* - -*a • w o.o.- * 4//FINAL REP'ORT to AIR FORCE OFFICE OF SCIENTIFIC RESEA,(UJ Project Manager: Dr. A. T. RoAensteinl I':'Grant 140. AFOSR 76-29286 Q!4 CREEP-FATIGUE ENVIRONMENT INTERACTIONS IN SUPERALLOES Principal Investigator Regis M. Pelloux professor of Materials Engineering Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, MA 02139 April 1981 upp1'@Y for pnbl 10 relL98iSI 8d15 tribdttOn 69l.tod- 815 12 069
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069 - Defense Technical Information · PDF filesign data before any new alloy system can be used. The problem of fatigue in nickel ... nickel-base super-alloy. ... billet, and the
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* - -*a • w o.o.-
* 4//FINAL REP'ORT
to
AIR FORCE OFFICE OF SCIENTIFIC RESEA,(UJ
Project Manager: Dr. A. T. RoAensteinl
I':'Grant 140. AFOSR 76-29286
Q!4 CREEP-FATIGUE ENVIRONMENT INTERACTIONS IN SUPERALLOES
Principal Investigator
Regis M. Pellouxprofessor of Materials Engineering
Department of Materials Science and Engineering
Massachusetts Institute of TechnologyCambridge, MA 02139
April 1981
upp1'@Y for pnbl 10 relL98iSI8d15 tribdttOn 69l.tod-815 12 069
SECURITY CLAIIIF#CWTI6C 6F TNTS PAE10fela
REPORfINK U-MENTATIO PAGE RZAD INSTRUCTIONS Is
RKPORK CO'PLZTt! ORN81 gr45 IOVTACCESSIONW AC No a M CATAO 061,1
Creep-Fat igue Envirorament Interactions in. Final7
Superalloys l .PROMIG04 _
Cambridge, MA 02139
II. CONTRORLING AGENCY1 NAME AN ADDRESSI Irwtio m~tn t~o)1.SCRT LS . A.me
Ballin APB, DCAYON 20332AQ 0.
16. 0ISTRISIJTION STATEMENT (.1 thmis Report)
Annve for public reluagedistribution unliulta.0
1S. SUPPLEMENTARY NOTES
III. KEY WORDS (Continue oan revese sse ait. necesoryc anid identify by block number)
fatigue, creep, crack growth, nickel base superalloy., Waspaloy, Astroloy,IN 100,, creep-fatigue-environment interactions, micromachanisma of fracture.
20. ABSTRACT (Continue an reveses mid* It necesuary~ and Identify by block niumber)
crack growth mode as a 4.*unction alloy chemistry and microstructure.
Three alloys, conventionally cast and forged Waspaloy, powder metallurgyas-hot isostatically pressed low carbon Astroloy, and powder metallurgysuperplasticall~y forged IN-100 were compared by room and elevated tempevE. Lurej
DO 3 1473 EO!TION OF I NOV 65 15 O@SOLFTE
SECURITY CLAS1SIiCATION 6P THIS PAGE ("v nls En fo
44 _____ _____ ____
CONTENTS
PageAbstract 2
Introduction 3I Summary of Work 5
1. Evaluation of LCF properties ofThree Superalloys 5
2. LCF Performance of L/C Astroloy 9
3. Fatigue and Creep Crack Growth forL/C Astroloy 13
References 19
"Conclusion 20
Achievements 22
Publications 22
Figures 23
Tables 33
AIfR FORd O7FICE OF SCIENTIFIC RESARCH (ASO)MOTION OF TRANSMITTAL TO DDOThis technical report 111 been reviewed and laapproved for public r3leaBe IAW AFR 190-18 (7b).DistributLion is unlimited.A. D. DLOSZTecbnical Insormtion Offloeor
L i_ _... II ... , ,v;A
ABSTRACT 2
The fatigue-creep-environment interactions in nickel base superalloys wereinvestigated from 6501C to 760C in the low cycle fatigue range and in thecrack growth mode as h function alloy chemistry and microstructure.
