mm diameter cylindrical 

specimens machined from the heat treated blanks. The 

specimens were heated to 704 °C and strained to 1 percent 

total strain. Subsequent stress relaxation was measured as a 

function of time for the 100 hr duration of the tests. 

Results and Discussion 

Non-Unified Visco-Plastic FEM Analysis of 

Notch Root Stresses 

The evolution of the notch root stresses was modeled using 

finite element modeling (FEM) as a function of the number of 

applied cycles for both max dwell and min dwell loading 

profiles. The notched specimen geometry was modeled with a 

two-dimensional axi-symmetric non-linear elements and the 

material modeled using a non-unified viscoplastic analysis 

wherein the strain is decomposed into separate elastic, plastic, 

creep, and thermal components. The analysis utilized material 

properties such as elastic modulus, flaw strength, and 

relaxation behavior which were determined through tensile 

tests performed at 704 °C for both ME3 heat treatments using 

a constant strain rate (8.5×10–5 1/s). Upon reaching 1 percent 

total strain, the tensile specimens were used to conduct 100 hr 

stress relaxation tests. The purpose of the present analysis was 

to capture the trend of stress state evolutions at the notch root 

and to help understand the effect of stress relaxation behavior, 

and consequently stress redistribution, on fatigue life during 

two different dwell loading profiles. Stress state at the notch 

root was monitored for the first 100 applied dwell cycles. 

While the analysis was performed to capture the above 

mentioned trends, more rigorous analysis will be performed in 

the future to capture the effect of the loading rate and 

creep/plasticity interactions. 

The plastic strain was computed using the Von Mises 

plasticity model within Abaqus, the required material 

parameters being obtained from the tensile curves are shown 

in Figure 1(a). The stress relaxation material response was 

correlated using the following creep law: 

 (1) ௠ݐ௡σܣ ൌ ሶߝ

where ߝሶ is the strain rate, σ is stress, t is time and A, n and m

are fitting constants. The creep coefficients which fit this 

 
NASA/TM—2012-217691 3 

equation for both heat treatments were determined and are 

shown in Figure 1(b). The figure shows the measured and 

analytically modeled axial stress relaxation response as a 

function of time for the two cooling rates after an application 

of a 1 percent total strain. As shown, the high strength (more 

creep resistant), fast cooled heat treatment resulted in 

consistently 50 to 100 MPa higher stresses throughout the test 

in comparison to the lower strength, slow cooled heat 

treatment. The non-unified visco-plastic creep analysis of the 

stress relaxation response resulted in an excellent correlation 

with the experimentally measured stress relaxation for both 

heat treatments. 

The above non-unified visco-plastic creep analysis was 

used to perform FEM modeling of the circumferential notch 

root stresses. The estimated maximum axial stresses generated 

at the notch root for both ME3 cooling rates as a function of 

the maximum and minimum dwell cycles are compared in 

Figure 2. As shown, the initial magnitude of the notch root 

stresses is similar for both dwell loading waveforms, however 

in the case of the max dwell waveform the notch root stresses 

decrease rapidly during subsequent cycles. In comparison, for 

the minimum dwell tests the maximum notch root cyclic stress 

remained almost constant throughout the duration of the test. 

Thus, as the number of dwell cycles accumulates, the 

differences in the notch root stresses between min and max 

dwell load profiles increase and therefore may have a 

substantial effect on the dwell NLCF behavior. Again, the 

magnitude of stress may not be accurate since some of the 

non-linear material responses were not precisely modeled. 

Yet, the trend of stress evolution during the max and the min 

dwell loading profile under the same 0.33 Hz loading rate 

should be qualitatively the same. 

The effect of the difference in the cooling rates on the notch 

root stresses is also significant. While the trends are very 

similar for both cooling rates, the actual magnitude of the 

notch root stresses is consistently 50 to 75 MPa higher for the 

fast cool heat treatment for both the min and the max dwell 

loading profiles (Fig. 2). 

