Low Cycle Fatigue of As-HIP and HIP + Forged Rene 95

SHAH]I) BASHIR, PHILIPPE TAUPIN, AND STEPHEN D. ANTOLOVICH

The continuous cycling and hold t ime low cycle fatigue 13roperties of the Ni base s u p e r - alloy Ren~ 95 were studied at 649~ using powder products (-60 mesh) in the a s -HiP and HIP + forged condit ions. It was shown that c racks were in i t ia ted by pores , by c e r a m i c pa r t i c l e s and by a c l a s s i c a l stage I me c ha n i sm for both ma t e r i a l s and for both cycle c h a r a c t e r s . F o r the cont inuously cycled a s - H i P ma te r i a l , deformat ion was r e s t r i c t e d to welI defined bands at low s t r a i n s and became homogeneous as the s t r a i n level i nc reased . The total energy to f r ac tu re i nc r ea sed abrupt ly in the low s t r a in r eg ime and this was also re f lec ted by a b reak in the Coff in-Manson plot. In al l cases c racks in i t ia ted at pores . The hold t ime spec imens exhibited an ex t r eme ly high d is locat ion densi ty and surface connected in i t ia t ion yet without a s igni f icant life reduct ion . The observa t ions were e s s e n - t ia l ly s i m i l a r for the HIP + forged m a t e r i a l except that deformat ion tended to be confined to well defined s l ip bands even at high s t r a i n s and to some extent even for the hold t ime tes t s . This behavior was a t t r ibuted to the fact that the r ' - p r e c i p i t a t e s were s m a l l e r , more coherent and more read i ly sheared by d is loca t ions which were s t rongly pa i red . The re was a marked tendency for c rack propagat ion to change from t r a n s g r a n u l a r to i n t e r g r a n u l a r (also observed for the a s - H i P mate r i a l ) at a unique combinat ion of c r ack length and plas t ic s t r a i n . The t r a n s i t i o n occur red at sho r t e r c rack lengths for the HIP + forged m a t e r i a l except when c rack in i t ia t ion was subsu r face . In this case the t r ans i t i on was delayed and the life was g rea t ly enhanced, indicat ing that the env i ronmen t plays a ma jo r role in de t e rmin ing the fatigue life.

I. INTRODUCTION

I~ENI~ 95 is a high s t reng th n ickel base supera l loy that, in powder form, has potent ia l appl ica t ion as a disc m a t e r i a l in jet engines . Since d i scs a re operated at high t e m p e r a t u r e and under va ry ing loads, it is i m- por tant that the m a t e r i a l exhibi ts both good c o r r o s i o n r e s i s t a n c e and r e s i s t a n c e to fat igue. In this paper the r e su l t s of a study d i rec ted towards a mechan i s t i c unders tanding of the low cycle fatigue ( L C F ) p r o p e r t i e s of Ren~ 95 a re d i scussed . P a r t i c u l a r emphas i s is given to meta l lographic examina t ion by optical, scanning e l ec t ron mic roscopy (SEM) and t r a n s m i s s i o n e l ec t ron mic roscopy (TEM).

II . EXPERIMENTAL PROCEDURE

A. Mate r i a l , P r o c e s s i n g and Heat T r e a t m e n t

The composi t ion of the ma te r i a l used in this study is shown in Table I. Powders of this composi t ion were produced by an a rgon a tomiza t ion p roce s s and only those pa r t i c l e s that would pass a 60 mesh s ieve were used to make the compact . The compacts were hot i sos ta t i ca l ly p r e s s e d (HIPed) at 1121~ (2050~ for 3 h at a s t r e s s of 103 MPa (15 ksi). After HIPing, LCF blanks were cut f rom the compact . The a s -HIP b lanks were subsequent ly solut ioned at 1149~ (2100~ for 1 h and quenched in a sa l t bath to 538~ (1000~

SHAHID BASHIR and STEPHEN D. ANTOLOVICH are Graduate Research Assistant and Professor of Materials Science, respectively, Department of Materials Science and Metallurgical Engineering, University of Cincinnati, Cincinnati, OH 45221. PHILIPPE TAUPIN, formerly Research Associate, Department of Materials Science, University of Cincinnati, is now with Framatome, Paris, France.

Manuscript submitted November 20, 1978.

METALLURGICAL TRANSACTIONS A

They were then aged at 871~ (1600~ for one h and at 649~ (1200~ for 24 h. The HIP + forged m a t e r i a l was forged into the form of a disc and the en t i re disc was aged at 760~ (1400~ for 16 h af ter solut ioning.

B. Tes t P r oc e du r e

Standard t ens i l e and longi tudinal LCF tes t b a r s were machined and low s t r e s s ground. Fat igue tes t ing was done in a i r at 649~ using a c losed loop u n i v e r s a l tes t machine ha the s t r a i n cont ro l mode at a f requency of 0.33 Hz. All tes t ing was done in a fully r e v e r s e d mode (RE = -1 , Ae = ~). F o r the hold t ime tes ts , there was a 10 s r a m p t ime to ma x i mum s t r a in , a 900 s hold at ma x i mum s t r a i n and a t0 s unloading t ime to zero s t r a in . H y s t e r e s i s loops were taken in t e rmi t t en t ly us ing an XY r e c o r d e r and the load was moni to red continuously by a s t r ip chart .

