1-The Role of Hydrogen in Corrosion Fatigue

8/6/2019 1-The Role of Hydrogen in Corrosion Fatigue

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Th e R ole of Hydrog en in Co rrosion Fat igueof H igh P ur ity A I -Zn-Mg Exposed to W ater VaporR . E . R I C K E R a n d D. J . D U Q U E T T EC o r r o s i o n f a ti g u e t e s ts w e r e p e r f o r m e d o n s a m p l e s o f a h i g h p u r i t y A I - Z n - M g a l l o y i n h u m i d n i t r o g e ng a s a f t e r p r e - e x p o s u r e t o e i t h e r v a c u u m o r h u m i d a i r. T h e r e s u lt s o f t h e s e t es t s w e r e c o m p a r e d t o t h er esu l t s o f f a t igue tes t s pe r f or med in dr y n i t r ogen , used as an ine r t r e f e r ence envi r onment , a f te r thes a m e p r e - e x p o s u r e tr e a t m e n t s. T h e p r e - e x p o s u r e ti m e s w e r e c a l c u l a t e d b y a s s u m i n g t h a t b u l k d i f f u -s i o n o f h y d r o g e n w a s t h e r a te l i m i t i n g p r o c e s s i n e i th e r h y d r o g e n a d s o r p t i o n o r d e s o r p t i o n . W a t e r v a -p o r i n t h e t e s ti n g e n v i r o n m e n t r e s u l t e d i n r e d u c e d f a t i g u e l iv e s ; h o w e v e r , p r e - e x p o s u r e t o h u m i d a i rw a s j u s t a s d e t r i m e n t a l a s w a t e r v a p o r i n t h e t e st e n v i r o n m e n t . T h e p r e - e x p o s u r e e m b r i t t le m e n t e f f e c to f h u m i d a i r w a s f o u n d t o b e c o m p l e t e l y re v e r s ib l e w h e n t h e s a m p l e s w e r e s t o r e d i n a v a c u u m l o n ge n o u g h t o r e m o v e h y d r o g e n , a s s u m i n g a b u l k d i f fu s i o n c o e f fi c i e n t o f 1 x 10 -13 m 2 / s e c . T h e s e r e s u l t sc o n f i r m t h e h y p o t h e s i s t h a t t h e r e d u c e d f a t i g u e l iv e s o f A I - Z n - M g a l l o y s i n w a t e r v a p o r i s d u e t oh y d r o g e n e m b r i t t le m e n t .

I . I N T R O D U C T I O NALUMINUMl loys , and in pa r t i cu la r the h igh s t r ength A 1-Z n - M g a l l o y s , a r e s u s c e p t ib l e t o e n v i r o n m e n t a l l y a s si s te dc r a c k g r o w t h . T h e f i r s t m e c h a n i s t i c m o d e l s d e v e l o p e d t o e x -p la in s t r es s cor r os ion c r ack ing ( SCC) of A 1- Zn- Mg a l loysw er e based on e i th e r s tr es s as s i s ted d i s so lu t ion o f the h igh lys t r a ined p las t ic zone a t the c r ack t ip or on pas s ive f i lm r up-ture and disso lut ion at a ct ive s l ip s tep s , t1"2] Th e lo cal iza t iono f t h e c r a c k p a t h t o t h e g r a in b o u n d a r i e s w a s t h o u g h t t o b edue to e i the r s egr ega t ion of a l loy ing e lements or impur i t i e sto the gr a in bou ndar ies or to pr e f e r en t ia l s l ip in the sof t p r e -cipi tate f ree z ones , t1 '2] Late r , i t was pro po sed that s tress cor -r os ion c r ack ing could be caused by a h ydr ogen em br i t t l ementp r o c e s s r e s u l t i n g f r o m t h e c a t h o d i c a l l y e v o l v e d h y d r o g e nw h i c h i n v a r i a b l y a c c o m p a n i e s t h e c o r r o s io n p r o c e s s o n a l u -[34 5 ]m i n u m a ll o y s . " H y d r o g e n g a s b u b b l e n u c l e a t i o n a n dgr ow th a t g r a in boundar ies has been o bser ved in the t r ansmis -s ion e lec t r on mic r oscope . t6] H o w ev er , For d 7'81 dem ons t r a tedtha t a ca lcu la t ion of anodic d i s so lu t ion r a tes can quant i t a -t i v e l y e x p l a i n t h e o b s e r v e d c r a c k g r o w t h r a t e s , w h i l e h y -d r o g e n e m b r i t tl e m e n t , a t l e a s t b y t h e g a s b u b b l e n u c l e a t o nm e c h a n i s m o f Z a p f f e a n d S i m s , tgl c a n n o t .S t u d i e s o n p r e - e x p o s u r e o f A 1 - Z n - M g a l l o y s t o e n v i r o n -m e n t s c o n t a i n i n g w a t e r a n d w a t e r v a p o r h a v e s h o w n t h a t t h ea l l o y s a re e m b r i t t l e d b y e x p o s u r e t o e i t h e r l i q u i d o r v a p o rphase water , t~~ Tests have shown tha t there is a s trong rela-t ionship between pre-exposure, tes t environment, s train rate ,and duc t i li ty , t1~ H o l r oyd and H ar d ie ]151 te ste d an A1-Zn-Mg a l loy [7049] in dr y a i r a t va r y ing s t r a in r a tes a f te r a th r eed a y p r e - e x p o s u r e t o s e a w a t e r, a n d f o u n d a m i n i m u m i n d u c -t i l i ty at a s train rate of 1 x 1 0 - 4 m / ( m 9 s ) . T h e y a t t ri b u t e dthe min imum in duc t i l i ty to inc r eas ing embr i t t l ement w i thlow e r s t ra in r a tes un t i l the r a te of hydr og en desor p t ion dur -ing tes t ing r esu l ted in the los s o f the embr i t t l ing spec ies be -fore fai lu re, t151 Th ey repo r ted th at s torage of the samp les inv a c c u u m w a s n o t e f f e c t i v e i n r e s t o r i n g t h e d u c t i l i t y a n dc o n c l u d e d t h a t d i sl o c a t io n m o t i o n w a s r e q u i r e d t o r e m o v e

hy dro gen . I151 Swan n e t a l . [16.17] fo un d tha t pre -ex po su re tom o i s t g a s e s r e s u lt e d i n e m b r i t tl e m e n t a n d h y d r o g e n a b s o r p -t i o n a s d e t e c t e d b y s t r a in i n g i n a v a c u u m c h a m b e r e q u i p p e dw i th a mass spec t r omete r .

I n cont r as t to SC C w her e c r ack ing i s v i r tua l ly a lw ays in te r -gr anula r , t r ansgr anula r c r ack ing or mixed in te r gr anula r andt r a n s g r a n u l a r c r a c k i n g i s u s u a l l y o b s e r v e d f o r c o r r o s i o nf a t ig u e ( C F ) o f a l u m i n u m a l l o y s a l t h o u g h t h e s a m e m e c h a -n i s m s h a v e b e e n p r o p o s e d t o e x p l a i n t h e r o l e o f t h e e n v i r o n -me nt in the f r ac tur e pr oce ss , t18-211 To d i s t ingu ish be tw eena n o d i c d i ss o l u t io n a n d h y d r o g e n e m b r i t t l e m e n t m e c h a n i s m sin cor r os ion f a t igue , S to l tz and P e l loux 22j s tud ied the e f f ec to f e l e c t r o c h e m i c a l p o t e n ti a l o n c r a c k g r o w t h r a t e s o f p r e -c r a c k e d s a m p l e s o f a l lo y 7 0 7 5 a n d c o n c l u d e d t h a t d i s s o lu -t i o n w a s r e s p o n s i b l e f o r t h e a c c e l e r a t e d c r a c k p r o p a g a t i o n .H o w e ver , in a s imi la r s tudY 240n smo oth sam ples of a l lo y123 2 ]7075 , J acko and D uq ue t te ' show ed tha t the f a t igue l ivesof s amples w er e reduced by the addi t ion of a hydr og en r ecom -bina t ion poison ( A s ) and f ound tha t cor r os ion f a t igue w as r e -duced , bu t no t e l imina ted , in to r s iona l load ing ( Mode I I I ) .Ho we ver , Fo rd 251 dem onstrated that , as in the case for SC C,anodic d i s so lu t ion i s quant i t a t ive ly capab le of exp la in ing theobser ve d CF c r ack gr ow th r a tes in an A 1- 7 pc t Mg a l loy .U nambiguous d i s t inc t ion be tw een anodic d i s so lu t ion m ech-a n i sm s a n d h y d r o g e n e m b r i t t le m e n t m e c h a n i s m s i s d i ff i c u l tf o r a l u m i n u m a l l o y s e x p o s e d t o a q u e o u s s o l u ti o n s . A l u -m i n u m i s a n a c t iv e m e t a l ( E ~ = - 1.71 V~h~) and th e Flad epoten t ia l i s ac t ive w i th r espe c t to the hy dr oge n e lec t r ode , 26jA s a r e s u l t , h y d r o g e n e v o l u t i o n a c c o m p a n i e s d i s s o l u t i o neve n d ur ing anod ic p olar iz at ion , t26' 27] Als o, cath odic pola r -iza t ion r esu l t s in the accumula t ion of hydr oxyl ions a t thesur f ace w hich a t tack the pas s iva t ing f i lm caus ing d i s so lu-t ion o f the metal.128-31]. As a result , a tes t pro gra m w as dev el-o p e d u t i l iz i n g w a t e r v a p o r a n d a n i n e r t g a s e o u s e n v i r o n m e n t(dry ni trogen) to tes t the hypo thesis that hyd roge n in sol id so-lu t ion could acce le r a te f a t igue f a i lu r e of A 1- Zn- Mg a l loy s .