NLCF Testing 

The results of the NLCF tests conducted in the laboratory 

air environment at a maximum stress of 855 MPa for the three 

load histories investigated are shown in Figure 3. The 

specimens tested without dwells at a 0.333 Hz frequency 

produced the longest lives, while the applied max and min 

dwell tests resulted in a pronounced fatigue debit for both 

waveforms. The cyclic lives were in the order of 80,000 to 

110,000 cycles, while the max dwell tests duration was 

approximately 20,000 cycles with the min dwell tests lives 

being in the 10,000 cycle range. As shown in the figure, the 

cooling rate did not significantly alter either the 0.333 Hz or 

the min dwell tests (specimens with only the fast cool 

condition were tested under the applied 855 MPa max dwell 

(a)

(b)

Figure 1.—Simulation of the experimentally measured 

tensile and stress relaxation tests. (a) Tensile testing and 

modeling. (b) Stress relaxation testing and modeling. 

Figure 2.—FEM modeling of the notch root axial stresses 

as a function of applied dwell waveform and number of 

cycles for both slow and fast cooled heat treatments. 

 

strain, %

0.0 0.2 0.4 0.6 0.8 1.0

Stress, MPa

0

200

400

600

800

1000

1200

SC Experiment

SC Model

FC Experiment

FC Model

Time, hours

0.001 0.01 0.1 1 10 100

Stress, MPa

600

700

800

900

1000

1100

1200

SC Experiment

SC Model

FC Experiment

FC Model

dot = Ant

m

 A n m

FC: 1.1E-27, 8.0, -0.5

SC: 2.2E-27, 8.0, -0.5

N, cycles

0 20 40 60 80 100

Stress, MPa

900

1000

1100

1200

1300

1400

Fast Cool Max Dwell 

Slow Cool Max Dwell 

Fast Cool Min Dwell 

Slow Cool Min Dwell 
NASA/TM—2012-217691 4 

Figure 3.—NLCF lives for the three different waveforms at 

an applied 855 MPa max stress. 

Figure 4.—NLCF lives for the three different waveforms 

at an applied 89 MPa max stress. 

waveform). These observed trends were similar to those 

reported earlier for both the subsolvus LSHR and RR1000 

P/M alloys with the min hold waveform exhibiting the 

shortest lives (Refs. 1 and 2). 

The results of the NLCF tests conducted in air at a 

maximum stress of 896 MPa were more difficult to interpret 

with regards to the data trends. As shown in Figure 4, the 

NLCF fatigue lives of the cyclic tests conducted at 0.333 Hz 

still resulted in the longest lives (ranging from 25,000 to 

75,000 cycles) in comparison to the dwell tests, even though 

the life scatter was greater than for the 855 MPa max stress 

cyclic tests. The 90 sec min dwell tests exhibited fairly 

uniform short fatigue lives in the range of 3,500 to 

7,000 cycles. However, the 90 sec max dwell test results were 

more complex. The NLCF fatigue lives of the max dwell tests 

exhibit a bimodal distribution with one group showing failure 

lives of approximately 4,000 cycles while the other group of 

specimens failed at approximately 15,000 cycles. 

Figure 5.—The NLCF results for the max dwell tests performed 

at 896 MPa, show that the cooling is an important variable. 

Figure 6.—For the min dwell tests at 896 MPa, the NLCF 

results are not a function of the cooling rate. 

The NLCF dwell data for the 896 MPa max dwell tests is 

replotted in Figure 5, this time with the cooling rate from the 

solutioning temperature assigned for each specimen. The 

replotted results show very clear trends. All the slow cooled 

max dwell specimens resulted in NLCF lives of 

approximately 15,000 cycles while all the fast cool specimens 

failed at approximately 4,000 cycles. 

In case of the 896 MPa min dwell tests (Fig. 6), the fatigue 

lives were very similar regardless of the cooling rate. These 

results suggest that not only the dwell waveform but also the 

cooling rate has a pronounced effect on NLCF behavior. 

Furthermore, in the case of the max dwell waveform, the fast 

cooled specimens fatigue lives were also very sensitive to the 

applied net section stress. The increase in the applied dwell 

stress from 855 to 896 MPa resulted in an almost 5× reduction 

in cycles to failure. In comparison the same increase in the 

max stress for the min dwell tests resulted in only a 2× 

reduction in NLCF lives. 