C. Meta l lography

1. Optical Metal lography. Standard optical me ta l - lographic techniques were used to evaluate the in i t ia l s t r u c t u r e s of both the a s -HIP and the HIP + forged m a t e r i a l . W a l k e r ' s reagent was used for etching.

2. Scanning E lec t ron Microscopy (SEM). Af ter t e s t - ing, the f r ac tu re su r faces of spec imens were examined by SEM. The goal of this phase of the study was to both identify in i t ia t ion s i tes and to de t e r mi ne the mechan i sm involved in c rack propagat ion.

3. T r a n s m i s s i o n E lec t ron Microscopy (TEM). Af ter tes t ing and examining by SEM, sma l l wafers were cut pe rpend icu la r to the spec imen axis f rom the gauge sect ion about 2 m m f rom the f r ac tu re su r face . These wafers were subsequent ly e lec t ropol i shed using a so lu- t ion of 45 pct acet ic acid, 45 pct butyl ce l lusolve , 10 pct pe rch lo r i c acid at 20~ and 52 V, by s t andard twin

ISSN 0360-2133/79/1011-1481500.75/0 �9 1979 AMERICAN SOCIETY FOR METALS AND VOLUME 10A, OCTOBER 1979-1481

THE METALLURGICAL SOCIETY OF AIME

Table I. Composition of Ren~ 95 Alloys Used in This Study

As - HIP HIP and Forged Element Wt. Pct Wt. Pct

C 0.050 0.08 Mn <0.01 0.01 Si 0.08 0.06 S 0.005 0.002 P <0.005 Cr 12.86 12.80 Co 8.28 8.05 Mo 3.53 3.56 Fe 0.05 0.39 Ta <0.01 <0.0 l Cb 3.50 3.60 Zr 0.04 0.053 Ti 2.49 2.36 AI 3.61 3,57 B 0.009 0.01 W 3.42 3.59

Ni Balance Balance (a)

j e t t e c h n i q u e to p r o d u c e fo i l s that w e r e s u f f i c i e n t l y th in fo r v i e w i n g by T E M . The g o a l of the e l e c t r o n m i c r o s c o p y was to c h a r a c t e r i z e the ~ ' s t r u c t u r e , the g r a i n s t r u c t u r e , c a r b i d e s , and the d i s l o c a t i o n s u b - s t r u c t u r e . In g e n e r a l , the c r y s t a l l o g r a p h i c o r i e n t a t i o n of a p a r t i c u l a r a r e a of i n t e r e s t was d e t e r m i n e d and m i c r o s c o p y was done u s ing s o - c a l l e d two b e a m c o n d i - t i o n s . In add i t ion , d a r k - f i e l d t e c h n i q u e s w e r e used to b r i n g out the n a t u r e and s i z e of the T' p r e c i p i t a t e s .

III. R E S U L T S AND DISCUSSION

A . In i t i a l S t r u c t u r e s

1. O p t i c a l M e t a l l o g r a p h y . The s t r u c t u r e s of both the a s - H i P and the HIP + f o r g e d m a t e r i a l s a r e shown on the o p t i c a l l e v e l in F i g . 1. Note the c l e a r d e n d r i t e p a t t e r n tha t can be s e e n in F ig . l (a ) a s w e l l a s the l a r g e u n s o l u - t i o n e d y ' - p a r t i c l e s . T h e m i c r o s t r u c t u r e s w e r e g e n - e r a l l y qu i t e h o m o g e n o u s and i ndependen t of o r i e n t a t i o n .

2. T E M . a) A s - H I P . T h e i n i t i a l T ' - m o r p h o l o g i e s and d i s l o c a t i o n s u b s t r u c t u r e s a r e shown in the T E M m i c r o - g r a p h of F i g . 2. T h e g r a i n s i z e a v e r a g e d f r o m 5 to 50 ~ and s m a l l c a r b i d e s w e r e f r e q u e n t l y s e e n d i s t r i b - u ted t h r o u g h o u t t he s t r u c t u r e . In t he m a t r i x the d i s - l o c a t i o n d e n s i t y was qu i te low and t h e r e w e r e both s m a l l (~0.2 ~ d i a m ) and i n t e r m e d i a t e (~2 # d i a m ) y ' - p r e c i p i t a t e s . T h e i n t e r m e d i a t e T ' - p r e c i p i t a t e s c o n - t a in n e t w o r k s of n e a r - e d g e d i s l o c a t i o n s on the i n t e r - f a c e s . T h e s e d i s l o c a t i o n s a r e s e s s i l e and a c c o m m o d a t e t he m i s f i t s t r a i n b e t w e e n the p r e c i p i t a t e and m a t r i x . S i m i l a r d i s l o c a t i o n n e t w o r k s have b e e n d e s c r i b e d in d e t a i l e l s e w h e r e . 1 We have d e t e r m i n e d , u s ing a 6- f r i n g e t e c h n i q u e 2 that the s i g n of the m i s f i t is p o s i t i v e . We have a l s o d e t e r m i n e d by e x t i n c t i o n e x p e r i m e n t s tha t the B u r g e r s v e c t o r s a r e of the f o r m a / 2 <110>. T h u s one c a n app ly B r o o k s 3 f o r m u l a to c o m p u t e the m i s f i t s t r a i n :

+ l b l = d [1]

w h e r e b = B u r g e r s v e c t o r and d = s p a c i n g b e t w e e n d i s l o c a t i o n s . T h e m i s f i t s t r a i n t u r n s out to be ~0 .25 p c t .

