NASA Contractor Report 175050 

Grain Boundary Oxidation and Oxidation 

Accelerated Fatigue Crack Nucleation 

and Propagation 

HUCLEAFICM AHD EBOPAG &TIOM fSpracuse Univ, , 

' N- P, I 19 p HC A02/8Pf A03 CSCL 333 ffnclas 

G3,/26 0565Y 

H. W. Liu and Yoshiki Oshida 

Syracuse University 

Syracuse, New York 

January 1986 

Prepared for 

Lewis Research Center 

Under Grant NAG 3-348 

Nat~onal Aeronautics and 

Space Administration 

Grain boundary oxidation may accelerate fatigue crack nucleation and 

propagation. McMahon and Coffin [I] studied oxide rupture as the process 

of fatigue crack nucleation in cast Udirnet 500. 

Antolovich et al.[2] have sham that, when an oxide reaches a &tical 

size at an applied stress level, the oxide will fracture. The oxide crack 

will serve as a fatigue crack nucleus to graw by the subsequent cyclic 

load. Coffin [3], Solaron [4], and Solcmn and Coffin [S] shcswed that fatigue 

crack growth in air has a strong frequency dependence, which is absent in 

vacuum. They attributed the accelerated fatigue crack growth rate and the 

frequency &pendence in air to environmmtal effects. 

Remy et al.[6] have sham that the oxidation of carbides accelerated 

fatigue crack grclwth in Mar-M509. Oshida and Liu [7] have studied the kinetics of grain boundary oxidation in a nickel-base superalloy, TAZ-8A. They 

treated the grain boundary oxide penetration as a precrack, and analyzed 

the effects of the precrack on the remaining fatigue life. Liu and -an 

[81, MScwan and Liu [9] , Liu and Oshida [lo] analyzed the effects of 

grain bundaq oxidation on the accelerated fatigue crack growth. A -1 

of intermittent microruptures of the grain boundaxy oxi& was proposed 

an2 analyzed. In the law frequency region, the mdel shows that the fatigue 

crack growth rate is inversely proportional to cyclic frequency as observed 

in a n&er of materials. 

In this paper, the earlier studies on grain boundary oxidation kinetics 

[7], the effects of grain boundary oxidation on fatigue life [ll], and the 

mckl of intermittent microruptures of grain boundary oxide [lo] will be 

reviewed and synthesized. 
11. GRAIN BOUNDARY OXIDATION KINETICS 

Figure la shows a cross-section of an unoxidized cocqpn of the cast 

nickel-base superalloy, TAZ-8A. The microstructure etched by a mixed solution 

of HC1 and HP2 (by 10:l wlm ratio) shcws the y/yl matrix and the needle 

like pmokent Chinese-script M2 carbides, primary MC particles (white particles) and C type carbide colonies (dark particles). Only the &$,3C6 33 6 

carbides wexe found along grain bouikies, It is noted that very fw of 

these carbides are found in the thin &ace layer of the test coupon. 

Figure lb shms the picture of a cross-section of an oxidized cylindrical cmpn- The surface is covered by a thin layer of &ace oxide. The 

grain boundary oxide penetrated much deeper into the spech than the thickness of the surface oxide. When the applied stress is high enough the grain 

boundary oxide my crack, and the cracked grain boundaxy oxide may serve as 

a nucleus to graw by the subsequent fatigue cycles. If the grain hundary 

oxide crack is foe quickly, the fatigue crack nucleation period as well 

as the overall fatigue life will be shortened. 

Oshida and Liu [ll] measured the grain boundary oxide penetration depth, 

a, as a function of the oxidation temperature T and the expsure time t. 

At a given oxidation tenperatme and exposure time, the grain boundary penetration depth varies widely fm one grain boundary to another, The statistical distribution of the grain boundary oxide penetration depth was studied, 

and its effect on the statistical scatter of the fatigue life will be analyzed, 

Cylindrical test coupons were oxidized in a laboratory air at temperatures of 600, 800, and 1000~~ for -sure times fmn 100 to 1000 hours. The 

oxidized disk coupons were sectioned, and the deepest grain boundary oxide 

penetration depth, ami, of the ith sectioned surface was mawed. Mter the 

measurerent, a thin layer of the coupon approximately 80 m thick was ground 

off and another ami of the new section was rreasured. This pracess was repeated twelve tirrres for each oxidized coupon to collect twelve data points for 

the statistical analysis. 