Specimen

1234567

Cycles to Failure

0

20x103

40x103

60x103

80x103

100x103

120x103

0.333 Hz; Air; 855 MPa

90 sec Max Hold; Air; 855 MPa

90 sec Min Hold; Air; 855 MPa

SC

SC

FC

FC FC SC FC
NASA/TM—2012-217691 5 

Vacuum Testing 

Numerous vacuum tests were conducted to ascertain the 

role of environment on the NLCF behavior of this alloy. The 

fatigue results for all three load histories tested in vacuum 

at a max stress of 896 MPa are shown in Figure 7. The fatigue 

lives of the cyclic specimens were on the order of 

70,000 cycles and remained considerably longer than those of 

the min and max dwell specimens. There was no longer a 

substantial difference between the fatigue lives of min and 

max dwell specimens with the failures occurring at 

approximately 18,000 to 20,000 cycles regardless of the dwell 

waveform or the cooling rate. The most dramatic difference 

between air and vacuum is exhibited by the min dwell tests as 

shown in Figure 8. The NLCF life was increased 

approximately by a factor of four by changing the test 

environment from air to vacuum with no clear influence of the 

cooling rate. 

For max dwell tests of the slow cooled heat treatment, the 

change in the environment from air to vacuum had a less 

dramatic impact on the NLCF lives, with the vacuum max 

dwell specimens exhibiting approximately a 25 percent increase 

in life (Fig. 9). However, as shown in the same figure, for the 

fast cooled specimens the fatigue lives of the max dwell tests 

were four to five times longer in vacuum than in air. 

Fatigue Damage Mechanisms 

The NLCF testing produced a number of interesting results, 

which required further analysis to explain the observed trends. 

Extensive fractographic analysis of the microstructural 

damage mechanisms was performed using scanning electron 

microscopy (SEM). The focus was to identify and compare 

the failure mechanisms of min and max dwell NLCF tests in 

both air and vacuum environments so that the differences in 

the NLCF fatigue lives could be explained. Additionally, the 

role of the cooling rates from the solutioning treatment on the 

prevailing damage mechanisms was also examined. 

The fractography performed on the failed NLCF specimens 

revealed a variety of operative fatigue initiation and 

propagation modes that depended on the applied loading 

waveform, test environment and the cooling rate. 

Max Dwell Testing—Air 

Fast cooled NLCF specimens tested under max dwell 

loading, exhibited a predominant intergranular crack initiation 

and propagation failure mode for all specimens tested under 

this waveform as shown in Figure 10(a) and (b). This was the 

case for the specimens tested at the 896 MPa max stress 

(Fig. 10(a)) which resulted in relatively short lives as well as 

the specimens tested at 855 MPa (Fig. 10(b)) which resulted 

in much longer NLCF lives. The extensive intergranular notch 

root surface damage which occurred during the test is clearly 

visible in the surface region adjacent to the primary crack 

(Fig. 10(b)). 

Figure 7.—Vacuum NLCF results for the three waveforms 

tested at 896 MPa. 

Figure 8.—Comparison of air and vacuum NLCF results for 

the min dwell waveform for tests conducted at 896 MPa. 

Figure 9.—Max dwell NLCF lives in vacuum and air for the 

two heat treatments tested at 896 MPa.

Specimen

1234567

Cycles to Failure

0

5000

10000

15000

20000

25000

MIn Hold; Air; 896 MPa

Min Hold; Vacuum; 896 MPa

SC

SC SC

SC

FC FC

FC

Specimen

0 2 4 6 8 10

Cycles to Failure

0

5000

10000

15000

20000

25000 Air; Slow Cool; Max Hold 896 MPa

Air; Fast Cool:: Max Hold; 896 MPa

Vacuum; Slow Cool; Max Hold; 896 MPa

Vacuum; Fast Cool; Max Hold; 896 MPa

Slow Cool

Fast Cool
NASA/TM—2012-217691 6 

(a) 

(b) 

Figure 10.—Intergranular initiation, propagation and secondary 

cracking for fast cooled specimens tested using the max 

dwell waveform, loading axis horizontal. (a) Max dwell test 

performed with an 896 MPa hold stress on a fast cooled 

NLCF specimen; Nf = 3,969 cycles. (b) Max dwell test 

performed with an 855 MPa hold stress on a fast cooled 

NLCF specimen; Nf = 18,942 cycles. Back scatter image. 