R. E . RI C K ER i s M eta l lu r g i s t , I n st i tu te f o r M ate r i a l s S c ience and Eng i -nee r ing , N a t iona l Bu r eau o f S tandar ds , G a i the r bu r g , M D 20 899 . D . J .D U Q U ETTE i s p r o f es s o r , D epar tmen t o f M ate r i a l s Eng inee r ing , Rens s e -laer Polytechnic Ins t i tu te , Troy, NY 12181.M anus c r ip t s ubmi t t ed A ugus t 24 , 1987 .

I I . T H E O R YT h e c h e m i c a l r e a c t i o n s w h i c h o c c u r b e t w e e n w a t e r a n da l u m i n u m m e t a l t o y i e l d h y d r o u s a l u m i n u m o x i d e s a n d / o rh y d r o x i d e s c a n b e r e p r e s e n t e d b y t h e f o l l o w i n g e q u a ti o n :

U . S . G O V E R N M E N T W O R KMETALLURGICAL TRANSACTIONS A NOT PROTECTED BY U.S. COPYRIGHT VOLUME 19A, JULY 198 8-- 1775


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2A l( s ) + ( 3 + X ) H 20 ~ A 1203 9X ( H 20) ( s ) + 3H 2( g)[1 ]

W h e r e t h e d e g r e e o f h y d r a t i o n o f t h e r e s u lt i n g o x i d e ( X )var ies f r om 0 to 3 . Th e d i f f e r en t phases tha t f or m the cor r e -s p o n d i n g d e g r e e o f h y d r a t i o n a r e g i v e n i n T a b l e 1. [32,33,34]S c a m a n s a n d T u c k [371 s tudied the react io ns and the prod -u c t s th a t f o r m w h e n a l u m i n u m i s e x p o s e d to w a t e r v a p o r a t7 0 ~ T h e y fo u n d t h a t e x p o s u r e to w a t e r v a p o r r e s u lt e d i nt h e fo r m a t i o n o f a d u p l e x f i l m o f p s e u d o b o e h m i t e ( X = 2 )and bayer i te ( X = 3 ) . The y a l so obser ved b l is te r ing of thef i l m c a u s e d b y t h e h y d r o g e n g a s e v o l v e d d u r i n g t h e r e a c -t i on s . A c c o r d i n g t o t h e t h e r m o d y n a m i c i n f o r m a t i o n g i v e n i nT a b l e I , t h e e q u i l i b r i u m p a r t i a l p r e s s u re o f h y d r o g e n f o r t h ef o r m a t i o n o f b a y e r i t e o n a l u m i n u m a t 8 7 p c t r e l a t i v e h u -midity and 25 ~ is greater than 1053 Pa ( 1 0 4 8 a tm) and gr ea te rthan 1039 Pa ( 1034 a tm) f o r the f or mat ion o f boehm i te . A lso ,

a t th e i n t e r f a c e b e t w e e n t h e b a y e r i t e a n d t h e a l u m i n u m m e t a lthe f o l low ing r eac t ion i s poss ib le :2Al(s) + 6A1203 9 3H 2 0( bay er i te ) ~ 4A 1203( cor undum)

+ 3H2(g) [2]T h e s t a n d a rd f r e e e n e r g y c h a n g e f o r t h e f o r m a t i o n o f c o r u n -d u m b y t h i s r e a c t i o n i s - 8 4 1 . 3 k J / m o l e f o r w h i c h t h e e4 qu i-l ib r ium par t ia l p r es sur e of hydr o gen i s g r ea te r than 105 Pa(104 9 a tm ). 132.33,34]

H y d r o g e n e v o l u t i o n o c c u r s ra p i d l y a n d S c a m a n s a n dTuc k 371 obse r ved b l i s te r ing of the sur f ace ox ide f i lm w i th in1 5 s ec o n d s o f e x p o s u r e t o w a t e r v a p o r a n d b a y e r i t e g r o w t hw i t h i n 5 m i n u t e s o f e x p o s u r e . H o w e v e r , t h e r a te o f a b s o r p -t ion of hydr ogen w i l l be l imi ted by so l id s ta te d i f f us ion ofhydr ogen in to the m eta l . Ext r apola t ion of hydr ogen d i f f us ionmeasur ements made a t e leva ted temper a tur es ( 450 to 625 ~to 25 ~ ind ica tes tha t the d i f f us ion cons tan t f or hyd r ogen in12 14 2 [35 36]a l u m i n u m i s b e t w e e n 1 0 - a n d 1 0 - m / s e c . ' S c a m a n san d Tucks I37 '38 '39] and Ge s t and Tr o iano 1~ have mea sur edthe d i f fus ion of hydr ogen in A 1-Zn-Mg a l loys a t o r nea r r o omt e m p e r a t u re . S c a m a n s a n d T u c k u s e d w a t e r v a p o r s a t u ra t e da i r ( 70 ~ as the hyd r oge n sour ce in a pe r m ea t ion ce l l andf o u n d t h e h y d r o g e n d i f f u s i o n c o n s t a n t t o b e a p p r o x i m a t e l y

10-1 4 m Z /s ec . [37.38,39]Ges t and Tr o iano used aqueous so lu t ionsi n a n e l e c t r o c h e m i c a l p e r m e a t i o n c e l l to e s t i m a t e t h e h y d r o -gen d i f f us ion coef f ic ien t a s 2 10 -13 m2/se c a t 25 ~ t~~S i n c e t h e r a t e d e t e r m i n i n g s t e p i n h y d r o g e n a b s o r p t i o n i sso l id s ta te d i f f us ion of the d i s so lved hydr ogen in to the a l -l o y , p r e - e x p o s u r e t re a t m e n t s i n t e n d e d t o c h a r g e s a m p l e sw i th hyd r ogen m us t a l low suf f ic ien t t ime f o r so l id s ta te d i f -f u s i o n o f h y d r o g e n i n t o th e s a m p l e . A l s o , s i n c e a lu m i n u ms a m p l e s c a n a b s o r b h y d r o g e n d u r i n g f a b r i c a t i o n o r f r o mlaboratory air pr ior to tes t ing, control samples m ust be pu rgedo f h y d r o g e n p r i o r t o t e s ti n g b y a p r e - e x p o s u r e t r e a tm e n t t ov a c u u m . T h e v a c u u m p r e - t r e a tm e n t o f t h e c o n t r o l s a m p le smus t be f or a t ime per iod w h ich w i l l a l low so l id s ta te d i f fu-s ion of the in te r na l hydr ogen to the sur f ace w her e i t can bed e s o r b e d .T h e r e a r e f o u r d i f f e r e n t w a y s t h a t e x p o s u r e a n d / o r p r e -e x p o s u r e t o w a t e r v a p o r c a n i n f l u e n c e t h e f a t i g u e r e s i s ta n c eo f A I - Z n - M g a l l o y s . T h e s e a re :1 . P r e - E x i s t i n g In t e r n a l H y d r o g e n : S o l i d s ta t e d i s s o l v e dh y d r o g e n p r e s e n t i n t h e a l l o y p r i o r t o f a t i g u e l o a d i n g w i l lbe ava i lab le to d i f f use to the p las t ic zon e ahead of a f a t iguec r a c k o r t o t h e s u r f a c e a n d c a u s e e m b r i t t le m e n t .2 . P r e - E x i s t i n g C o r r o s i o n D a m a g e : D u r i n g e x p o s u r e t ow a t e r v a p o r p r io r t o f a ti g u e l o a d i n g , c o r r o s i o n d a m a g e o c -cur s . A lso , the pr esen ce of a l a r ge hydr o gen par t ia l p r es sur ein the meta l could r esu l t in i r r ever s ib le damage such as then u c l e a t i o n o f i n t e r n a l v o i d s . T h e p r e s e n c e o f t h i s d a m a g em a y c o n t r i b u t e t o t h e i n i t a ti o n a n d p r o p a g a t i o n o f f a t i g u ecr acks .3 . O x i d a t i o n E f f e c ts : O x i d a t i o n o f a l u m i n u m m e t a l a n d t h egr ow th of a b r i t t l e ox ide f i lm on the sur f ace or a t the c r ackt ip could acce le r a te f a t igue c r ack in i t i a t ion and pr opaga t ion( inc ludes ox ide induced c losur e e f f ec t s ) .4 . E x t e r n a l l y G e n e r a t e d H y d r o g e n : H y d r o g e n is g e n e r a t e da t the sur f ace and a t a c r ack t ip as a by p r odu c t o f the cor r o-s i o n r e a c t io n b e t w e e n w a t e r v a p o r a n d u n f i l m e d a l u m i n u mm e t a l . T h u s , r u p t u re o f t h e p a s s i v e f i l m b y e m e r g i n g s l i ps teps a t the c r ack t ip w i l l r e su l t in hydr ogen pr oduc t ion a tt h e c r a ck - t ip . T h i s h y d r o g e n c a n b e a b s o r b e d i n t o t h e a l l o yand in te r ac t w i th the sur f ace or p last ic de f or m at ion to causeembr i t t l ement .