Altogether twelve sets of meas-ts at twelve carbinations of T and t 

were collected. It is assunu3d that the relation between a t, and T has the mi' 
The linear regression analysis of the data gives the e~npirical relation 

=m 

in an. t in seconds, the activation energy in Kcdl/ml, and T in OK. The 

coefficient of correlation is 0.96. The plot of ln (a,i/tO. 25) versus (In) 

is shown in Figure 2. A sizeable scatter is shown in the figure. c0-g and 

Wluchowski [12] and Turnbul and Hoffman [13] have sbm that grain bunday diffusion is a function of the anqle of the mis-orientation of the two 

neighboring grains. Therefore it is reasonable to expect that a statistical 

scatter of ami exists. 

Equation (2) is an empirical equation obtained by the regression analysis. The deviation of each individual measuremnt £ran the empirical relation 

can be expressed in terms of the variation ai 

With the masured ami at twrature T and -sure tirre t known, n = 0.25, and 

Q = 4.26 ~cal/ml. , the value of ai can be calculated from -tion (3 1 . The 

values of a.'~ for the 144 measurerrents are ranked in the i~creasing order. The 

1 

probability of finding a value less than ai on a sectioned surf.ce is 

P(a.1 .= ~~/(l + N) . Mi is the ranking nmker of ai and N is the total number 

17 

of the nutasuremnts. 

The probability of finding an a-value on a sectioned surface equal to 

or more than a is [l - P (a) .I. The Weibull plot of In In [l/ (1 - P (a) . ) ] versus 

3 I 

a of all of the 144 data points is shown in Figwe 3. 

The Weibull distribution function is 

For the data. b = 1.85 is the shape parameter or We;hull dulus; aU = 0.53 x 

- 6 , the location par-ter ; and I = 4.8 x 10 , tke scale prmter. 0 

3 
The maxhnnn value of a dong the periphery of a sectioned surface can be 

considered as the maxirrnrm penetration of an exposed area of rDd of the test 

coupon. D is the coupon diameter and d is the grain size. Another sectioned 

surface at a distance one grain dieter away contains an entirely different 

set of grain boundaries and it is another independent sample of the total 

exposed area of the test awn. 

LRt P(a) be the pmbability to find the maxirrnan value less than a on a 

unit surface area of a test coupon, P (a) the probability of finding a value 

less than a on a surface area S, and P (a) . the probability for a sectioned 

7 

surface. 

For an area of ITD~, 

ccnrtsining (4) and (7) , 

The value of P(CL)~ can ke taken as the value of P(a)S, +he probability to 

find a grain boundary penetration depth "a" on an qsed surface 

area S. It has been known that fatigue lives of gas turbine engine 

c~nents have very wide statistical scatter. It is also known that the 

statistical scatter of fatigue crack propaga+don is rat?= narmw. Therefore 

grain bundary oxidation and its effect on fatigue crack nucleation auld 

be one of the contributing factors to the wide variation the fa^^^ 

lives of gas turbine engine components. 
111. GRAIN BOUNDMN OXIDATION AND IXM CYCLE FATIGUE LIFE 

In the intermediate AJ region, fatigue crack grorwth rate is related 

to AJ by a per relation, 

Shih and Hutchinson have shown that for a srrall crack in a RmkqOsgocd elastic-plastic solid in general yielding [14,15] 

where n1 and n2 functions of strain hardening enent "n". For a given 

mterial, they are amstants. The value of n2 is usually much smller than 

the first tern. WD is the deformation work density 

a and E are the applied stress and strain away £ran the cracked region. 

Zheng and Liu [16] have studied the crack tip fields for small cracks in 

plates in tension for piecewise per hardening materials. a = EE for 

n 

u a and (o/uY) = (E/E~) for o > aye Y smali single edge u-acks (a/w 

= 0.1) in both stress and plane strain were studied. They found that 

Equation (10) is also appLicable to the piecewise per hardening materials. 

For cyclic loading, Equation (10) is rodified to the form 

Crack tip stresses and strains will cause a crack to grm, only when 

the crack tip is om. Take t?e sbple assuirption that under a wletely 

reversed cyclic axial loa&c;, a crack tip is ow hben *e aqlied s+~ess 

becanes tensile during the loading half of the cycle (see Figure 4). 
Theref ore 

o and E are the applied stress and strain. For a piecewise per hardening 

material, AW is the cross-hatched area in Figure 4, which can be appm~ted D 

by 

Substituting (12) into (9) and integrating, 

?he remaining fatigue life after a crack reaches the size a is (Nf - N ) . a 

0 0 f 

is mre or less a constant. Therefore the fatigue life with a precrack, N , 

is a function of the precrack size, ao. f"o 

The gmin boundary oxide crack can be considered. a precrack. If th, re 

lati02 (16) is deterministic, t5e probability to have a fatigue life of N fa 

is also the probability to have an oxide crack size E on a speck sarfac8. 