The initiation and propagation failure modes in the slow 

cooled max dwell specimens differ somewhat in comparison 

to the fast cooled specimens. As shown in Figure 11, both 

transgranular and intergranular crack initiation regions were 

present in the slow cooled specimens (Fig. 11(a)). The 

transgranular region typically extended up to approximately 

100 μm into the specimen, after which intergranular 

propagation dominated (Fig. 11(b)). This behavior was 

consistent for all specimens examined. 

The presence of intergranular initiation and propagation 

failure mechanisms in max dwell tests was expected. It has 

been shown repeatedly (Refs. 3 to 5) that the presence of a 

prolonged tensile holds in air promotes intergranular failure 

mechanisms which are known to accelerate grain boundary 

(a) 

(b) 

Figure 11.—Mixed mode initiation and propagation in slow 

cooled max dwell NLCF specimens tested at 896 MPa max 

stress. (a) Intergranular initiation (upper arrow) and 

transgranular initiation (lower arrow), horizontal loading axis. 

(b) Region of transgranular initiation surrounded by 

intergranular propagation. 

damage. The observed differences in the amount of 

intergranular initiation and propagation between the two 

cooling rates may be due to a combination of higher notch 

root stresses in the case of fast cooled material (Fig. 2) and the 

possibility that the higher cooling rate may produce a grain 

boundary structure which is more susceptible to intergranular 

failure. 

Combination of the above described mechanisms may 

explain the observed differences in the max dwell NLCF lives 

between the fast and slow cooled heat treatments tested 

at 896 MPa. A greater propensity for grain boundary 

environmental attack tied in with higher notch root stresses for 

the fast cooled material could have led to early intergranular 

crack initiation and failure for this heat treatment in 

 
NASA/TM—2012-217691 7 

comparison to slow cooled specimens which were subjected 

to lower notch root stresses and may possess microstructure 

less sensitive to grain boundary attack. 

Min Dwell Testing—Air 

The min dwell NLCF fatigue lives for both fast cooled and 

slow cooled specimens tested in air were very short and did 

not appear to be correlated with the cooling rate (Figs. 3 

and 6). The failure mode of the slow and fast cooled min 

dwell NLCF specimens are shown in Figures 12 and 13, 

respectively. For the slow cooled specimens, predominantly 

transgranular initiation and propagation failure modes were 

observed (Fig. 12). 

The fractography of the fast cooled specimens tested using 

the min dwell waveform showed that crack initiation occurred 

through a mixture of both transgranular and intergranular 

crack failure modes (Fig. 13). Interestingly, even after a 

transgranular initiation, a 50- to 75-μm-deep region of 

intergranular crack propagation was present which 

transitioned again to a transgranular failure mode for the rest 

of the test (Fig. 13(b)). Thus, while there were some 

differences in the failure mode between the two cooling rates 

for the min dwell tests, the transgranular failure mode was 

active for both cooling rates. 

Interrupted Testing 

Transgranular initiation and propagation are usually 

considered more benign failure mechanisms than intergranular 

attack. However in the case of the min dwell cycle tests which 

mostly exhibited transgranular failure mechanisms, their 

NLCF lives were either equivalent or in many cases much 

lower than the comparable max dwell tests which tended to 

exhibit an intergranular failure mode. It is clear from these 

Figure 12.—Predominant transgranular initiation and 

propagation for slow cooled specimens tested using 

min dwell waveform, horizontal loading axis. 

(a) 

(b)

Figure 13.—Mixed mode initiation and propagation in fast 

cooled specimens tested using min dwell waveform. (a) 

Intergranular initiation (upper arrow) and transgranular 

initiation (lower arrow), horizontal loading axis. (b) Transgranular initiation, small region of intergranular propagation 

(dashed lines) followed by additional transgranular propagation (arrow). 

results that simplistic assumptions regarding transgranular 

versus intergranular failure mechanisms cannot be used to 

explain the current NLCF results. More in-depth knowledge 

of the failure mode mechanisms and the variables which 

control crack initiation and propagation is required. 

In order to gain a better understanding of the operative 

crack initiation mechanisms and how they can influence 

NLCF behavior, interrupted tests were conducted in air and 

vacuum for NLCF specimens subjected to the min dwell 

loading waveform. Only specimens given the slow cooled 

heat treatment were used. All tests were interrupted after 
NASA/TM—2012-217691 8 

1000 cycles, the specimens removed from the test rig and 

placed in the SEM for examination. 