T a b l e I . T h e O x i d e P h a s e s o f T h e A l u m i n u m - W a t e r S y s t e m ~32'33'34jG e n e r a l R e a c t io n : 2 A l ( s ) + ( 3 + X ) H 2 O -' -- > A I 2 0 3 " X ( H 2 0 ) ( s ) - t- 3 H 2 ( g )

Degree ofHydration(X ) Chemical Com position A GPhases (kJ /mol. )0 anhydrous aluminum oxideA12031 aluminum oxyhydroxideA1203 " H20 or A1OOH2 hydrated aluminumoxyhydroxide

A 1 2 0 3 " 2(H20) or A1OOH 9 (H20)3 tri-hydrated aluminum oxideA1203 9 3(H20)3 aluminum hydroxide2[Al(OH)3]

corundum - 864.8boehmite - 864.8diasporepseudoboehmite

bayerite - 888.3nordstranditegibbsite (hydrargillite) -8 9 7. 2amourphous hydroxide -8 52 .0

1 7 7 6 - - V O L U M E 1 9 A , J U L Y 1 98 8 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


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T h e g o a l o f t h is i n v e s t i g a t i o n w a s t o e v a l u a t e t h e r e l a t i v ec o n t r i b u t i o n s o f e a c h o f t h e s e f a c t o r s i n d i v i d u a l l y a n d i nc o m b i n e d a c t i o n a n d , t h e r e b y , t e st t h e h y d r o g e n e m b r i t t le -ment hypothes i s . A s a r esu l t , a t e s t p r ogr am w i th f ive d i f -f e r e n t c o m b i n a t i o n s o f p r e - e x p o s u r e t r e a t m e n t a n d t e s te n v i r o n m e n t w a s e m p l o y e d . T h e s e a r e :A . V a c u u m / D r y N i t r o g e n : S a m p l e s p u rg e d o f h y d r o g e n b ya v a c u u m p r e - e x p o s u r e w e r e t e s t e d i n a n i n e rt e n v i r o n m e n ta s a s t a n d a r d r e f e r e n c e fa t i g u e l i fe c u r v e ( n o e n v i r o n m e n t a lcont r ibu t ion to f a i lu r e ).B . H u m i d A i r / H u m i d N i t r o g e n : S a m p l e s p r e - c o r r o d e d a n dp r e - c h a rg e d w i t h h y d r o g e n b y p r e - e x p o s u r e t o h u m i d a i rw e r e t e s t e d in a c o r r o s i v e c h e m i c a l e n v i r o n m e n t t o t e s t t h ec o m b i n e d e f f e c t o f a l l fo u r o f t h e p o s s i b l e m e c h a n i s m s d i s -c u s s e d a b o v e .C . V a c u u m / H u m i d N it ro g e n : S a m p l e s p u r g e d o f h y d r o g e nb y v a c u u m p r e - e x p o s u r e w e r e t e s te d i n a c o r r o s i v e ( h u m i d )envi r onmen t tha t a l so pr ovided hyd r ogen a t the sur f ace of th es a m p l e . F o r t h e s e s a m p l e s , o n l y m e c h a n i s m s 3 a n d 4 c a ncont r ibu te to f a i lu r e .D . H u m i d A i r /D r y N i t r o g e n : S a m p l e s p r e - c o r r o d e d ( a n dt h u s p r e - c h a r g e d w i th h y d r o g e n ) w e r e t e s t e d i n a n i n e r t e n -v i r o n m e n t t o t e s t th e e f f e c t t h a t h y d r o g e n i n i n t e r n a l s o l i dso lu t ion w ould have on the f a t igue pr oper t ie s of th i s a l loy .F o r t h e s e s am p l e s , o n l y m e c h a n i s m s 1 a n d 2 c a n c o n t r i b u t eto f a t igue f a i lu r e .E . H u m i d A i r T h e n V a c u u m O u t g a s s e d / D r y N i t ro -~ e n : S a m p l e s p r e - c o r r o d e d a n d p r e - c h a r g e d w i t h h y d r o g e nb y a p r e - e x p o s u r e t re a t m e n t t o h u m i d a i r w e r e p u r g e d o f h y -d r o g e n b y a v a c u u m e x p o s u r e a n d f a t i g u e t e s te d i n a n i n e r te n v i r o n m e n t t o t e s t th e e f f e c t o f t h e p r e - c o r r o s i o n d a m a g eand to te s t the r ever sib i l ity of the pr e - exposur e embr i t t l e men te f f e c t ( m e c h a n i s m 2 o n l y ) .

I I I . E X P E R I M E N T A LS a m p l e s 8 0 m m l o n g a n d 2 0 m m w i d e w i t h a c e n t r al g a g e

s e c t io n 1 0 m m w i d e a n d 1 3 m m l o n g w e r e m a c h i n e d f r o m as h e e t o f h i g h p u r i ty t e r n a r y A I -5 . 6 4 p c t Z n - l . 9 4 p c t M g a l -loy , so lu t ion t r ea ted in a r gon a t 465 ~ f or an hour , qu ench edin co ld w a te r , and aged in s i l i con o i l a t 120 ~ f or 24 hour s toachieve peak hardness. J291 This r esu l ted in equ iaxed gr a insw i t h a n a v e ra g e d i a m e t e r o f 0 . 1 2 7 m m ( A S T M s i z e 3) .T h e s a m p l e s w e r e g r o u n d i n o i l a n d i n a k e r o s e n e / p a r a ff i nmixtur e to the equiva len t o f a 600 gr i t f in i sh , c leaned , ande lec t r opol i sh ed in a pe r ch lor ic ac id /e th anol so lu t ion . I4~ Ther e s u lt i n g s a m p l e s w e r e a p p r o x i m a t e l y 1 . 0 m m t h i c k . A f t e re lec t r opol i sh ing , the samp les w e r e s tor ed e i the r in a vacu umc h a m b e r , t o r e m o v e h y d r o g e n a n d t o p r e v e n t h y d r o g e n a b -sor p t ion , o r in a 10 l i t e r cham ber w her e the r e la t ive humid i tyw as he ld cons tan t a t 87 pc t to char ge the samples w i th hy-drogen. t13 ' 37-39] A rotary van e oi l se aled me chan ical va cu ump u m p w a s u s e d t o e v a c u a t e th e v a c u u m c h a m b e r a n d th e h u -m i d i t y w a s m a i n t a i n e d c o n s t a n t i n t h e h u m i d i t y c h a m b e r b yi n c lu d i n g a b e a k e r c o n t a in i n g - 5 0 0 m l o f sa t u r a te d s o d i u mc a r b o n a t e s o l u t i o n ( w i t h e x c e s s s o l i d ) f o r w h i c h t h e e q u i -l ibr iu m relat iv e humid ity at 25 ~ is 87 pct . t4~]

I n A p p e n d i x 1 , th e r a t e o f h y d r o g e n a b s o r p t i o n ( o r r e -mov a l ) i s ca lcu la ted f or d i f f e r en t hyd r ogen d i f f us ion coef f i -c ien ts w hich enco mpa ss the va lues f ou nd in the l i t e ra tur e f orh y d r o g e n i n a l u m i n u m . T a b l e I I g iv e s t h e p e r c e n t a g e o f t h et o t a l h y d r o g e n a b s o r b e d ( o r r e m o v e d ) f o r d i f f e r e n t e x p o s u r et i m e s a n d d i f f u s io n c o e f f i c ie n t s . F a t i g u e l i v e s w e r e f o u n d

T a b l e H . E x p o s u r e T im e R e q u i re d t o C h a r g eo r R e m o v e H y d r o g e n f r o m t h e S a m p l e s fo rD i f f e r e n t D i f fu s i o n C o e f f i c i e n t s (A p p e n d i x )PercentageAbsorbed Exposure Time Required (Days)(Mt/M=) Dn = 1 10 -12 Dn = 1 10 -13 Dn = 1 X 10 -14 100 Pct (m2/sec) (mZ/sec) (m2/sec)

99.5 5.87 58.7 58796.0 3.52 35.2 35289.0 2.35 23.5 23570.2 1.17 11.7 11750.8 0.59 5.9 59DH = h y d ro g e n d i f fu s i o n c o e f f i c i e n t a t 2 5 ~ i n (m 2 / s e c )M t = a m o u n t o f h y d r o g e n a b s o r b e d i n e x p o s u r e t im e ( t )

M ~ = a m o u n t o f h y d r o g e n a b s o r b e d w i t h i n f in i t e e x p o s u r e t im e

t o b e a f u n c t i o n o f p r e - e x p o s u r e t im e s f o r p r e - e x p o s u r e s o fl es s t h a n 3 0 d a y s b u t n o t o f l o n g e r e x p o s u r e s . A c c o r d i n g l y ,t h is m i n i m u m e x p o s u r e p e r i o d w a s a d o p t e d f o r b o t h t h ev a c u u m p r e - e x p o s u r e ( t o o u t g a s h y d r o g e n a b s o r b e d d u r i n gs a m p l e p r e p a r a t i o n ) a n d f o r t h e w a t e r v a p o r p r e - e x p o s u r e( to c h a r g e s a m p l e s w i t h h y d r o g e n ) .

T h e f a t i g u e t e s ts w e r e c o n d u c t e d w i t h a f o u r p o i n t b e n d i n gm a c h i n e i n a 1 . 4 l it e r p l e x i g la s s c h a m b e r a t a f r e q u e n c y o f2 . 7 7 8 H z . T h e e n v i r o n m e n t a l c h a m b e r w a s u s e d to c o n t r o lt h e e n v i r o n m e n t f o r a l l t e s ts . T h e c y c l i c b e n d i n g m o m e n tw as appl ied to the sample such tha t the c r acks pr opaga teda c r o ss t h e s h o r t d im e n s i o n o f t h e s a m p l e . T o h e l p p r e s e r v ef r ac togr aphic f ea tur es , the bending mo men t w as no t r ever seda n d th e m i n i m u m b e n d i n g m o m e n t d u r i n g a l o a d c y c l e w a sz e r o . T h e s t r o k e w a s c o n t r o l l e d a n d m o n i t o r e d w i t h a nLV D T and the load w as moni tor ed w i th a load ce l l. The maxi -mum s t r a in a t the sur f ace w as ca lcu la ted f r om the equa t ionf or de f lec t ion of e las t ic beams r e la ted to the measur ed ge-omet r y of each ind iv idua l spec imen. [29,42] T h e a c c u ra c y o f t h eca lcu la t ions w as conf i r med w i th s t r a in gages pr ior to envi -r onmenta l t e s t ing .