0 

If the oxidation process is fst, the fatigue life m~lZ 5e reciud considerably. It is well known that the fatigue life of an engine coiinerlt has a 

wide sta+dsticzl scatter. Perna~s grain Sounciary oxi&-don and ~~ bo.s\.dxry 

oxide craclcing contrikute to a siqnificant part of t?e setter. 

The reciuction in fatLcpe life is ca~ed by shorten& "aack nucleation" 

prid. The accelerateci .fatigue crack grOWC& due to oxidation will 5e &iscussed 

in the next sectdon. 
IV. GRAIN BOUNDARY OXIDATION AND FATIGUE CRACK PIiDPAGATION 

Fatigue crack growth at elevated taperatures is sensitive to cyclic 

frequency. Figure 5 shms the frequency effect on fatigue crack growth rate 

at a mnstant AK level and at a constant mrature T. The cyclic wsve 

shaps of these tests are also shown. Several of +& tests had hold tin-e 

at K-. The others had sawtooth wave £om. The crack gmwth rates of 

Inconel 718, Astroloy, and Waspaloy, in the low frequency region, are inversely proportional to cyclic frequency, v, and are linearly pro~rtional 

to the tirre duration at the K-. The tim rate of fatigue crack gruwtt., 

da/dt = a (da/dN) (l/v) is a constant. a is a constant, its value &pn& on 

the wave shape of the fatigue cycle. In the low frequency region, fatigue 

crack growth is intergranular. 

For a sustain4 load test, i..e, constant-K test, da/dt is anscat 

and crack growth is intergramla-. Crack ur&r a sustzineci co~s'antK test is cmnly known zs Cree? crack m. ?"ne fatigue cra& growths 

in the lw frequency region have these two growth characteristics. Thus khe 

faciqce crack gmkkl in the 1w fre~rumcy region is often referred t~ as CZ~D - 

crack m, - even thougn fatigue crack gxmi r&es at elevated '-a?.xes 

in vacuum an6 in &Y are quite different, ad the Eifference CE~D~ be explzined in te-rms of creep bge. 

The qain bounikry - n? oxi5e =.ne=a+tion or inZ-8' 15s 9ivei-1 b2- -Son (1) . 

Pe-~n~s, +he oxi& =ietza-2on ciesth zt a crack ti? cz? 5e h~it~~i i: the 

fom 

wkre 9 is the qzin b~~Ci;_77 c-3-f fusion ef ficiexz and in is m- e~iriczl 

$3 

mns-ait . 5 add ke a 5xx+2~n of 3e ---- L-~ll. - +A? 5ieX m5 &. -7s c:.z79~? ancentrxion ii +5e abienz. 

Anmlovi&rl hx fm-rld <?at, at cz?. mslied stress, +Ae 3~i52 Ir: a %-cot5 

spch-rl will rqt~ye, -den it zeaches 2 critical size [2] . T5s criticzl 

crzck ti;> oxide size at -v?=e mat he related tc X. 
These tests had a hold time at K-. At K-' the crack tip oxide will 

rupture when the oxide reaches a critid size, 6a, and the crack will 

gm by the same amurat. Once the crack tip grcws to its new position, this 

pmaess of garaira diffusion, grain bundary oxidation, grain boundary 

oxide mq?twhg, and the heremmtdl crack will repeat again. This 

press ean be repeated ths during the hold tirre at K-. 

At a given K-level, the th~ incremznt, 6ti, necessary for the oxide to 

zeach the critical size 6ai are related directly to ai by a relation such as 

The number of microrup-s, n, during the hold thz, At, is n = At/&t = 

~a/&a, Aa is the crack grolwth during the hold the of one fatigue cycle. "n" 

is proportional to At but inversely proportion& to frequency, v. Fatigue crack 

gmwf21 per cycle is the sum of the intermittent microruptures per cycle. 

) is the crack grm rate at a reference fr-cy vo. &/dN is inversely 

vo proportional to v. This agrees with the qirical data for Inwnel 718, 

Astroloy, and Waspaloy in Figure 5 in tle law frequency region. The sm 

conclusion that da,/dN is inversely praportional to cyclic frequency, can be 

derived for other wave forms if the frequency does not affect crack ti? 

field [lo 1 , 

The micronptures, which take place alonq oxiZized grain Soundaq, result 

in intergranular crack grawth. Therefore bo%\ the co,?sL3nt tine rate of fa+Ac;~ 

crack growth and the intergranular fracture are not necessarily caused by Cree? 

damage, They can be the result of grain bundary oxidation. Cree? cracking qraJth 

has a conxiotation of crack growth by creep deformation, grain boundaq void 

formation, grain boundaxy cavitation, and/or the qrow"Lh of such voids an6 cavities, At least for som materials, to call fatigue crack graJth as Cree? 

cracking in the 1m frequency region is a rnisnoaner. 
V. liIJMMkIrY AND CX)IJCLUSIONS 

1. Grain boundary may accelerate fatigue crack nucleation and propagation. 

2. Grain boundary kinetics of a nickel-base superalloy, TAZ-8A was studied. 

The grain boundary oxide penetration depth can be expressed in a dfied 

form of the Arrhenius relation. 