(b) Fig. 1-Microstructure of as-HIP (a), and HIP + forged (b), Ren6 95. Note the dendritic structure in (a). In (b) forging has produced a more homogeneous structure.

Fig. 2-Initial structure of as-HIP Ren6 95. Note the interfacial dislo- cations on the large ~,'-particles. These have been determined to be near edge having a Burger's vector of a/2 < 110 >. The combination of dislocation spacing and Burger's vector gives a misfit parameter of % 0.25 pct.

1482-VOLUME 10A, OCTOBER 1979 METALLURGICAL TRANSACTIONSA

In addit ion to the i n t e rmed ia t e 7 ' - p r e c i p i t a t e s , l a r g e r ) / - p a r t i c l e s were f requent ly obse rved at g r a in boundar i e s . They were about 5 ~ in s i ze , contained g e o m e t r i c a l l y n e c e s s a r y in t e r f ac i a l d i s loca t ions and w e r e coheren t with one of the g r a in s . These r e m a i n e d unsolut ioned, g rew during subsequent heat t r e a t m e n t and have the ef fec t of l imi t ing the g r a i n s i z e . In no ins tance w e r e any in t e r r ac i a l d i s loca t ions obse rved on the fine m a t r i x y ' indicating that t he r e is comple te cohe rency between the m a t r i x and the p rec ip i t a t e for y ' up to at l eas t 0.2 ~ d iam.

b) HIP + Forged . In Fig. 3, the morphology of the fo rged m a t e r i a l is shown. Again the g r a in s ize is gen- e r a l l y on the o rde r of a few mic rons , ranging any- where f rom one mic ron to approx ima te ly 50 m i c r o n s . L a r g e ca rb ides d i s t r ibu ted randomly throughout the m a t r i x can be seen as wel l as numerous s m a l l e r c a r b i d e s . F i g u r e 4 shows the s m a l l m a t r i x ~' , the i n t e rmed ia t e ~' with the i r a s soc i a t ed in t e r r ac i a l d i s - locat ions , a v e r y large carb ide pa r t i c l e , and a number of s m a l l e r ca rb ide p a r t i c l e s . Note the numerous d i s -

locat ions around the l a rge ca rb ide p a r t i c l e . They probably a r e a r e s u l t of the d i f f e r ence in t h e r m a l ex- pansion between the carb ide and the m a t r i x which r e s u l t s in s t r e s s e s dur ing cooling which were suff i- c ient to nuclea te the d i s loca t ions . F igu re 5 shows a l a rge y ' - p a r t i c l e , a boundary made up of a v e r y c lo se ly packed a r r a y of d i s loca t ions as we l l as nu- m e r o u s s m a l i ca rb ide p a r t i c i e s . Such boundar ies were n e v e r obse rved for the a s - H i P m a t e r i a l . The same boundary is seen in da rk - f i e l d in F ig . 6, where it can a lso be seen that the s ize of the ~' in the m a t r i x is app rox ima te ly 0. t ~. In F ig . 6 the d i f f rac t ing condi- t ions on e i the r s ide of the boundary were somewhat d i f ferent , indicat ing a s m a l l d i f f e rence in or ien ta t ion .

B. T ens i l e and LCF T e s t Resu l t s

1. T ens i l e P r o p e r t i e s and Fa t igue L i v e s . The t ens i l e t e s t r e s u l t s a r e shown in Table II while Coff in-Manson d i a g r a m s for the a s - H i P and the HIP + forged m a t e r i a l s a r e shown in F i g s . 7 and 8. In F i g . 7, t he r e is r e l a -

Fig. 3-Same as Fig. 2 except the material is HIP+ tbrged. Note that the carbide concentration has increased compared to the as-HIP material.

Fig. 5-Same as Fig. 4 except different region. Note the boundary in the center of the micrograph which was formed by the forging process and which was never seen in the as-HIP material.

Fig. 4 Typical TEM micrograph of HIP+ forged material showing carbides (large and small), fine 7' and intermediate sized 3" sur- rounded by interfaeial dislocations. Note also that dislocations appear to have been generated by the large carbide particle.

Fig. 6-Same area as Fig. 5 except dark-field. Note that the matrix 7'-particles are smaller than for the as-HIP material and that the diffracting conditions on either side of the boundary are slightly different.