Three alloys, conventionally cast and forged Waspaloy, powder metallurgy
as-hot isostatically pressed low carbon Astroloy, and powder metallurgysuperplastically forged IN-100 were compared by room and elevated temperature
low cycle fatigue tes in under control of total strain range. It was found
that at 427 C and 649 C Oaspaloy had the longest fatigue lifetimes followed bylow carbon Astroloy, then IN-lO0, but IN-100 had the highest cyclic flowstress and Waspaloy the lowest,
On the basis of its relatively good combination of fatigue properties, homo-genity and i--tropy resulting from powder processing, and because it can beheat treated to a wide variety of different microstructures, one alloy, lowcarbon Astroloy, was selected for a program to systematically determine theeffects of changes in microstructure on elevated temperature fatigue behavior.Low carbon Astrcloy was heat treaý,. to four very differegl microstructuren withvarious combinations of fine (500A) and coarse (2000-8000A) matrix ~Y'and grainboundaries with and without carbides and primaryj,ý).
Microstructure I in low carbon Astroloy, which is kn to offer the Laetcombination of tensile strength and stress rupture properties, was shown hereto have the best intermediate and elevated temperature low cycle fatiguestrength.
The rates of fatigue crack growth and of creep crack growth were measured inL/C Astroloy in the range of temperatures from 650*C (.64 Tm) to 760*C (.76 Tm).The fgtigue crack growth rates are strongly frequency dependent from 10 Hzto 10-3 Hz (1 cycle/15 min.). Decreasing frequency promotes intergranular creepcavitation during cyclic loading. At very low frequencies, fatigue crack pro-pagation is completely intergranular, and the cyclic crack growth rates areessentially equivilent to the creep crack growth rates.
Creep crack growth in low carbon Astroloy proceeds in a creep-brittle manner inthe range of temperatures from .64 TM to .76 Tm and in the range of nominalstress from . 4 Oy to , 7 ay. Creep crack growth occurs by a process of nucleationand growth of cavities ahead of the crack tip. The kinetics of cavity growth iscontrolled by pow-r law creep deformation.
Oxidation at the crack tip contributes to the creep crack growth rates in the
low temperature range (650*C), where the crack growth rates are small. In thehigh temperature range (760°C), the acceleration of creep crack growth due tooxidation is negligible. Fractography showed that the role played by oxidationis to enhance creep cavitation.
The results of sequential fatigue-creep cracking periods demonstrated that asimplistic linear superposition damage rule cannot be used to predict the crackgrowth rates under combined fatigue-creep loading. The fatigue crack growthrates at a frequency of I liz following a creep loading period are accelerated ab
much as one order of magnitude. This acceleration of fatigue cracking aftercreep cracking is attributed to the creep damage in the crack tip region of the
creep crack.
3
I. Introduction
Fatigue damage limits the useful life of nickel-base
superalloys in many applications. Today there is considerable
emphasis on fatigue in design of gas turbine components. Lack of
a good mechanistic understanding of low cycle fatigue and fatigue
crack growth causes time consuming, expensive generation of de-
sign data before any new alloy system can be used.
The problem of fatigue in nickel-base superalloys is very
complex because these alloy systems are used at temperatures from
0.4 to 0.8 TM. Up to about 0.6 TM the flow stress of y' precipi-
tation strengthened superalloys remain constant with increasing
temperature, while the ductility and tensile strength show only
slight reductions. By contrast the fatigue strength decreases
rapidly with temperature, and the fatigue lifetime can be reduced
by a factor of twenty from room temperature to 760 0 C (0.67 TM).
Obviously, fatigue limits the full utilization of the elevated
temperature strength of nickel-base superalloys.
It was the intent of this program to further the understand-
ing of the mechanisms of fatigue of superalloys. First, different
alloy systems, including Waspaloy, Lxca', and IN-100, ranging
in y' volume fraction from 22 to 55 percent w,!i evaluated and
compared by room and elevated tenipe. '_,.re .low 1.yclc It-ique test-
ing under controlled total strain range. Then on t'e 1basis of
this investigation, powder processed, as-hoi. ,.- 11Eat~ica~ly ,
~ carbon Astroloy was selected for a systematic study
,; & of the influence of grain boundary structure and y'
The main achievements of the research work reported here
are:
1. Development and refinement of the test procedures
used to evaluate low cycle fatigue and fatigue-creep crack
growth performance of high strength nickel base superalloys.
2. Detailed evaluation of the role of the intergranular
and transgranular microstructures of Astroloy on its per-
formance in low cycle fatigue.
3. Measurements of the fatigue and creep crack growth
rates in Astroloy as a function of temperature, frequency,
wave shape and hold times.