3. Grain boundary oxide penetration depth varied widely fm one boundary 

to another. The Weibull' s distribution function of grain baundary oxide 

penetration was studied. The variation in oxide penetration may contribute to the wide statistical scatter of the fatigue lives of gas turbine 

engine canponents . 

4. The grain boundary oxidation kinetics are essential for the develapmt of 

a quantitative life prediction methodology based on a mchanistic &el 

for those materials susceptible to fatigue Life ifipairment by oxidation. 

5. The effects of grain boundary oxide crack on fatigue life ue analyzed 

and discussed in tern of the reduced fatigue crack "nucleation" period. 

6. An intermittent grain boundary oxide microrcp- &el is proposed for 

the fatigue crack growth in the low frequency region. The &served inverse relation between &/dN and cyclic frequency, v, and the observed 

intergranulv fatigue crack growth are consistent with the propsed 

rrodel. 

7. To call fatigue crack grad in the low frquency region "trees cracking" 

because of mns"at &thw rate of crack growth a12 intergrar.ular fracture, 

is a misnorer for those materials susceptible to fatigue life irpzirment 

by the o~idation process. 
[l] C. J-McMahon and L. F. Coffin, Jr. , ; Met. Trans, 1, 3443/3450 (1970) 

[2] S.D.Antolovich, E.Wsa and A.Pineau; Mat. Sci. and Engr., Feb. (1981) 

[3] L.F.Coffin, Jr.,; Trans. ISM, - 56, 339/344 (1963) 

[4] H.D.Solamn; Sour. of Mats., - 7, 299/306 (1972) 

[5] H.D.So1-n and L.F.Coffin, Jr.,;"The Effects of frequency and environment of fatigue crack growth in A286 at ~OOO~F", PSTM STP-520, 112/122 

(1972) 

161 J. Reuchet and L. Remy; Met. Trans. , - 14A, 141/149 (1983) 

[7] Y.Oshida and H.W.L~U; "Grain Boundary Oxide Crack, Oxidation Accelerated 

Fatigue Car& Grmth and Fatigue Life" , Pm. of Mechanical Failures 

Prevention Group Syrrp~si~m, Gaithersburg, MD, (April 1985) 

[8] H.W.Liu and J.J.MSman; Scripta Met., 1, 507/512 (1981) - 

[9] J.J.McGowan and H.W.Liu; Jour. of Eng. Mats. and Tech., Trans. of ASME, 

- 103, 246/252 (1981) 

1101 H.W.Liu and Y.Oshida; "Grain Boundary Oxidation and Fatigue Crack Growth 

at Elevated Terrperatures" , to be published (1985) 

[ll] Y.Oshida and H.W.Liu; "Grain Boundary Oxidation and an Analysis of the 

Effects of Pre-Oxidation on Subsequent Fatigue Life", prescqL& at AS'M 

Syrrposiuin on "m Cycle Fatigue - Direction for the Future", Bolton 

Landing, NY (Sep./Oct. 1985) 

1121 L.Couling and R.Srroluchmski; J.Qpl.Phys., - 25, 1538 (1954) 

1131 D.Tumbull and R.Hof%; Acta Met. , - 2, 419 (1954) 

[14] C. F. Shih and J. W. Hutchinson; Jour. filats . a?d Tech. , Trans. of AS-%, 

- 98, 289/295 (1976) 

[15 ] C. F. Skih; " J-integral estimates for stzain hardaming raterizls in antiplane shear usirg f u l l y p l a s t i c solution", in "Mez;rlznics of Crack Grme" 

ASTM STP-590, 3/26 (1976) 

[16] M.Zhen9 and H.W.Liu; "Crack Tip Fieid anti Fatigue Crack Gr-I. in Gerisral 

Yielding and Lazr Cycle Fatigue" NASA Con",-ac"tr Iieprt No. 174686 (Se?. 