METALLURGICAL TRANSACTIONS A VOLUME 10A, OCTOBER 1979-1483

T a b l e I I . M o n o t o n i c T e n s i l e P r o p e r t i e s a t 6 4 9 ~

0.2 Pct Y.S, UTS, Elongation, Reduction in Area, Alloy MPa MPa Pct Pct

As-HIP 1119.7 1514.1 16.4 17.3 HIP + Forged 1122 1480 12.8 14.2

d

,~ 16: Z

~E

_Z

_.9. if) 5

1(1'

�9 CONTINLIOOS ~, h ~ INITIATION AT PORE

6. HOLD "riME

�9 TRANSGRANULAR

i i , ~ ] i l I l L i i ~ [ I ~ ~ J i J I i

10 ~ 104 10

NO. OF CYCLES TO FAILURE. N F

Fig. 7 - C o f f i n - M a n s o n represen ta t ion of the fatigue behavior of as-HIP material.

format ion . As expected, the HIP + forged mater ia l shows a somewhat greater s t r e s s re sponse for a g iven plast ic s tra in ampl i tude . This can be understood in t e r m s of d i s locat ion boundaries that have been put into the HIP + forged mater ia l , F ig s . 5 and 6, that are not present in the a s - H i P mater ia l .

3. Energy A n a l y s i s . Typica l h y s t e r e s i s loops were examined and it was shown that the energy per loop can be v e r y accurate ly represented by the equation:4

Aa • Zxcp [2] AW = 1 + n '

where , Zx~ = s t r e s s range, A e p = plas t ic s tra in range, and n' = c y c l i c s tra in hardening exponent. Equation [2] was used to ca lculate the total energy to fracture for both the a s - H i P and the HIP + forged s p e c i m e n s and the re su l t s are shown in F ig . 10. It i s c l ear that in both ins tances , there is a tendency for there to be l e s s total work to fracture for mater ia l subject to high plast ic s tra ins as compared to low plast ic s t ra ins . In addition, for the a s - H i P mater ia l , there appears to be two dis t inct r e g i m e s of behavior in which the energy is constant. The impl icat ions of this observat ion wi l l be d i s c u s s e d in t e r m s of deformation mode in a sub- sequent sec t ion . Note that the break in energy behavior occurs at a s tra in l eve l corresponding to the break in the Coff in-Manson curve , Fig. 7.

10 ~

u] (9 7

_Z

p- cO

0 F- 69

1(J'

L~

2

S

�9 CON'[LNUOtJS

a HOLD lIME

, , , , , , J I i i h i i ~ l i I I i i I i[ 10 ' ~r

NO. OF CYCLES TO FAILURE, N,

Fig. 8 -Cof f in -Manson representation o f the fatigue behavior o f the HIP+ forged material. The numbers beside the data points refer to the specimen number.

t ive ly l i tt le scatter to the data provided that the data are represented by a two segment curve . The i m p l i c a - tion of a two segment curve is that different de forma- tion m e c h a n i s m s are operating at high and low strains.~-~ In F ig . 8 there is cons iderably more data scat ter and the re la t ive ly s traight forward interpre- tation g iven for the a s - H i P mater ia l is not pos s ib l e . The reasons for the data scat ter in F ig . 8 compared to 7 are d i s c u s s e d in a subsequent sec t ion and are re lated to d i f f erences in crack init iat ion behavior .

2. Cyc l i c S t r e s s - S t r a i n Behav ior . The Cyc l ic S tres s Strain curves for both the As -HIP and the HIP + forged mater ia l are shown in F ig . 9. In both c a s e s the mate - r ia l i s s tra in hardening compared to monotonic de-

b 100

LU" O D

g_ <

cO u.l ~ 2(/ I--

I I I I 1 [ 1J

PLASTIC

[ A E ~ -11 o A S - H ,,~o = 1 7 9 6 | ~ _ . ~ 1 2 k2),

" H * F ~ = 2 6 2 0 ( - ~ . ~ ~z

I I I i I I [ l l I

STRAIN A M P U T U D E , A C / 2

ld

Fig. 9-Cyclic stress/strain curves for as-HIP and HIP+ forged Ren6 95.

•

.c

LL~ r r =,

o

O

5 f.,-

�9 HIP* FORGED

. o AS HIP

o o

o �9 �9

II I I I i I1,11 , I . . . . . . I lo ~ lo" 1~

NO. OF CYCLES TO FAILURE, N F

Fig. 10 Total energy to failure as a func t ion o f life. Note that the data for the as-HIP material can be represented by two constants corresponding to the two deformat ion modes that were observed.