As observed in the SEM for the two air tested specimens, 

numerous cracks were already visible in the notch root section 

after the application of 1000 cycles which corresponds to 

approximately 20 to 25 percent of the expected lives (Fig. 14). 

The detailed SEM and EDS evaluation of these interrupted 

tests revealed two different crack initiation mechanisms which 

were responsible for the early crack initiation in the lab air 

environment. 

One of these crack initiating mechanisms is discernible in 

Figure 14. It is apparent in the back scatter image shown in 

Fig. 14(a), that all the visible cracks are associated with the 

irregularly distributed darker regions present on the notch 

surface. No cracks appeared to have initiated in the lighter 

regions of the image. Secondary electron images and the EDS 

analysis of these crack initiating regions showed that they 

consisted of oxide-like scales (Fig. 14(b) and (c)) which grew 

preferentially during the first 1000 cycles of min dwell 

testing. These oxide scales were found to be considerably 

higher in Cr, Ti and oxygen content in comparison to the 

adjacent uncracked regions (light regions of Fig. 14(a)). 

Preliminary results from a recently performed x-ray 

diffraction analysis positively identified these phases as 

chromium oxide, Cr2O3, and titanium oxide, TiO2. 

It is proposed that the early formation and growth of these 

irregularly distributed oxide colonies results in the creation of 

brittle phases which can crack when the scales reach some 

critical thickness and are subjected to sufficiently high applied 

stresses. The cracks emanating from these oxides propagate 

into the surrounding metal and eventually lead to premature 

failure. Since these oxide scales are not associated with grain 

boundaries, transgranular cracks can form as was observed in 

the tested specimens. As shown in the visco-plastic FEM 

analysis (Fig. 2), very little notch root stress relaxation occurs 

as the min dwell cycles are accumulated, thus the maximum 

cyclic stress remains high for the duration of the test while the 

oxide scales form and grow during the min holds. This 

combination of high max cyclic stresses and prolonged 

exposures in an environment conducive to the growth of 

brittle phases may account for the reduced NLCF life of the 

min dwell tested NLCF specimens. 

A second crack initiation mechanism was also detected in 

the min dwell NLCF tests performed in lab air. The notch root 

surface contained a large number of very small particles 

which appeared to have already been broken up after 

1000 cycles (Fig. 15). These particles, which are 

approximately 0.3 to 1 μm in diameter, were not easily 

detected prior to start of the tests, but were clearly visible on 

the surface after the application of 1000 dwell cycles. While 

the identification of these particles has not been finalized, they 

are most likely MC carbides which have reacted with oxygen, 

became brittle, and fractured during testing. EDS analysis 

showed them to be rich in Ta, Ti, Mo and O. A small 

percentage of these fractured particles have initiated cracks 

(a) 

(b) 

(c) 

Figure 14.—Observations of transgranular initiating from the 

preferentially oxidized notch surface regions. Vertical loading 

axis. (a) Back scatter image of a min hold specimen tested 

in air. Extensive cracking confined only to the darker phases 

present on the notch surface. (b) Secondary image showing 

the cracked regions corresponding to irregularly spaced 

oxide scale on the surface. (c) Higher mag image of cracking 

from the oxide scale. 

 
NASA/TM—2012-217691 9 

Figure 15.—Multiple broken up small particles (probably MC 

carbides). Few of these particles initiated cracks. Vertical 

loading axis. 

which started to propagate into the surrounding metal during 

the initial 1,000 cycles (Fig. 15). 

As was the case for the oxide scale cracking, the 

propagation of the cracked and oxidized carbides into the 

surrounding metal may require the higher max cyclic stresses 

present for the min dwell loading profile. Thus both of these 

mechanisms can contribute to early failure of the NLCF min 

dwell specimens. It should be noted that the frequency of the 

cracks starting from the oxidized MC carbide particles was 

considerably lower than those which initiated from the 

previously described mechanism shown in Figure 14. 

While the growth of the brittle oxide-scale phases and 

breakup of the carbides was also noted in the max dwell tests, 

the lower magnitude of notch root stresses due to visco-plastic 

stress relaxation may delay early crack initiation from these 

phases and thus prolong the lives of the max dwell NLCF 

tests as was observed in the test program. Interrupted max 

dwell tests are currently being performed to confirm this 

hypothesis. 