1964) 

1171 ~.Tqeerasooriya and T-Nicholas; "Hol6-Tins Effects in E1eva-d Terrp=rakure 

Fatigue Crack Pro?agationn AFWX-TR-64-4184 (WG\ 1-985) 
[18] F.Gabrielli and R.M.Pelloux; &t. Trans., - 13A(6), 1083/1090 (1982) 

[19] Ming zhou and F. Gabrielli; Z.Metallkde. , - 74 (10) , 639/642 (1983) 

[20 ] R. M. Pelloux and Jia S . Huang ; " Creep-Fatigue- Interactions in As troloy" 

in "Creep-Fatigue-Environent Interactions", ed. by R.M.Pelloux and 

N. S. Stoloff , AIME, 151/164 (1980) 

[21] ~.M.~ikbin and G.A.Webster; "Prediction of Crack Gruwth Under CreepFatigue Loading C0nditions'~ASTM Symposium on "- Cycle Fatigue - 

Directions for the Future", (Sep. /Qct. 1985) 

[22] L.A.Jams; "The Effects of Frequency upon the Fatigue-Crack Grawth of 

Type 304 Stainless Steel at 1000~~" in "Stress Analysis and Grcwth of 

Cracks", ASTM STP-513, 218/229 (1972) 

[23] J.M.Larson, B.J.Schwartz and C.G.Annis, Jr.; "Cumulative Damage Fracture 

Pkchanics Under mgine Spectra" , AFWL-TR-79-4159 (Jan. 1980) 
Figure la cross-section microstructure of unoxiciized test coupon, TAZ-8A 

primay MC carbids 

prominent Chhese -scr/Df 

MC carbide 

MZ3 c6 cotBide colonies 

- 
Figure 1S Cross-section of oxidized test mqn 
temperature PC1 

800 

Fipe 2 Arrehius plot of arm, based on Equation (2) in the text 


materlal temp.f'C) AK (MParm) wave-form 

0 INCONEL 718 649 28 /-v-v-(17) 

INCONEL X-750 6 50 30 /q/VVVV (18) 

@ INCONEL X-750 6 5 0 

50 /3V-7r (20) 

50 (20) 

I0 M[21) 

a0 w (229 

30 /7NV7/V (23) 

V ~CF-MO-v STEEL 5 6 5 I0 W(21) 

Frequency (cycle/sec 

j - 5 p~rluency deperidencies of hi.gI1 temperature lw cycle fatigue 

c:ra(:Ic qrrxlrt 11 of variotts mteri a1 s 
 o or sale by the National Technical Information Service, Springfield, Virginia 22161 

1. Report No. 

NASA CR-175050 

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

4 Title and Subtitle 

Grain Boundary Oxidation and Oxidation Accelerated 

Fatigue Crack Nucleation and Propagation 

7 Author(s) 

H.W. Liu and Yoshikl Oshida 

5. Report Date 

January 1986 

6. Performing Organization Code 

8. Performing Organization Report No. 

None 

10. Work Unit No. 

9. Performing Organ~zat~on Name and Address 

Syracuse University 

Dept. of Mechanical and Aerospace Engineering 

Syracuse, New York 13210 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546 

11. Contract or Grant NO. 

NAG 3-348 

13. Type of Report and Period Covered 

Contractor Report 

14. Sponsoring Agency Code 

533-04-1 2 

15. Supplementary Notes 

Project Manager, Jack Tel esman, Structures Di vi sion, NASA Lewis -~esearch 

Center, Cleveland, Ohio 44135. This material was presented at the MInnowbrook 

<Conference on Life Prediction for High Temperature Gas Turbine Materials, Blue 

Mountain Lake, New York, August 27-30, 1985. This research was sponsored by 

the HOST Program. 

16. Abstract 

Fatigue life at elevated temperatures is often shortened by oxidation. Grain 

boundary oxidation penetrates deeper than the surface oxidation. Therefore, 

grain boundary oxide penetration could be the primary cause of accelerated 

fatigue crack nucleation and propagation, and the shortened fatigue life at 

elevated temperatures. Grain boundary oxidation kinetics was studied and its 

statistical scatter was analyzed by the Weibullis distribution function. The 

effects of grain boundary oxidation on shortened fatigue life were analyzed and 

discussed. A model of intermittent microruptures of the grain boundary oxide 

was proposed for the fatigue crack growth in the low frequency region. The proposed model is consistent with the observations that fatigue crack growth rate 

In the low frequency region with hold time at Kmax is inversely proportional 

to cyclic frequency and that crack growth is intergranular. 

17. Key Words (Suggested by Author(s)) 

Oxidation; Fatigue crack growth; 

Crack initiation 

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unclassiaied 21. No. of ages 

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Cleveland, Ohio 44135 

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