! J

'E

1 4 8 4 - V O L U M E 10A, OCTOBER 1979 M E T A L L U R G I C A L T R A N S A C T I O N S A

C. SEM Ana lys i s of F r a c t u r e Surfaces

1. A s - H I P . Typica l c rack in i t ia t ion s i tes for the a s - HIP m a t e r i a l a re shown in F ig . 11. In a l l i n s t ances , it was found that fatigue c racks in i t ia ted f rom pores located within approximate ly one pore d iam of the s u r - face. This type of in i t ia t ion si te is cons idered to be a sur face connected pore . Since the in i t ia t ion s i t e s were s i m i l a r in a l l cases , the effect of va r i ab l e in i t ia t ion s i tes is e l imina ted and the data shown in the Coffin- Manson plot of F ig . 7 can be analyzed in fundamenta l t e r m s . Since the amount of sca t t e r was s m a l l and al l in i t ia t ion s i tes were s i m i l a r , changes in slope can be a t t r ibuted to e i ther d i f fe rences in the deformat ion mechan i sm 4-6 or to d i f fe rences in the seve r i ty of en- v i ronmen ta l attack. 7 In i t ia l c rack propagat ion gen- e r a l l y occu r r ed t r a n s g r a n u l a r l y . However, at some point, the mechan i sm changed f rom t r a n s g r a n u l a r to i n t e r g r a n u l a r . The t r a n s i t i o n f rom t r a n s g r a n u l a r to i n t e r g r a n u l a r is shown in F ig . 12. As far as hold t ime LCF is concerned , typica l SEM mic rog raphs a re shown in F ig . 13. Crack in i t ia t ion gene ra l ly occu r red at the sur face , e i ther at a sur face connected pore, F ig . 13(a), or by t r a n s g r a n u l a r in i t ia t ion, Fig . 13(b).

When c racks in i t ia ted at a pore , the life was c o m p a r a - ble to that observed for cont inuous cycl ing. On the other hand, t r a n s g r a n u l a r in i t ia t ion was assoc ia ted with a somewhat i nc r e a se d life. Crack propagat ion

Fig. 12-Crack initiation region in HIP+ forged specimen showing transgranular initiation and propagation followed by intergranular cracking.

(a) (a)

(b) Fig. 11 -Typical crack initiation site for continuously cycled as-HIP material is shown in (a). Surface-connected pores were observed in all specimens. In (b), a microcrack apparently initiated at a spherical inclusion. Note the initial well defined stage I propagation.

(b) Fig. 13-Typical hold time crack initiation and propagation features shown for as-HIP material. In (a) initiation took place at a surface connected pore and propagation tended to be intergranular. In (b) initiation took place by a transgranular mechanism and the life was correspondingly increased.

METALLURGICAL TRANSACTIONSA VOLUME 10A, OCTOBER 1979-1485

in hold t ime fatigue was gene ra l l y i n t e r g r a n u l a r . In hold t ime tes t ing the f r ac tu re sur face was always mac roscop ica l l y flat whereas for cont inuous cycl ing f inal f r ac tu re took place on macroscop ic shear p lanes .

2. HIP + F o r g e d . ' W h e r e a s the in i t ia t ion s i tes for the cont inuously cycled a s - H i P spec imens were a l l s u r - f ace -connec ted pores , this was not the case for the HIP + forged spec imens . Crack in i t ia t ion took place at pores (either sur face or subsurface) , at c e r a m i c par t i - c les located at the sur face and by what was apparen t ly a c l a s s i c a l stage I mechan i sm. Var ious types of c r a c k in i t i a t ion s i tes a re shown in Fig. 14. It is i n t e res t ing to note that for a p las t ic s t r a i n of approximate ly 0.03 pct, there was a va r i a t ion in life f rom approx imate ly 4500 cycles to approximate ly 96,000 cyc les . For spec i - men 16, shown in Fig. 14(a), in i t ia t ion occur red at a sur face pore while for spec imen 14, shown in F ig . 14(b), in i t ia t ion occur red at a pore of the same size but located in the spec imen i n t e r i o r . Since the s t r a i n range for each was v i r tua l ly the same , the d i f ference in life can be a t t r ibu ted to the locat ion of the in i t ia t ion s i te and this provides a s t rong indicat ion that the en- v i ronmen t plays an ex t r eme ly impor tan t ro le in d e t e r - mining the total LCF life of Ren~ 95, in ag r eeme n t with what has been observed for other Ni base sy s t e ms

(a)

tes ted at high t e m p e r a t u r e s . 8,9 With the exception of spec imen 14, in i t i a t ion was always assoc ia ted with a su r face connected fea ture , and in a l l ins tances , c rack propagat ion in i t i a l ly took place in a t r a n s g r a n u l a r mode. However at some point, as was the case for the a s - H i P ma te r i a l , there was a d i s t inc t change f rom t r a n s g r a n u l a r to i n t e r g r a n u l a r . The c rack length at which this change occurs has been plotted in F ig . 15 as a function of p las t ic s t r a i n for both the a s - H i P and HIP + forged ma te r i a l . It is noteworthy that al l of the continuous cycl ing points lie on s t ra ight l ines , the except ion being that spec imen 14, in which c rack in i - t ia t ion occur red in te rna l ly , l ies well above the l ine. F i g u r e 15 indica tes that there is a c r i t i c a l combinat ion of c rack length and s t r a in at which the t r a n s i t i o n occu r s . Such a p a r a m e t e r is a me a su r e of the d i la ta - t ion ahead of the m i c r o c r a c k . At some point the d i la ta- t ion is probably large enough to promote excess oxygen accumula t ion in the boundar i e s . The weakened bound- a r i e s then provide a low energy c rack path and a mechan i s t i c t r a n s i t i o n occurs . In the as -HIP ma te - r ia l , the t r a n s i t i o n was not as c lea r and this , along with the fact that the curve for the a s -HIP l ies above the HIP + forged is an indicat ion that the boundar ies for the a s - H i P m a t e r i a l a re less sens i t ive to env i ron - menta l a t tack.