Vacuum Testing 

Max and min dwell vacuum testing produced very similar 

NLCF fatigue lives (Fig. 7) which typically exceeded the 

NLCF lives of tests performed in air (Fig. 4). The crack 

initiation and propagation failure modes in vacuum were 

strictly transgranular for both types of dwell waveforms were 

(Fig. 16). The difference in the failure mode between air and 

vacuum are especially striking when the comparison between 

the max dwell tests is attempted. The high oxygen content lab 

air resulted in predominant intergranular failure mode 

(Fig. 10) while the low oxygen vacuum environment resulted 

in a transgranular failure appearance. 

(a) 

(b) 

(c) 

Figure 16.—Strictly transgranular initiation and propagation for 

both min and max dwell NLCF tests conducted in vacuum. 

(a) Transgranular initiation failure mode in a max dwell test 

performed in vacuum. Horizontal loading axis diffraction. 

(b) Transgranular propagation in a max dwell test conducted 

in vacuum. (c) Transgranular initiation propagation in a 

vacuum min dwell test. 
NASA/TM—2012-217691 10 

A more unexpected and interesting finding was the 

comparison of the notch surfaces of the specimens tested in 

air and vacuum. Figure 17 shows a comparison of air and 

vacuum notch regions for two tests for which the NLCF lives 

were comparable (18,000 to 20,000 cycles). The differences 

are striking, the air test shows previously described localized 

colonies of oxide scale and broken up carbides (Fig. 17(a)), 

while the vacuum tested specimen shows a dense homogenous 

coating of the notch surface with fine particles (Fig. 17(b)). A 

cross section through the vacuum tested specimen performed 

in a focus ion beam (FIB) microscope (Fig. 17(c)) shows these 

particles to be approximately 0.5 to 1.0 μm thick. Initial 

assessment of these vacuum grown particles was done using 

x-ray diffraction. 

The results from this preliminary analysis indicated that two 

types of Cr-Co metallic phases formed on the vacuum 

exposed surface. The predominant phase had a tetragonal 

structure with a 0.6Cr-0.4Co composition, while the presence 

of a 0.25Cr-0.75Co hexagonal phase was also detected. No 

oxide phases were observed. Further work is in progress to 

determine the origin of these phases. 

The surface of the 1000 cycle vacuum interrupted min 

dwell test showed that the continuous scale of these phases 

was already present on the notch surfaces at this early stage of 

the test (Fig. 18). In contrast to the lab air interrupted tests, no 

cracking after 1000 cycles was detected in the vacuum 

interrupted min dwell test. 

(a) (b) 

(c) 

Figure 17.—Comparison of the notch root surfaces for tests performed in air and vacuum at an approximately similar testing 

time. (a) Notch surface of a specimen tested in air for approximately 460 hr. Max dwell at 855 MPa. Vertical loading axis. 

(b) Notch surface of a specimen tested in vacuum for approximately 500 hr. Min dwell at 896 MPa. Vertical loading axis. 

(c) Cross section through the notch surface of a vacuum tested specimen. Sectioning performed using a FIB microscope. 
NASA/TM—2012-217691 11 

Figure 18.—Notch surface after 1000 min dwell cycles in 

vacuum. Continuous scale of small particulates already 

present. No cracking was detected (horizontal line seen in 

the top half of image is a left over machining groove). 

Vertical loading axis. 

Summary 

A study was conducted to determine the processes which 

govern dwell NLCF behavior of a P/M ME3 disk alloy. The 

emphasis was placed on the environmentally driven 

mechanisms which may embrittle the highly stressed notch 

surface regions. In conjunction with the environmentally 

driven notch surface degradation processes, the visco-plastic 

creep driven mechanisms which significantly change the 

notch root stresses were also considered. 

The notch root max stresses were found to be significantly 

lower for the max dwell loading profile due to creep induced 

stress relaxation in comparison to the min dwell waveform. 

Slow cool heat treatment also produced lower max notch root 

stresses than the fast cooled heat treatment for the two dwell 

load profiles investigated. 

These differences in the notch root stresses had a major 

effect on the lives of the tested NLCF specimens, especially 

when the environmentally driven notch surface degradation 

mechanisms became active. Thus for the lab air tests, the min 

dwell NCF lives were very short and were found for a 

majority of the test conditions to be considerably lower in 

comparison to the max dwell tests. This occurred despite a 

prevailing transgranular crack initiation process being active. 