The dr y n i t r ogen envi r onmen t w as main ta ined by a co nt inu-o u s fl o w o f p u r e n i t ro g e n ( < 3 p p m w a t e r a n d < 0 . 5 p p mo x y g e n ) a t 0 . 2 2 l i t e r s p e r m i n u t e t h r o u g h a c o l d t r a p i m -mer sed in a r ese r voi r fi l l ed w i th so l id CO 2. The hum id n i -t r o g e n e n v i r o n m e n t w a s m a i n t a i n e d b y b u b b l i n g n i t r o g e n a tthe same r a te th r ough d i s t i l l ed w a te r , and then thr ou gh a so-l u t i o n o f s o d i u m c a r b o n a t e s a t u r a t e d w i t h e x c e s s s o l i d t omain ta in the r e la t ive humidi ty a t 87 pc t ( 25 ~T h e f r a c tu r e su r f ac e s w e r e e x a m i n e d i n a s c a n n i n g e l e c -t r o n m i c r o s c o p e ( S E M ) . T o e v a l u a t e th e r e l a t i v e i n f lu e n c eo f e n v i r o n m e n t a n d s t re s s i n t e n s it y o n t h e m o d e o f f r a c t u r e ,l in e s w e r e d r a w n o n S E M m i c r o g r a p h s p e r p e n d i c u l a r t o t h ed i r e c ti o n o f c r a c k p r o p a g a t i o n a t t h r e e d i f f e r e n t c ra c k d e p t h

to sample th ickness r a t ios ( 0 .25 , 0 . 50 , and 0 .75 ) . To min i -mize e r r ors w hich may be in t r oduced by edge e f f ec t s and i so-la ted in i t i a t ion events , a l ength of l ine r epr esen t ing 25 pc to f t h e w i d t h o f th e s a m p l e ( 2 .5 m m ) i n t h e c e n t r a l r e g io n o ft h e s a m p l e w a s u s e d f o r t h i s d e t er m i n a t i o n . T h e r a t i o o f t h elength of th i s l ine w hich t r aver sed in te r gr anula r f r ac tur e tothe to ta l l ength of l ine w as taken as the pe r ce ntag e of in te r -g r a n u l a r f r a c t u r e . T w e l v e m e a s u r e m e n t s ( f o u r a t e a c h o ft h r e e d if f e r e n t c r a c k l e n g t h s ) w e r e m a d e f o r e a c h t e s t in g e n -v i r o n m e n t . T h e s t r e ss i n t e n s i t y f a c to r s w e r e e s t i m a t e d f o re a c h m e a s u r e m e n t u s i n g a n a s y m p t o t i c a p p r o x i m a t i o n f o r ath in strip" w it h a sin gl e, edge[43]c r a c k i n b e n d i n g a s s u m i n gs t ra igh t l ine c r ack geom et r y . V ar ia tions in c r ack shape andMETALLURGICAL TRANSACTIONS A VOLUME 19A, JULY 198 8-- 1777


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other f ac tor s w i l l in f luence the ac tua l s t r es s in tens i ty , andthese r ough es t imates o f the ave r age s t r es s in tens i ty a r e in -c luded to i l lus t r a te the t r end in the magni tude of the s t r es sin tens i ty w i th c r ack length .

I V . R E S U L T SA. Tests in Dry Nitrogen Gas after Vacuum Pre-Exposure

Figur e 1 show s the r esu l t s o f f a tigue tes t s in dr y n i t r ogenaf te r a p r e - exposur e tr ea tment to vacuum of 30 days or m or e .R e d u c e d f a t i g u e li v e s w e r e f o u n d f o r v a c u u m p r e - e x p o s u r e sof le s s than 30 days . A s a resu l t , on ly samples w hich w er es t o re d i n a v a c u u m f o r l o n g e r t h a n 3 0 d a y s w e r e u s e d f o r t h isi n e r t re f e r e n c e e n v i r o n m e n t f a t i g u e l i f e c u r v e . T h e s e d a t aa r e used as a base - l ine compar i son ( bes t f a t igue r es i s tance)f or subsequen t t e s t s.B. Tests in Hu mid Nitrogen after Pre-Exposure to Hu mid Air

T h e r e s u l t s o f c o r r o s i o n f a t i g u e t e st s p e r f o r m e d i n h u m i dn i t r o g e n a f t e r a p r e - e x p o s u r e t r e a t m e n t i n h u m i d a i r( > 3 0 d a y s a t 87 p c t R . H . ) a r e a ls o s h o w n i n F ig u r e 1 . F o rth i s t e s t ing condi t ion , p r e - ex is t ing in te r na l hydr ogen , p r e -e x i s ti n g c o r r o s i o n d a m a g e , o x i d a t i o n e f f e c ts , a n d e x t e r -n a l l y g e n e r a t e d h y d r o g e n c a n c o n t r i b u t e t o r e d u c i n g f a t ig u el i f e . T h e r e s u l t s o f t h e s e t e s t s s h o w t h a t b o t h t h e f a t i g u el ives a t a f ixed s t r a in r ange an d the 106 cyc les f a t igue l imi ta r e s i g n i fi c a n t l y r e d u c e d b y t h is c o m b i n a t i o n o f p r e - e x p o -sur e t r ea tment and tes t envi r onment .C. Tes t s in Hum id Ni t rogen a f ter Pre-Exposure to Vacuum

T w o s a m p l e s w e r e t e s t e d i n h u m i d n i t r o g e n w i t h o u t p r e -e x p o s u r e t o h u m i d a i r (v a c u u m p r e - e x p o s u r e ) a n d c o m p a r e dt o t h e a f o r e m e n t i o n e d t e s ts w h i c h s h o u l d r e p r e s e n t th e m a x i -m u m a n d m i n i m u m p o s s i b l e e n v i r o n m e n t a l i n f l u e n c e o fhum id a i r ( 87 pc t re la t ive humidi ty) . O ne samp le w as te s teda t a s t ra i n r a n g e o f 0 . 0 0 4 m / m w h i c h i s n e a r t h e v a c u u mpr e- exp osur e /d r y n i t r ogen 106 cyc les f a t igue l imi t w hi le theo ther w as te s ted a t a l a r ger s t ra in r ange . F or th i s te s t ing co n-d i t ion , the r e is ne i the r p r e - ex is t ing in te r na l hyd r ogen d i s -s o l v e d i n th e s a m p l e n o r c o r r o s i o n d a m a g e o n t h e s u r fa c e o f

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Cycl es t o Fa i l ur eF i g . 1 - - C y c l e s t o f a i lu re fo r v a c u u m o u t g a s s e d s a m p l e s te s t ed i n d ry n i t ro -g e n g a s a n d f o r s a m p l e s p r e - e x p o s e d t o h u m i d a i r t e st e d i n h u m i d n i t r o g e ng a s . E a c h p o i n t r e p re s e n t s a s i n g l e t e s t a n d t h e e r ro r b a r s r e p re s e n t a twos t a n d a rd d e v i a t i o n r a n g e o f t h e e r ro r a s d e t e rm i n e d b y r e g re s s i o n a n a l y s i s .

the sample a t the onse t o f te s t ing . A s show n in F igur e 2 , thef a t igue l ives of these samples w er e es sen t ia l ly the same ast h a t o f s a m p l e s p r e - e x p o s e d t o w a t e r v a p o r a n d t e s t e d i nw ate r vapor . This demons t r a tes tha t the pr e - exposur e t r ea t -ment i s no t r equi r ed to ob ta in the r educ t io n in f a t igue pr op-e r t ie s o b s e r v e d a b o v e .D. Test s in Dry Ni t rogen a f ter Pre-Exposure to Humid A ir

Fat igue tes t s w er e a l so conduc ted in dr y n i t r ogen on sam-p l e s w h i c h r e c e i v e d t h e s t a n da r d p r e - e x p o s u r e t r e a t m e n t t ohumid a i r . These samples conta ined in te r na l so l id s ta te d i s -s o l v e d h y d r o g e n a n d s u r f a ce c o r r o s i o n d a m a g e a t t h e o n s e to f t e s ti n g a s a r e s u l t o f t h e p r e - e x p o s u r e t r e a t m e n t . H o w -e v e r , n o h y d r o g e n w a s a v a i l a b l e f r o m t h e e n v i r o n m e n t d u r-i n g t h e t e st a n d n o c o r r o s i o n p r o c e s s e s c o u l d o c c u r d u r in gthe te s t w hich d id no t occ ur in the base - l ine r e f e r en ce envi -r onment t e s t s . These da ta , show n in F igur e 2 , ind ica te tha tthe f a t igue l ives of these samp les w e r e es sen t ia l ly iden t ica lto the f a t igue l ives of s amples t e s ted in hu mid n i t r ogen a f te rp r e - e x p o s u r e t o w a t e r v a p o r . T h e o n l y d e v i a t i o n o c c u r snear the f a t igue l imi t w her e the dur a t ion of the te s t in dr yn i t r o g e n i s s u c h t h a t s i g n if i c a n t h y d r o g e n d e s o r p t i o n m a yoccur .E. Test s in Dry Ni trogen and Pre-Exposure to Hum id A irand Vacuum

T o t e s t th e r e v e r s i b i l it y o f t h e e f f e c t o f p r e - e x p o s u r e t ow a t e r v a p o r , te s ts w e r e c o n d u c t e d i n d r y n i t r o g e n o n s a m p l e st h a t w e re p r e - e x p o s e d t o h u m i d a i r f o r m o r e t h a n a m o n t ha n d t h e n p l a c e d in a v a c u u m f o r v a r y i n g t im e p e r i o d s . T h ev a c u u m e x p o s u r e w i l l r e m o v e t h e h y d r o g e n a b s o r b e d i n t ot h e a l l o y d u r in g t h e p r e - e x p o s u r e t o h u m i d a i r b u t , i t w i l ln o t r e m o v e t h e s u r f a ce d a m a g e c a u s e d b y c o r r o s i o n d u r i n gthe humid a i r exposur e . For s amples p laced in the vacuum f ormor e than a month , the f a t igue pr oper t ie s in dr y n i t r ogen ,a l so show n in F igur e 2 , w er e the same or be t te r than thosef ound f or s amples no t expos ed to humid a i r. Tha t i s , the f a -t i g u e p ro p e r t ie s w e r e c o m p l e t e l y r e s t o r e d b y t h e v a c u u m e x -p o s u r e o r r o o m t e m p e r a t u r e " b a k e o u t " . T o e x a m i n e t h e r a t eof th i s r ecover y , t e s t s con duc te d a f te r va r ious s tor age t imes

Ee

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F i g . 2 - - T h e e f f e c t o f d i f f er e n t p r e - ex p o s u r e a n d t e st e n v i r o n m e n t c o m -b i n a t i o n s o n f a t i g u e l if e a s c o m p a r e d t o t h e r e s u l t s s h o w n i n F i g u re 1 fo rv a c u u m o u t g a s s e d s a m p l e s t es t e d i n d r y n i t r o g e n g a s a n d f o r s a m p l e s p r e -e x p o s e d t o h u m i d a i r a n d t e s t e d i n h u m i d n i t ro g e n g a s . Ea c h p o i n t r e p re -s e n t s a s i n g l e t e s t a n d t h e e r ro r b a r s r e p re s e n t a t wo s t a n d a rd d e v i a t i o nr a n g e o f t h e e r r o r a s d e t e r m i n e d b y r e g r e s s i o n a n a l y s i s .