The t r a n s i t i o n c rack s izes (in mic rons ) a re given by the following equat ions:

a H = 15.3 (ix Ep) -~ (as-HiP) [3]

aH +F = 1.16 (A s (HIP + forged) [4]

where ix ~p = p las t ic s t r a i n . Apparen t ly the tendency for the m a t e r i a l to t r a n s i t i o n to i n t e r g r a n u l a r propagat ion occurs much e a r l i e r for spec imens in which the in i t ia t ion s i te is sur face con- nected and in which the env i ronmen t can play an im- por tant ro le as seen in Fig . 15.

In hold t ime LCF, the observa t ions e s sen t i a l l y p a r - a l le led those of the a s - H i P m a t e r i a l . When ini t ia t ion took place in u t r a n s g r a n u l a r mode there was an in- c r e a s e in the l ife.

(b) Fig. 14- Crack initiation in continuous cycling HIP+ forged specimen. In (a) crack initiation occurred at a surface connected pore while in (b) initiation occurred at a similar feature in the interior. In (b) the life was an order of magnitude larger than in (a).

-I- t- (9 z LU . J

C~ .< n"

z _o p -

z

o A S H a =15.3(&t ;p} "~

2o0 �9 H+F a,= 1.16(/tEpj "~

2 0 I I I I 1 I I 1 1 i I ] I I

15' 16 ~

PLASTIC STRAIN RANGE Agp

Fig. 15-Transition crack length as a function of plastic strain for as-HIP and HIP + forged materials. The point lying off the corresponding line is for specimen 14 in which initiation was subsurface.

1486 VOLUME 10A, OCTOBER 1979 METALLURGICAL TRANSACTIONSA

(a) Ca)

(b) Fig. 16-Deformat ion character for as-HIP specimen continuously cycled at low plastic strain (0.017 pct). In (a) well defined slip bands can be seen. In (b) the same region is shown using a superlattice reflection to form the dark-field image. The absence of contrast along the slip bands indicates that local order has been destroyed and strongly implies a shearing mechanism.

(b) Fig. 17 -Same as Fig. 16 except at high plastic strain (0.067 pct). In (a) the deformation appears essentially homogeneous while in (b), which was a rather rarely observed feature, a high density o f individu- al slip bands is seen.

Fig. 18-Deformat ion substructure associated with 0.058 pct plastic strain as-HIP hold time fatigue specimen. This area was somewhat atypical in that the dislocation density was very high. In many grains there was no evidence of dislocation activity. At higher strains the dislocation density was homogeneous and so high that imaging was extremely difficult.

Fig. 19-Typica l dislocation substructure for HIP+ forged material continuously cycled at 0.059 pct plastic strain. Note the deforma- tion bands and the strong tendendy for dislocation pairing.

METALLURGICAL TRANSACTIONS A VOLUME 10A, OCTOBER 1979-1487

(a)

(b) Fig. 20-Same as Fig. 19, except for specimen 2 cycled at 0.028 pct plastic strain. In (a) typical slip bands are seen while (b) is a high magnifica- tion stereo pair of the bands seen in (a).

D. TEM Ana lys i s of LCF Subs t ruc tu res

1. As -HIP Mate r i a l . A typical TEM mic rog raph for a low s t r a i n spec imen is shown in F ig . 16. In F ig . 16(a), it can be seen that the fatigue deformat ion oc- c u r r e d in d i s c r e t e bands . In addit ion, de format ion propagated f rom one g r a in to another g ra in apparen t ly by nuclea t ing s l ip sources in the second g ra in due to the high s t r e s s e s assoc ia ted with the s l ip bands . It was f requent ly observed that de format ion bands were tan- gent to the la rge or in t e rmed ia t e s ize 7 ' - p a r t i c l e s and this is a lso c l ea r ly shown in F ig . 16. The impl ica t ion is that there were high s t r e s s e s assoc ia ted with the

la rge y ' - p a r t i c l e s which were suff icient to in i t ia te the deformat ion p r o c e s s . F igu re 16(b) is a da rk - f i e ld TEM mic rog raph of the a r e a shown in F ig . 16(a). It is no te - worthy that in the a r e a s co r respond ing to the de fo rma- t ion bands there is a total lack of imaging of the r _ pa r t i c l e s indicat ing that there has been a des t ruc t ion of the local o rder .5 This would imply that de format ion occurs by a shea r ing me c ha n i sm and is comple te ly cons i s t en t with the observed lack of i n t e r f ac i a l d i s loca - t ions around the s m a l l y ' - p a r t i c l e s . In F ig . 17, the d i s - locat ion s u b s t r u c t u r e a s soc ia t ed with high s t r a i n s is shown. In gene ra l , deformat ion was much more homog- eneous ly d i s t r ibu ted as shown in Fig . 17(a), although

1488-VOLUME IOA, OCTOBER 1979 METALLURGICAL TRANSACTIONSA

Fig, 21-Same as Fig. 20 except for specimen 14 cycled at 0.028 pct plastic strain. While the deformation mode was similar to that seen in Fig. 19, the life was an order of magnitude greater.