Two different environmentally driven mechanisms were 

identified to be responsible for the early crack initiation and 

premature failure. Formation of highly localized oxide scale 

regions and break up of small carbide particles after they have 

reacted with oxygen, created brittle phases on the surfaces of 

the notch which led to early crack initiation due to the high 

cyclic notch root stresses. While these phases also exist in 

max dwell tests, significantly lower notch root stresses 

prevent early crack initiation in these phases. Vacuum testing 

produced significantly longer dwell NLCF lives, especially 

for the min dwell loading profile. 

The cooling rate from the solutioning heat treatment was 

found to be an important factor governing the NLCF lives of 

max dwell tests. Fast cool heat treatment resulted in 

considerably shorter NLCF lives in comparison to the slow 

cooled specimens. Both the higher notch root stresses of the 

fast cooled specimens and possible changes to the grain 

boundary microstructure due to the fast cooling rate produced 

conditions which favored the observed intergranular crack 

initiation and lower NLCF lives. 

References 

1. T.P. Gabb, J. Telesman, L.J. Ghosn, A. Garg, J. Gayda, “Factors 

Influencing Dwell Fatigue Cracking in Notches of Powder 

Metallurgy Superalloys,” NASA/TM—2011-217118, Washington, 

D.C., June, 2011. 

2. M.R. Bache, J.P. Jones, G.L. Drew, M.C. Hardy, N. Fox, 

“Environment and time dependent effects on the fatigue response 

of an advanced nickel based superalloy,” Int. J. of Fatigue, vol. 31, 

pp. 1719–1723, 2009. 

3. D.F. Smith, E.F. Clatworthy, D.G. Tipton, W.L. Mankins, 

“Improving the Notch-Rupture Strength of Low-Expansion 

Superalloys,” Superalloys 1980, ed. J.K. Tien et al., TMS, 1980, 

pp. 521–530. 

4. R.H. Bricknell and D. Woodford, “The Embrittlement of Nickel 

Following High Temperature Air Exposure,” Met. Trans. A, 

vol. 12, March 1981, pp. 425–433. 

5. U. Krupp, P. Wagenhuber, W.M. Kane, T. Jacobs, C.J. McMahon 

Jr., “Environmentally Assisted Brittle Fracture of Nickel-Base 

Superalloy at High Temperatures,” Proc. of the 11th Int. Congress 

of Fracture, Turin, Italy, 2005. 
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Dwell Notch Low Cycle Fatigue Behavior of a Powder Metallurgy Nickel Disk Alloy 

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Telesman, J.; Gabb, T., P.; Yamada, Y.; Ghosn, L., J.; Hornbach, D.; Jayaraman, N. 

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14. ABSTRACT

A study was conducted to determine the processes which govern dwell notch low cycle fatigue (NLCF) behavior of a powder metallurgy 

(P/M) ME3 disk superalloy. The emphasis was placed on the environmentally driven mechanisms which may embrittle the highly stressed 

notch surface regions and reduce NLCF life. In conjunction with the environmentally driven notch surface degradation processes, the viscoplastic driven mechanisms which can significantly change the notch root stresses were also considered. Dwell notch low cycle fatigue 

testing was performed in air and vacuum on a ME3 P/M disk alloy specimens heat treated using either a fast or a slow cooling rate from the 

solutioning treatment. It was shown that dwells at the minimum stress typically produced a greater life debit than the dwells applied at the 

maximum stress, especially for the slow cooled heat treatment. Two different environmentally driven failure mechanisms were identified as 

the root cause of early crack initiation in the min dwell tests. Both of these failure mechanisms produced mostly a transgranular crack 

initiation failure mode and yet still resulted in low NLCF fatigue lives. The lack of stress relaxation during the min dwell tests produced 

higher notch root stresses which caused early crack initiation and premature failure when combined with the environmentally driven surface 

degradation mechanisms. The importance of environmental degradation mechanisms was further highlighted by vacuum dwell NLCF tests 

which resulted in considerably longer NLCF lives, especially for the min dwell tests.

15. SUBJECT TERMS

Notch low cycle fatigue; Dwells; Oxidation; Vacuum; Alloy ME3 

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