1778- -VO LU ME 19A, JULY 1988 METALLURGICAL TRANSACTIONS A


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i n t he vacuum w ere p lo t t ed a s a pe rcen t o f r ecove red f a t iguel i fe (on the logar i thm ic sca le) vs t he exposure t ime , a s show nin F igure 3 . Al so , show n on t h i s g raph i s the pe rcen t age o fh y d r o g e n t h a t w o u l d b e d e s o r b e d f o r d i f f e r e n t ef f e c ti v e h y -d rogen d i f fus iv i t ie s (Append ix 1 ). Fro m th i s , i t c an be seentha t t he r a t e o f r ecove ry co r re sponds t o t he r a t e a t wh ichhydrog en wou ld deso rb f rom the sample a ssuming an e f fec -t i ve d i f fus ion coe f f i c i en t o f 1 x 10-~3 m2/sec.F. Fractography

The f r ac tu re su r faces we re exam ined i n t he SEM and re -g ions o f i n t e rg ranu la r and t r ansgranu la r c rack p ropaga t ionwere obse rved on a l l sample s . The pe rcen t age o f i n t e rg ran -u l a r f rac tu re in t he t e s t i ng env i ronment s was de t e rmin ed byl inear in tercept a t constant c rack length on SEM frac tog raphs .The ave rage pe rcen t age o f i n t e rg ranu la r f r ac tu re fo r eachenv i ronm ent is p lo t t ed aga ins t t he c rack l eng th t o t h i ckness(a/h) r a t io i n F igure 4 . In t h i s f i gu re , e ach po in t r ep re sen t sthe ave rage o f fou r measurem en t s , t he e r ro r ba r s r ep re sen tthe s t anda rd dev i a t i on o f t he measuremen t s , and t he l i ne sth rough the po in t s a re t he r eg re ss ion l i ne s de t e rm ined f romt h e m e a s u r e m e n t s . T h e s t r e s s i n t e n s i t i e s w e r e e s t i m a t e dfo r each sample and c rack l eng th t o t h i ckness (a/h) r a t i o ,a ssuming s imple s t r a igh t l i ne c rack geome t ry . The ave rageso f t h e s e v al u e s a re i n c l u d e d i n F i g u r e 4 t o s h o w h o w t h es t re ss i n t ens i t y can be expec t ed t o va ry w i th t he (a/h) ra t io .In t h i s f i gu re , i t c an be seen t ha t t he pe rcen t age o f i n t e rg ran -ular f rac ture a t each (a/h) r a t i o was l ower i n d ry n i t rogen gasthan i n humid n i t rogen gas . In d ry n i t rogen gas , t he pe r -cen t age o f i n t e rg ranu lar f r ac tu re d id n o t change s ign i f i can t lywi th i nc rea s ing cyc l i c s t re ss i n t ens i t y wh i l e i n h um id n i t ro -gen i t i nc rea sed s ign i f i can t ly . The r eg re ss ion l i ne s i nd i ca t etha t t he pe rcen t age o f i n t e rg ranu la r f r ac tu re i s no t s i gn i f i -can t ly d i f f e ren t fo r shor t c rack l eng ths and , a s t h i s f i gu reimpl i e s , c rack i n i t i a t ion was p r imar i l y t r ansg ranu la r occur -r ing a t persis tent s l ip bands.F igure s 5 t h rough 10 a re scann ing e l ec t ron mic rographs

of t yp i ca l f r ac tu re su r faces , and i n t he se f i gu re s t he mac ro -scop ic d i r ec ti on o f c rack p ropaga t ion i s f rom the bo t tom ofthe f i gu re to t he t op . A typ i ca l r eg ion o f t he t r ansg ranu la rc rack p ropaga t ion obse rved i n d ry n i t rogen gas i s show n in

0.

10 0

80

60

4 0

20

0 0

~ D =10 "12 m2 /s

N__ D=10~4m2/s

, _ , . , . l . , . . . . .

20 40 60 8 0 100 120 140 160Time, Days

Fig. 3 - Comparison of the rate of fatigue ife recovery o the calculatedpercent of the h ydrogencontentwhich would be desorbed for differenthy-drogen diffusioncoefficients.

,=

g.

t-

_r

30Average E st imated Stress Intensity (MPs.m o.5)

oa.

20

10

0O.Oq

E] HumidN~trogenGas

i I , I i I i

0 .25 0 .50 0 .75 1 .00Crack Length to Thickness R at io (a/h)

F ig . 4 - - The pe r cen tage o f in t e r g r anu la r f r ac tu r e de te r mined by l inea r in -tercept at cons tant crack length v s crack length and average es t imated s tressintens i ty .

F ig . 5 - The t r ans g r anu la r c l eavage- l ike c r ack p r opaga t ion in tw o g r a in so f a vacuum ou tgas s ed s amp le t e s t ed in d r y n i t r ogen gas .

F ig . 6 - - A r eg ion o f s mal l t r an s g r anula r c l eavage- like f ace t s on the f r ac -tu r e s u r f ace o f a vacuum ou tgas s ed s amp le t e s t ed in d r y n i t r ogen gas .

METALLURGICAL TRANSACTIONS A VOLUM E 19A, JULY 198 8-- 1779


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F ig . 7 - - The f ace o f the s t ep be tw een tw o f ace t s o f t r ans g r anu la r c l eavage-l ike f racture on the f racture surface of a vacuum outg assed sample tes ted indry ni trogen gas .F ig . 10 - - A r eg ion o f in te r g r anu la r c r ack p r opaga t ion in a s amp le t e st ed inhumid n i t r ogen gas.

F ig . 8 - - A r eg ion o f in t e r g r anu la r c r ack p r opaga t ion in a vacuum ou t -gassed sample tes ted in dry ni trogen gas .

Figure 5 . In t h i s f i gu re , t he c rack p ropaga t ed ac ross a g ra inor i en t ed f avorab ly fo r t he fo rma t ion o f sma l l c l eavage - l ikeface ts i n to a g ra in o r i en t ed fo r t he fo rma t ion o f l a rge r f ace t s.The reg ion o f sm a l l f ace t s is show n a t a h ighe r magn i f i ca t i onin F igure 6 a nd s t r i a ti ons can be seen on t he f r ac tu re su r face .These s t r i a ti ons a re approx ima te ly 0 .1 /x m apa r t , i nd i ca ti nga c rack g rowth ra t e o f 1 x 10 -7 mete r s / cyc l e . The f ace o f as t ep be tween tw o reg ions o f c l eavage - l ike f r ac tu re i s show nin F igure 7 . Ev idence o f duc t i le de fo rma t ion and co r ros ioncan be seen on this surface . The corrosion may have occu rreda f t e r t e s t ing , bu t t he n i t rogen env i ronm ent i s no t comple t e lyo x y g e n f r ee ( < 0 . 5 p p m o x y g e n ) a n d f r e t ti n g co r r o s io n o fth i s su r face ma y have obscu red t he f ea tu re s on t h i s su r face ,i nd i ca ti ve o f t he ac tua l f r ac tu re mechan i sm . T yp ica l l y , t hein t e rg ranu la r f r ac tu re su r faces fo rmed in t h i s env i ronmentwere sm ooth and a lmos t f ea tu re l e ss , a s shown in t he uppe rhal f of Figure 8 . The fea tures show n on the gra in boun dary inthe l ower l e f t o f t h i s f i gu re we re no t obse rved on any o the rg ra in bounda ry .Tes t i ng i n humid n i t rogen re su l t ed i n a s l i gh t i nc rease i nthe pe rcen t age o f i n t e rg ranu la r fr ac tu re , e spec i a l l y a t l onge rc rack l eng ths . T he t r ansgranu la r c l eavage - l ike f r ac tu re ob-served in this envi ronment is show n in Figure 9 . The s urfacesappea r f l a t and s moo th , i nd i ca t ing t ha t l i tt l e p la s t i c de fo rma-t i on i s a ssoc ia t ed wi th c rack p ropaga t ion . T he i n t e rg ranu la rf r ac tu re su rfaces o f sample s t e s t ed i n t h i s env i ron men t ap-peared similar to the intergranular fracture surfaces producedin d ry n i t rogen . F igure 10 shows an i n t e rg ranu la r f r ac tu resu r face fo rmed in hu mid n i t rogen and , excep t fo r some cor -rosion damag e, i t does n ot appear s igni f icant ly di f ferent f romtha t shown in t he uppe r ha l f o f F igure 8 . The ex t e rna l su r faceof a sample t e s t ed in hum id n i t rogen i s shown in F igure 11.Th i s f i gu re shows the b l i s t e ri ng and c rack ing o f t he su r facef i lm tha t re su l t ed f rom the com bined ac t i on o f cyc l i c l oad ingand wa te r vapor .

F ig . 9 - - A r eg ion o f tr an s g r anu la r c r ack p r opaga t ion in a s amp le te s t ed inhumid n i t r ogen gas .