Fig. 22 Deformation character for hold-time HIP+ forged specimen fatigued at a plastic strain of 0.066 pct. Note that deformation still tends to occur in bands and that there is extensive faulting as evidenced by tl~e fringes.

a r ea s in which there was a tendency to form sl ip bands could occas iona l ly be seen, F ig . 17(b). In co r r e l a t i ng the observed deformat ion mode with the LCF life it can be seen that there was a tendency for the fatigue life to i n c r e a s e when the deformat ion was contained in a few d i sc re t e bands . This can perhaps be under - stood on the bas i s of local ized s t r a i n s and the pur i ty of the m a t e r i a l . Fo r high s t r a i n s the i n t e r n a l s t r e s s e s a r e la rge and homogeneously d i s t r ibu ted and the re will be a high a v e r a g e boundary s t r e s s which under appro- pr ia te condit ions will give r i s e to boundary fa i lu re . We have p rev ious ly demons t r a t ed that there is a marked tendency for fatigue to proceed by a boundary fa i lure mechan i sm. On the other hand at low s t r a in s de fo rma- t ion is contained in a few bands and even though the s t r e s s at the t ip of a s l ip band is high, the probabi l i ty of encounter ing a weak region (e .g . boundary par t i c le ) is low and there is then a tendency for s l ip to t r a n s f e r

METALLURGICAL TRANSACTIONS A

f rom one g r a i n to another and for c rack propagat ion to occur by a t r a n s g r a n u l a r mode. Clea r ly , it is des i r ab le at high t e m p e r a t u r e s to have t r a n s g r a n u l a r r a the r than i n t e r g r a n u l a r c rack propagat ion.

The subs t ruc tu r e assoc ia ted with hold t ime is shown in F ig . 18. In this case , while the d is loca t ion dens i ty is much higher than that shown for the cor responding continuous cycl ing case, the LCF life is not s igni f i - cant ly affected. This impl ies that the accumula ted d i s - locat ion debr i s , though v e r y high, is not suff icient to in i t ia te fa i lu re . It can be envis ioned that while the deformat ion re la ted damage is bui lding up to its l im i t - ing value, the env i ronmen t has caused suff icient damage to in i t ia te fa i lure .

2. HIP + Forged Mater ia l . Even at high s t r a i n s there was a tendency for deformat ion to be concent ra ted in s l ip bands . This can be seen in Fig . 19 where not only a re deformat ion bands observed, but it is a lso ex- t r e m e l y c lea r that the d is loca t ions occur in p a i r s . This s t rong pa i r ing of d is loca t ions is assoc ia ted with a shea r ing me c ha n i sm 1~ and would be expected to be more pronounced in the HIP + forged m a t e r i a l s ince the ~ ' - p a r t i c l e s a re s m a l l e r than in the a s - H i P ma te r i a l . F o r low s t r a in s , the s i tuat ion is somewhat s i m i l a r . A typical d i s loca t ion subs t ruc tu r e is shown in F ig . 20(a) which co r r e sponds to spec imen 2. F i g u r e 20(b) is a high magnif ica t ion s te reo pa i r taken f rom the same g ra in . Again the d i s loca t ion pa i r ing and slip band format ion a re quite evident . In F ig . 21 c o r r e - sponding to spec imen 14, it is evident that there is no di f ference in the deformat ion mode compared to ~pecimen 2, F ig . 20. In compar ing F i g s . 21 and 20, note that although there is an equivalence of the de- fo rmat ion mode, the life d i f fers by approx imate ly a factor of 10. Since the deformat ion modes were iden- t i ca l and the only d i f ference in the spec imens was subsur face vs sur face in i t ia t ion (the in i t ia t ion types and s i zes were the same) , one is again led to conclude that there is a ve ry s igni f icant effect of env i ronmen t on the LCF life.