4. Extensive correlations between the micromechanisms
of fracture and the continuum mechanics parameters on the
one hand and the alloy microstructure on the other hand.
The following research tasks were identifieds
1. A quantitative separation of the initiation and pro-
pagation stages is badly needed. A systematic study of the
creep fatigue growth of short cracks (< 1 mm) may help resolve
this problem.
2. The use of K as a fracture mechanics correlation
parameter at elevated temperatures is purely arbitrary, even
if the correlation of K with the crack growth rates appear
acceptable. The strong time and temperature dependence of
elaRtic stresses at a crack tip make the use of K meaningless.
"There is need fcr a detailed analysis of time-dependent stress
' vid strain distributl.ns at the tip of a growing creep crack.
._ ..
21
3. A great deal of fundamental work remains to be done
to understand the effect of environment on cavitation and
cracking at crack tips.4. The life prediction methodology is at this time
purely empirical. There is a need for an integrated life-
prediction methodology which can account not only for fre-
quency and wave shape effects, but also for sequential periods
of creep and fatigue and for microstructural effects.
Ii
U~.i1
22
Achievements
J. Runkle, Elevated Temperature Fatigue of Nickel BaseSuperalloya, ScD thesis, MIT, February 1978.
J. S. Huang, Fatigue Crack Growth and Creep Crack Growth ofP/M HIP Low Carbon Astroloy at High Temperature.ScD thesis, MIT. February 1981
Publications
J. C. Runkle, R. M. Pelloux, Micromechanisms of Low CycleFatigue in Nickel Base Superalloys at Elevated Temperature.ASTM STP 675, Fatigue Mechanisms, 1979. p. 501-527.
J. S. Huang, R. M. Pelloux, Low Cycle Fatigue Crack PropagationIn Hastelloy X at 25 and 7609C. Met. Trans. A., Vol. 11A,June 1980j p. 899-904.
R. M. Pelloux., J. S. Huang, Creep-Fatigue-Environment Inter-actions, Editors, Pelloux, Stoloff, 1980.
J. S. Huang, R. M. Pelloux, Creep Crack Growth in Astroloy:Experimental Data and Theory. In preparation to besubmitted to Met. Trans. A.
J. S. Huang, R. M. Pelloux, Effect of Frequency and Temperatureon Fatigue Crack Growth in Astroloy, in preparation. Tobe submitted to Met. Trans. A.
R. M. Pelloux, N. S. Stoloff (RPI) organized an AIME symposiumon Creep-Fatigue-Environment Interactions, Fall meeting,AIME, September 18-19, 1979. The proceedings werepublished by the Metallurgical Society of AIME.
• ..... •: •~~ ~ ~~I,• ,,::',: : .•S• s • • • • • • • i,
250
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210 0.04 Hz
1A ** ......... 3. 0%1j - .. e T *' ..C~~J .....0 ..1
130-
1101iII100 0' 102 O
N, cycles
Fiure 1a -T-he"c~ycl-ic -r~esponse of Waspaloy at room temperature underdifferent strain ranges shows a peak in stress range at about 10 cycles.
260
240 Room temp,
220 0.05Hz
go'I -0 100
Waspaly, L/ Astrloy, nd IN L /C Athe same s--triragshw ii
KI 6I4a0adnngsfeig eairI2 to 0 0 0
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ow
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00
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a8lo o a3'L; eqn ;
26
I
Figure 5. Sketches of low carbon Astroloy grain boundaries.
Hi
.1 .,. A
.40
0 O0UnU
0 r
(D ..JLL.
di
f.IN"W 44.
0 0 Lo toL of
%e3 V
28
T/Tm0.19 0.45 0.52 0.59 0.66
4000
0- 1 WAVY G.B. BIMODAL,'U-U U PLANAR G.B. BIMODALy
Figure 7. The variation on number of cycles to failure withtemperature for the four microstructures of low carbon Astroloytested at a constant total strain range of 2.2%.
29
H I P- Astrol oy"650' C, airR z.05
-.~ ~ - iNHZun FAIN •v:u.
10
/ V
EV
"-CYCLICCTO D/
/ • /
00
10 20 30 40 50 6070 100
A K M paoV/
Fig. 8 . Fatigue crack growth rates vs AK for L/C Astroloy in air,a, 650°C.