V . D I S C U S S I O NIn the t heory sec t i on o f t h i s pape r, i t was po in t ed o u t t ha tt he re a re fou r d i f f e ren t ways t ha t exposure o r p re -exposureto wa te r vapor can acce l e ra te f a t i gue f a i l u re . These a re :

1780--V OLUM E 19A, JULY 1988 METALLURGICAL TRANSACTIONS A


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F i g . 1 1 - - B l i s t e r i n g a n d c r a c k i n g o f t h e e x t e r n al e l e ct r o p o li s h e d s u r f a ceo f a s a m p l e a f t e r e x p o s u re t o h u m i d a i r a n d c y c l i c l o a d i n g i n h u m i d n i t ro -g e n g a s .1 . p re -ex i s ti ng i n t e rna l hydro gen ,2 . p re -ex i s ti ng co r ros ion dam age ,3 . ox ida t i on e f fec t s , and4 . ex t e rna l l y gene ra t ed hydrog en .Cor ros ion damage gene ra t ed dur ing p re -exposure t o wa te rv a p o r c a n n o t b e r e m o v e d b y a s i m p le v a c u u m e x p o s u re a troom t em pera tu re , w h i l e the p re sen t r e su lt s show tha t a vac -uum exposure can r e su l t in com ple t e r ecove ry o f f a t i gue re -s i s ta n c e . T h e r e f o r e , s u r f a c e d a m a g e d u e t o p r e - e x p o s u reto wa te r vapor canno t be r e spons ib l e fo r t he obse rved l ossin f a t i gue r e s i s t ance . Ox ida t i on e f fec t s , such a s ox ide i n -duced c losu re , wo u ld be env i ron men t dependen t r a the r t han[37]pre -exposure dependen t . Scam ans and Tuck show ed tha tt he k ine ti c s o f wa te r vapor adsorp t i on and hydrog en evo lu -t i on a re f a s t comp ared t o bu lk d i f fus ion o f hydroge n . S imi -l a r ly , t he k ine t i c s o f wa te r vapor de so rp t i on f rom the su r facewi l l be f a s t wi th r e spec t t o bu lk hydrogen d i f fus ion . Al so ,[37..38 39]Scamans and Tuck ' ' demo ns t ra t ed t ha t exposure t o wa -t e r vapor r e su lt s i n hydrogen ev o lu t i on , abso rp t i on , and d i f -f u s i o n i n t o t h e m e t a l . T h e r e s u l ts o f th e p r e s e n t s t u d ydemons t ra t e t ha t exposure t o w a te r vapor r educes t he f a t i guere s i s tance o f t h i s a l l oy , t ha t p re -exposure t o w a te r vapor i sa s damag ing a s wa te r vapor i n t he t e s t ing env i ronm ent , t ha tt he e f fec t o f wa te r vapor i s co mple t e ly r eve r s ib le , t ha t t hera t e o f r ecove ry o f t he p re -exposure e f fec t i s s l ow requ i r ingseve ra l days o f vacuu m exp osure , and t ha t t he r at e o f r e -c o v e r y i s e x a c t l y t h a t p r e d i c t e d b y h y d r o g e n d e s o r p t i o nk ine t i c s fo r t h i s sample geome t ry . The re i s no mechan i smothe r t han hydrogen embr i t t l emen t t ha t c an exp l a in t he seobse rva t i ons . Al so , s i nce the sam e fa t i gue l i fe r e su l t s f rompre -exposure t o wa te r vapor a s r e su l t s f rom wa te r vapor i nthe t e s t i ng env i ronment , h ydrog en p re sen t i n t he a l loy a t t heonse t o f t e s t i ng mus t be a s e f fec t i ve in r educ ing t he f a t i guel i fe a s c o r r o s i o n d a m a g e a n d h y d r o g e n g e n e r a t i o n d u r in gthe test .Hyd rogen ma y reduce t he f a t igue r e s is t ance o f A1-Zn-Mga l loys by e i t he r su r face adsorp t i on a t t he c rack t i p o r b y em -br i tt l ing t he p l a s t ic zone ahead o f t he c rack . Sur face adsorp -t i on and i n t e rac ti ons cou ld r educe f a t i gue l i ve s by r educ ingthe su r face ene rgy and e i t he r r educ ing t he ene rgy requ i redt441fo r b r i tt l e f r ac tu re a s sugges t ed by Uhl ig , o r by i nc rea s ingt h e n u m b e r a n d a c t i v i ty o f d i s l o c a t io n s o u r c e s p r o m o t i n g

shear a t the crack t ip as prop osed by Lynch.I451To embr i t t l ethe p l a s ti c zone ahead o f t he c rack t i p , wa t e r vapo r wo u ldreac t wi th the ba re me ta l su r faces exposed a t t he c rack t i p ,p r o d u c i n g s i g n i f i c a n t a m o u n t s o f m o b i l e h y d r o g e n a t o m swhich t hen d i f fuse i n to t he p l a s ti c zone ahead o f the g row ingc racks acce l e ra t i ng f a t i gue c rack g rowth by some in t e rna lh y d r o g e n e m b r i tt l em e n t m e c h a n i s m . F o r t h e c a s e o f a n a l l o ywhich i s sa tu ra ted wi th hydro gen and su bsequen t ly t e s t ed i na d ry env i ronment , t he p l a s t i c zone ahead o f t he c rack t i pwould be expec t ed t o ac t a s a s i nk fo r hydrogen , s i nce t hehydrog en so lub i li t y wo u ld be h ighe r i n t he c rack t ip p rocessz o n e . [ 4 6 1

The re su l t s o f the se expe r imen t s su ppor t t he concep t t ha thydrogen d i sso lved i n t he p l a s t i c zone and no t su r face ad -so rbed hydrogen i s r e spons ib l e fo r embr i t t l emen t . T he r a t e so f su r face reac t i ons and adsorp t i on a re mu ch fa s t e r t han t hera t e o f bu lk d i f fus ion . As a r e su l t , i f hydrog en adsorbed onthe su r face we re r e spons ib le fo r embr i t t l emen t , t hen t he f a -t i gue l i ve s o f hydrogen p re -cha rged sample s wou ld no t beiden t ica l to t he f a t i gue l ive s o f hydro gen f r ee sam ple s t e s t edin humid n i trogen . Ho weve r , i f embr i t t l emen t occurs by anin t e rna l embr i t tl emen t p rocess , t hen bu lk d i f fus ion f rom thesu r face o r f rom the i n t e r i o r t o t he p rocess zone w ou ld o ccura t abou t t he same ra t e . Al so , i f adso rbed hydrogen , e i t he rp r o d u c e d b y s u r f a c e r e a c t i o n s o r d i f f u s i n g f r o m t h e b u l kto t he c rack su r faces , we re r e spons ib l e fo r t he embr i t t l e -men t phenomenon , t hen t he co r re l a t i on be tween the ca l cu -l a te d a m o u n t o f h y d r o g e n d e s o r b e d a n d t h e e m b r i t tl e m e n t" r e c o v e r y " p r o c es s w o u l d n o t b e o b s e r v e d . R a t h e r , s i n c eadsorp t i on is r equ i red on ly a t t he c rack t i p fo r i t t o be e f fec -t i ve , t hen r e s idua l hydroge n in t he a l l oy wou ld sa tu ra t e t hesurface and cause embri t t lement for mu ch longer out - gass ingt imes t hen t hose p red i c t ed and obse rved i n t h i s i nves t i ga t i on .On the o the r hand , a r educ t ion i n t he gene ra l concen t ra t i onof hydrogen in t he a l l oy wou ld be expec t ed t o r educe em-br i t tl emen t i f it occur s wi th in t he c rack t i p p rocess zone bys o m e i n t e r n a l e m b r i t t l e m e n t m e c h a n i s m . F i n a l l y , t h e i n -c rea s ing dev i a t i on be tween the f a t i gue l ive s o f p re -cha rgedsample s and sam ple s t e s ted i n hum id n i t rogen wi th i nc rea s -ing t e s ti ng t ime (F igure 2 ) can be exp l a ined b y bu lk d i f fu -sion reducing the hydrogen concentra t ion in the sam ple w hi lethe oppos i t e tr end wo u ld be obse rved i f bu lk d i f fus ion t o t hesu r face we re a r equ i remen t fo r acce l e ra t ed f a t i gue f a i l u re .A mode l o f i n t e rna l r a the r t han su r face embr i t tl emen t i sa l so su~0or t ed by t he f a t i gue expe r imen t s o f Ho l royd andHard i e ~ ' j wh ich show ed tha t env i ronmenta l l y a ssoc i a ted f a -t i gue c rack g rowth i n a lum inum a l l oy 7017-T651 i s h igh lyf requency dependen t and re l a t ed to t he squa re roo t o f t imeava i l ab l e du r ing each cyc l e fo r c rack p ropaga t ion . A s t rongf requency dependence i s no t cons i s t en t wi th a su r face ad -s o r p ti o n p h e n o m e n o n s i n ce t h e k i n e t i c s o f a d s o r p t io n o nf re sh ly p roduced c rack su r faces shou ld be v i r t ua l ly i ns tan -t aneous . O n the o the r hand , i f t he re i s no con t r ibu t ion o fs t a ti c l oad s t r ess co r ros ion c rack ing dur ing each l oad cy c l e ,t hen a s t he f r equen cy dec rea se s t he p l as t i c zone ahead o f t hec rack t ip wi l l becom e sa tu ra t ed wi th hydrogen and t he c rackpropaga t ion r a t e pe r cyc l e w i l l approach a cons t an t va lue .W ei e t a l . i481s tud i ed t he co r ros ion f a t i gue c rack p ropaga t ionra t es o f a lumin um a l l oy 2219-T851 in wa te r vapor and foundtha t the re was a c r i ti c a l f r equency be low which c rack p rop-a g a t i o n w a s i n d e p e n d e n t o f f r e q u e n c y ; h o w e v e r , t h e y a t -t r ibuted this e ffec t to mass t ransport up the crack.