The d is loca t ion s t r u c t u r e assoc ia ted with hold t ime

VOLUME 10A, OCTOBER 1979-1489

f a t i g u e i s s h o w n in F i g . 22 . In c o m p a r i n g F i g . 22 to 19 n o t e t h a t t h e r e i s a s o m e w h a t h i g h e r d i s l o c a t i o n d e n s i t y f o r ho ld t i m e f a t i g u e . T h e i n c r e a s e d l i fe in t h i s c a s e i s p r o b a b l y due in p a r t to t he f a c t t h a t f o r t h e ho ld t i m e s p e c i m e n s i n i t i a t i o n t o o k p l a c e t r a n s - g r a n u l a r l y , w h e r e a s f o r t h e c o n t i n u o u s l y c y c l e d s p e c i - m e n i n i t i a t i o n o c c u r r e d a t a s u r f a c e c o n n e c t e d p o r e . T h e f a c t t h a t t he l i fe i s no t d e g r a d e d b y a ho ld a t m a x i - m u m s t r a i n t e n d s to c o n f i r m t he p o t e n t i a l of t h i s a l l o y f o r d i s k a p p l i c a t i o n s . F u r t h e r m o r e , s u c h b e h a v i o r h a s b e e n o b s e r v e d in o t h e r Ni b a s e s y s t e m s n and s u g g e s t s t h a t a n e g a t i v e c r e e p / f a t i g u e i n t e r a c t i o n =-14 w h i l e f r e q u e n t l y o b s e r v e d , i s no t a u n i v e r s a l p h e n o m e n o n a n d e a c h m a t e r i a l s h o u l d b e c a r e f u l l y e x a m i n e d on a c a s e - b y - c a s e b a s i s b e f o r e i n v o k i n g f a t i g u e l a w s t h a t m a y no t be a p p r o p r i a t e . In f a c t s u c h w o r k i s u n d e r w a y on a n u m b e r of Ni b a s e a l l o y s in t he a u t h o r ' s l a b o r a t o r y a n d w i l l be t he s u b j e c t of a f u t u r e r e p o r t .

CONCLUSIONS

A. For initiation at surface pores (as-HiP), changes in the slip mode lead to changes in both life and the absorbed energy. When slip is constrained to a small number of discrete bands there is an improvement in life.

B. When slip occurred in bands (HIP + forged), changes in the initiation site produced spectacular changes in the life. That is to say, subsurface initia- tion l e a d s t o a g r e a t l y e n h a n c e d l i fe l e a d i n g to the c o n - c l u s i o n t h a t t h e r e i s a v e r y l a r g e e n v i r o n m e n t a l e f f e c t on t he L C F of R e n 6 95 a t 650~

C . T h e r e i s a t e n d e n c y f o r c r a c k p r o p a g a t i o n to o c c u r by a b o u n d a r y m e c h a n i s m a n d a n e m p i r i c a l p a r a m e t e r i n c o r p o r a t i n g s t r a i n a n d c r a c k l e n g t h h a s

b e e n u s e d to d e f i n e t he p o i n t a t w h i c h t h i s c h a n g e o c c u r s . T h e c h a n g e o c c u r r e d m o r e r e a d i l y f o r t h e HIP + f o r g e d m a t e r i a l i n d i c a t i n g t h a t t he b o u n d a r i e s w e r e e i t h e r w e a k e r o r m o r e s u s c e p t i b l e to e n v i r o n - m e n t a l a t t a c k o r b o t h .

D . T h e f a t i g u e l i f e w i t h a 15 m i n d w e l l a t m a x i m u m s t r a i n w a s a t l e a s t a s g r e a t a s f o r c o n t i n u o u s c y c l i n g a n d t he c o n c e p t of a d e l e t e r i o u s c r e e p / f a t i g u e i n t e r - a c t i o n i s i n a p p r o p r i a t e f o r t h i s m a t e r i a l .

ACKNOWLEDGMENT

The authors wish to acknowledge the NASA Lewis Research Center (Grant #NSG-3147, Dr. R. Miner, grant monitor) for its support of this study.

REFERENCES

1. A. Lasalmonie and J. L. Strudel: Phil. Mag., 1975, vol. 32, p. 937. 2. J. M. Oblak and B. H. Kear: Trans. ASM, 1968, vol. 61, p. 519. 3. H. Brooks: Metallnterfaces, p. 20, ASM, Cleveland, OH 1952. 4. A. Saxena and Stephen D. Antolovich: Met. TransA., 1975, vol. 6, p. 1809. 5. D. Fournier and A. Pineau: Met. Trans. A., 1977, vol. 8A, p. 1095. 6. M. Clavel and A. Pineau: Change of Slope in the Endurance curve During

LCF Associated with a Change in the Deformation Mode for a Ni Base Alloy. Presented at International Conference on Fatigue, Sherbroke, Canada, July 1978.

7. L. F. Coffin: Fracture 1977, (ICF4), D. M. R. Taplin, ed., University of Waterloo Press, vol. 1, p. 263, 1977.

8. M. Gell and G. R. Leverant: ASTM STP 520, p. 37, 1973, 9. D. J. Duquette and M. Gell: Met. Trans., 1971, vol. 2, p. 1325.

10. N. SIoloff: The Superalloys, C. T. Sims and W. C. Haget, eds. pp. 79-111, John Wiley and Sons, New York, N.Y., 1972.

11. G. R. Halford and A. J. Nachtigall: NASA TM-78828, 1978. 12. S. S. Manson: ASTM, STP 520, p. 744, 1973. 13. L. F. Coffin: Proc. Second Internat. Conf. on Mech. Behavior of Materials.

p. 866, Boston, t9=/6. 14. L. F. Coffin, S. S. Manson, A. E. Carden, and L. K. Severud: ORNL 5073,

Jan. 1977.

1490-VOLUME 10A, OCTOBER 1979 METALLURGICAL TRANSACTIONS A