VIETALLURGICAL TRANSACTIONS A VOLUME 19A, JULY 198 8-- 1781


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The p re sen t s t udy does no t p rov ide d i r ec t ev idence fo r anyspec i f ic mechan i sm of i n t e rna l hydrogen embr i t t l emen t . Themixed i n t e rg ranu la r and t r ansgranu la r c rack ing obse rved i nth i s s t udy i nd i ca t e s t ha t the re i s no un ique f r ac tu re morpho-logy associa ted wi th the mechanism or mec hanisms of hydro -gen a ss i s t ed f r ac tu re . Tha t i s , i t was found tha t hydrogeninc rea sed t he pe rcen t age o f i n t e rg ranu la r f r ac tu re ; howeve r ,this type of f rac ture was ob served in the absence of dam agin ghydrog en spec i e s. Al so , w h i l e hydrog en in f luenced t he ap -pea rance o f t he t r ansg ranu la r f r ac tu re su r faces , t h i s f r ac tu remo de was obse rved ev en wi th s i gn i f i can t amoun t s o f hydro -gen d i sso lved i n t he a l l oy . In f ac t , Alb rech t e t a l . fm f o u n dtha t c a thod ic p re -cha rg ing 7075 t ens i l e sample s r educed t heduc t i l it y and i nc rea sed t he s i ze o f t he d imp le s obse rved onthe fracture surfaces. These observations indicate that neitherthe chemis t ry nor t he mo rpho log y o f g ra in bounda ry /p rec ip i -ta te in terac t ions i s c r i t ica l to en vi ron me nta l c racking9 Fo rexample , i t had been sugges t ed t ha t chemica l d i f f e rencesbe tween the g ra in bou nda ry p rec ip i t a te s a nd t he p rec ip i t a tef ree zone (PFZ) mig h t l e ad t o ga lvan i c co r ros ion o f t he g ra inboun da ry r eg ion [49-52] o r t o hydr ogen em br i t t l em en t ] 53'541Al t e rna t ive ly , i t ha s been sugges t ed t ha t t he pa r t i cu l a r mor -pho logy o f t he g ra in bounda ry p rec ip i t a t e s i s r equ i red fo rcrack propagation.lU-26'55J W hile both of these fa ctors m ay in-f l uence the c rack pa th an d /o r c rack g rowth ra t e s , appa ren t ly ,t hey a re no t r equ i red fo r env i ron menta l l y a ss i s ted c rack ing .

The f r ac tu re pa th appea r s t o be de t e rm ined by a va r i e ty o ffac to r s inc lud ing t he hydrog en concen t ra t i on , t he o r i en t a t ionor availabi li ty of a preferred path , and the load ing cond i t ions.Exam ina t ion o f t he f r actu re su r face r evea l ed t ha t f r equen t lyc rack p ropaga t ion was i n t e rg ranu la r when sma l l g ra ins p ro -v ided a conven ien t i n t e rg ranu la r pa th . S to l t z and Pe l l oux zzls tud i ed t he e f fec t o f r educ ing g ra in s i ze on co r ros ion f a t i guere s i s tance o f a l l oy 7075 and t h ey found tha t r educ ing g ra ins i z e p r o m o t e d i n t e r g r a n u l a r f r a c t u r e a n d i n c r e a s e d c r a c kpropaga t ion r a te s . Th i s e f fec t ma y be t he r e su l t o f hydrog ensegrega t ion t o t h i s r eg ion , t he h ighe r d i f fus iv i t y o f t he g ra inbound a ry a s compared t o t he m a t r i x o r s imply t he o r i en t a t ionof t he g ra in bound a ry wi th r e spec t t o t he s t re ss d i s t r i bu t i ona t the c rack t ip . G ra in bound a ry sepa ra t i on i n the p re sence o fso l id so lu t ion hydrog en has been obse rved i n t he t r ansmis -s ion e l ec t ron mic roscope t o occur a long non-cohe ren t p re -c ip i t a t e ma t r i x i n t e r face s wi th l i t t l e o r no accompany ingplast ic def orm at io n. I6'471 Th e prec ipi ta te- ma tr ix interf aceswi th in t he g rains cou ld a l so i n f luence hyd rogen s eg rega t ionand frac ture in a ma nner s imi lar to the gra in boun dary prec ipi -ta tes . This w ould resul t in lowering the t ransgranular f rac turer e s is t a n ce o f t h is a l l o y i n t h e p r e s e n c e o f h y d r o g e n m o r ethan tha t of other a luminu m al loys which are not prec ip i ta t ionha rdened o r wh ich a re no t ha rdened w i th p rec ip i t a t e phase swi th t he same l eve l o f cohe rency s t r a ins be twee n the m a t r i xand the prec ipi ta te .

V I . C O N C L U S I O N SI t ha s been show n tha t t he f a t igue r e s i s tance o f a h igh p ur -i t y A1-Zn-Mg a l l oy i s s i gn i f i can t ly r educed b y t he p re senceof wa te r vapor in t he t e s t ing env i ronmen t . Al so , an equ iva -l en t r educ t ion i n f a t i gue l if e was obse rved i n d ry n i t rogen fo rsample s p re -exposed t o wa te r vapor l ong enough fo r bu lkd i f fus ion t o sa tu ra te t he sample wi th hydro gen . W hen thesample s t ha t we re p re -exposed t o w a te r vapor we re s t o red i n

a vacuum , comple te recovery of fa t igue st rength resul ted an dthe r a te o f t h i s r ecove ry co r re sponde d to t he e s t im a ted r a t eo f h y d r o g e n d e s o r p t io n f o r a d i f f u s i o n c o e f f i c i e n t o f 1 x10-13m2/sec . These r e su l t s c an on ly be exp l a ined b y a bu lkh y d r o g e n e m b r i t t l e m e n t m e c h a n i s m . T h e r e f o r e , w e c o n -c lude t ha t t he r educ t ion o f t he f a t i gue r e s i s t ance o f t h i s a l l oyin wa te r vapor is due t o a fo rm o f hydrogen em br i t t l emen t .

A P P E N D I XFor t he absorp t i on o r de so rp t i on o f a spec i e s f rom a p l aneshee t whe re t he su r face concen t ra t i on i s no t i n equ i l i b r iumwith the a tmo sphe re , th e surface b oun dary cond i t ion i s : t561

_ D ( O C ~ =\ O x ] a ( C o - C , ) [A1]W h e r e C o i s t h e s u r f a c e c o n c e n t r a t i o n o f t h e s p e c i e s i nequ i l i b r ium w i th t he a tm osphe re a t i n f i n i te t ime , Cs i s t heac tua l su r face concen t ra t ion a s a func t i on o f t ime and a i sa p ropor t i ona l it y cons t an t . For a t h in shee t ( - h < X -< h )in i t ia l l y a t a un i fo rm concen t ra t i on C2 and t he exchang e de -f ined by Eq. [A1] for both surfaces, the solut ion for the con-cent ra t ion as a fun ct ion o f t ime ( t ) and posi t ion (X) i s : t561

u 2 c o s e xC ( X , t ) - C z ~ \ ^ / P \ A /= l - Z c o s t ko - C2 ,=l[A2]

Wh ere t he f t , t e rms a re t he pos i t i ve roo t s o f t he equa t ion :/ k

f l t a n f l = L = ( - ~ ) [ A 3 ]The t o t a l amo un t o f subs t ance , M, , en t e r ing o r le av ing t heshee t up t o t ime t i s de t e rmined a s a f r ac t i on o f t he t o t a lquan t i t y t ha t wi l l be absorbed o r de so rbed t o r each equ i -l ibr ium at in f ini te t ime, M=, by the re la t ionship: r561

2L 2 ex p ( ~ )M----Lt= 1 - ~'~ 2 2 L 2 [A4]M ~ , = l f t , ( f t , + + L )

For sma l l va lues o f t he d i f fus ion coe f f i c i en t , t he f t , t e rmsc a n b e a p p r o x i m a t e d b y t h e t e r m s f o r t h e a r g u m e n t a p -p roach ing i n f in i t y wh ich fo r t he f i r s t and second t e rms a re :f l l = 1 .5708 t2 = 4 .7124 [A5]

Th i s approx ima t ion i s good fo r va lues o f t he d imens ion l e sspa rame te r L g rea t e r t han 102. From th i s , i t c an be seen t ha tt he second and h ighe r o rde r t e rms a re sma l l compared t othe f i r s t t e rm o f t he se r i e s and t ha t a f i r s t t e rm approx ima-t i on wi l l be go od fo r a l l bu t shor t t imes . T he f i r s t t e rm ap-p rox ima t ion fo r t h i s r e l a t ionsh ip i s:

Mt _ 1 - (0 .81057) 2 .467 +L 2 + LM ~9 e x p - 2 . 4 6 7 ~ 2

For hydrogen in a luminum, d i f fus iv i t i e s be tween 10 -12 and10 TM mZ /sec are exp ecte d, t35-391 As a re sult , the dim ens ion -

1782- -VOLU ME 19A, JULY 1988 METALLURGICALTRANSACTIONS A


9/9

Th e C ompetit ion between M artensi teand Omeg a in Que nc he d Ti -Nb Al loysD . L . M O F F A T a n d D . C . L A R B A L E S T I E RA c a r e f u l e x p e r i m e n t a l s tu d y o f t h e p h a s e tr a n s fo r m a t i o n s w h i c h o c c u r i n a n n e a l e d / 3 p h a s e T i - N ba l l o y s d u ri n g q u e n c h i n g h a s b e e n c o m p l e t e d . T h e c o m p e t i t i o n o f th e a " a n d c o p h a s e s t o f o r m i n a l l o y sof 20 to 70 a t. pc t N b w as inves t iga ted as a f unc t ion of quen ch r a te and a l loy com pos i t io n . Par t icu la ra t ten t ion w as pa id to the in te r s t i t i a l conten t and chemica l homogene i ty of the a l loys . The mar tens i t i ca " p h a s e w a s f o u n d o n l y i n 2 0 a n d 2 5 a t . p c t N b a l l o y s , an d t h e n o n l y u s i n g f as t w a t e r q u e n c h e s o f- 3 0 0 ~ U n d e r s l o w e r q u e n c h c o n d i ti o n s, e . g . , - 0 . 3 t o 3 ~ co p h a s e p r e c i p it a t e s w e r ef oun d in these a l loys and in 30 and 35 a t . pc t N b a l loy s . Evid ence of "d i f f use" co phase p r ec ip i ta t ionw a s o b s e r v e d i n a l l o y s u p t o 5 0 a t . p c t N b . O n l y a l l o y s o f 6 0 a n d 7 0 a t . p c t N b w e r e f o u n d t o r e t a inthe s ing le phas e /3 s t r uc tur e upon q uench ing . T hese r esu l t s cons t i tu te the f i r s t pa r t o f a s tudy of thes tab le and metas tab le equi l ib r ia of the T i - N b a l loy sys tem.

I . I N T R O D U C T I O NIT g e n e r a l l y h a s b e e n a c c e p t e d t h a t o n l y t w o m a r t e n s i t i cs t r uc tur es ex i s t in the T i - N b a l loy sys tem . Th ese s t r uc tur es ,a ' a n d a " , h a v e b e e n o b s e r v e d in s e v e r a l o t h e r t i ta n i u m -based t r ans i t ion e lem ent a l lo ys ] ~'21 The s t r uc tur e of a ' i shcp t'3"41 w i th la t t i ce pa r a mete r s iden t ica l to those of a - T i .T h e a " s t r u ct u r e i s C - c e n t e r e d o r t h o r h o m b i c w i t h a C m c mspace grou p. D '3-6] Hat t and Rivlin [6] and Patta nay ak et a l . L7]h a v e r e p o r t e d t h e p r e s e n c e o f a t e t r a g o n a l p h a s e i n 2 5 a n d21.7 a t . pc t N b a l loys , r espec t ive ly ; how ever , the iden t i -f i ca t ion of th i s phase as mar tens i t i c i s uncer ta in .T h e a " s t r u ct u r e m a y b e v i e w e d a s b e i n g a tr a n si t io n f r o mt h e h c p s t r u c tu r e o f a ' t o t h e b c c s t ru c t u r e o f t h e / 3 p h a s e .T h e a t o m p o s i t io n s i n a " a r e ( 0 , 0 , 0 ), ( 1 / 2 , l / 2 , 0 ) , ( 0 , 1 -2y , 1 /2) , and ( 1 /2 , 1 /2 - 2y , 1 /2) w i th y be ing appr oxi -mate ly 0 .2 in T i - N b a l loys , t4'81 By var y in g b / a , c / a , a n d y ,a l l t h r e e s t r u c t u r e s m a y b e p r o d u c e d ] 4~ Fo r ex am ple , ani d e a l h c p s t r u c t u r e i s , o b t a i n e d w h e n y = 1 / 6 , b / a =V ' -3, c / a = V ~ . A b c c s t r u c t u r e i s o b t a i n e d w h e ny = 1 / 4 , b / a = X / 2 , c /a = X /2- . A com pi la t io n of thee x i s ti n g l a tt i ce p a r a m e t e r d a t a f o r t h e a ' a n d a " m a r t e n si t e si s show n in F igur es l ( a ) , ( b ) , and ( c ) . These da ta ind ica tetha t the a ' / a " " b o u n d a r y " o c c u r s a t 7 .2 a t . p c t N b . A l i n e a rex t r apola t ion o f the da ta tow a r d the bcc l imi t sugges t s tha t ,a t r o o m t e m p e r a t u r e , t h e u p p e r c o m p o s i t i o n a l b o u n d a r y f o ra " s h o u l d b e 3 0 . 7 a t . p c t N b . T h e d a t a o f M o r n i r o li a n dGan to is 41 f or a l loys of 25 to 28 a t . pc t N b , h ow e ver , do no tf o l low th i s ex t r apola t ion .T h e M s d a t a f o r th e T i - N b s y s t e m a r e g i v e n i n F i g u r e 2 .The ca lcu la t ion of K auf m an and Ber ns te in [9] o f T g ~" i s a l sos h o w n . F l o w e r et al. [m] noted tha t s ince the r e w er e tw omar tens i tes , the r e shou ld ex is t tw o Ms cu r ves , bu t the sca t te rin the da ta i s too gr ea t to d i s t inguish them . Ext r apola t ion ofthe da ta ind ica tes tha t a l loys of 26 , 3 0 .5 , and 32 a t . pc t N bs h o u l d h a v e M s t empe r a tur es of 273 , 77 , and 0 K , r espec-t i v el y . T h e u p p e r c o m p o s i t i o n a l l i m i t o f a " a s e x t r a p o l a te d

D .L. M O F F A T, f o r mer ly a G r adua te S tuden t in the M ate r i a l s S c ienceP r og r am a t the U n ive r s i ty o f Wts co ns in - M ad is on , i s a Res ea r ch A s s oc ia tein the Labo r a to r y o f N uc lea r S tud ies , Comel l U n ive r s i ty , I thaca , N Y14 85 3 -5 0 0 1. D .C . LA R BA L ES TI E R i s a P r o f es s o r in the M ate r i a l s S c i -ence P r og r am. the D epar tmen t o f M eta l lu r g ica l Eng inee r ing , and the A p-p l i ed S uper conduc t iv i ty Cen te r a t t he U n ive r s i ty o f Wis cons in - M ad is on ,M ad is on , WI 5 3 70 6Man uscr ipt submit te d July 13 , 1987.

f r o m t h e l a tt i ce p a r a m e t e r d a t a ( - 3 1 a t . p c t N b ) i s i n fa i ra g r e e m e n t w i t h t h e c o m p o s i t i o n h a v i n g a n e s t i m a t e d M s o f0 K . H o w e v e r , t h e M s ext r apo la t ion conf l ic t s w i th the r epor ttha t the 26 .6 and 28 .2 a t . pc t N b a l loys of M or ni r o l i andG a n t o i s w e r e m a r t en s i t ic a t r o o m t e m p e r a t u r e .I t i s n o t e w o r t h y th a t n o e x p e r i m e n t a l m e a s u r e m e n t s o f M sb e l o w 2 0 0 ~ o r f o r a l l o y c o m p o s i t i o n s g r e a te r t h a n 2 0 a t .pc t N b have been r epo r ted . O n e r eason g iv en f or th i s is tha tbe low 300 ~ the the r mal a r r es t s igna l beco me s too smal l toobse rve. [1~'~2] I t is a lso app aren t in F igu re 2 that incre as ingt h e n i o b i u m c o n t e n t m u c h b e y o n d 2 5 a t . p c t w o u l d l o w e rM s t o r o o m t e m p e r a t u r e o r b e l o w . A n a d d i t i o n a l c o m p l i -ca t ion i s tha t the mic r os t r uc tur es of a l loys in the 20 to 30 a t .pc t N b r ange a r e gr ea t ly a f f ec ted by quench r a te , a s w i l l bedemons t r a ted in th i s paper .T h e e f f e c t o f q u e n c h r a te o n t h e t y p e o f t r a n s f o r m a t i o n[3]w as s tud ied by Jepson et a l . T h e i r d a t a a r e s u m m a r i z e d i nF igur e 3 . I n the nuc l ea t ion and growt h r eg ion i t w as pr o-posed tha t the the r mal a r res t s w er e due to a nuc le i f o r min gby d i f f us ion . For a l loys and quench r a tes in the r eg ionlabe led s h e a r , M s w a s f o u n d t o b e i n d e p e n d e n t o f q u e n c hr a te . B y c h e m i c a l l y t h i n n in g s p e c i m e n s t o 1 30 to 5 0 / x m ,J e p s o n et a l . w e r e a b l e t o e f f e c t e x t r e m e l y h i g h q u e n c h r a t esw h i c h w e r e f o u n d t o s u p p r e s s t h e m a r t e n s i t i c t r a n s f o r -m a t i o n . T h e r e t a i n e d f l p h a s e c o u l d b e t r a n s f o r m e d r e a d i l yto f l + a" w i th de f or mat ion . The poss ib i l i ty ex i s t s , how -ever , tha t the th inn ing pr ocess in t r oduced s ign i f ican t hy-d r o g e n c o n t a m i n a t i o n , w h i c h m a y h a v e a l t e r e d t h e p h a s et r ans f or mat ions .

O t h e r i n v e s ti g a t io n s h a v e s h o w n t h a t t h e q u e n c h e d m i c r o -s t r uc tures of 14 to 30 a t . pc t N b a l loys a r e m or e com pl ica tedt h a n t h o s e p r o p o s e d b y J e p s o n et a l . Ha tt and Rivlin E6]obser ved tha t f ur nace cool ing a 20 .7 a t . pc t N b a l loy a t0 . 03 ~ r esu l ted in the pr ec ip i ta t ion of the co phase . A i rcool ing th i s a l loy y ie lded the same r esu l t s . O nly w ate rq u e n c h i n g p r o d u c e d a ' . M o r n i r o l i a n d G a n t o i s 4] n o t e ds imi la r r esu l t s in a l loys w i th mor e than 14 a t . pc t N b. Thu s ,i t may be tha t the the r mal a r r es t s obse r ved by Jepson et a l .in the nuc l ea t ion and g rowt h r eg ion w er e ac tu a l ly due to thepr ec ip i ta t ion of co, no t a .

P r ec ip i ta tes of the to phase have been obser ved in a l loysof t i t an ium and z i r conium w i th sever a l o the r t r ans i t ion e le -ments . T he co phase i s hexag ona l and has la t t i ce pa r ame te r sw hich a r e r e la ted to the f l phase la t t i ce , i . e . , a ~ = X / 2 a a ,METALLURGICAL TRANSACTIONS A VOLUME 19A, JULY 198 8-- 1677

1-The Role of Hydrogen in Corrosion Fatigue

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