Grain refinement through thermal cycling in an Fe-Ni-Ti cryogenic alloy
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R E C E N T r e s e a r c h in t h i s l a b o r a t o r y 1-4 h a s shown t h a t a l l o y s f r o m the F e - N i - T i s y s t e m ( e . g . , F e - 1 2 N i 0.25 Tit can b e p r o c e s s e d to have a p r o m i s i n g c o m b i nation of s t r e n g t h and t o u g h n e s s at c r y o g e n i c t e m p e r a t u r e . As e x p e c t e d , the d u c t i l e - b r i t t l e t r a n s i t i o n t e m p e r a t u r e s of t h e s e a l l o y s tend to d e c r e a s e if the e f f e c tive g r a i n s i z e is m a d e s m a l l . Hence a m a j o r focus of o u r r e s e a r c h h a s b e e n on the d e s i g n of p r o c e s s e s which accomplish significant grain refinement. P r e v i o u s w o r k 5-r e s t a b l i s h e d two t e c h n i q u e s f o r r e fining the g r a i n s i z e of F e - N i a l l o y s of m o d e r a t e n i c k e l content. F o l l o w i n g s t u d i e s b y G r a n g e , 8 P o r t e r and Dabkowski s d e m o n s t r a t e d g r a i n r e f i n e m e n t through a t h e r m a l c y c l i n g p r o c e d u r e which i n c l u d e d r e p e a t e d c y c l e s of r a p i d a u s t e n i t i z i n g and cooling. G r a i n s i z e a s sinai1 a s 3 to 5 ~ m (mean i n t e r c e p t length: ASTM #13-14) was obtained. Saul e t a l 6 a l s o o b s e r v e d g r a i n r e f i n e m e n t in m a r a g i n g s t e e l s on s i m p l e r e p e a t e d a u s t e n i t i z i n g ( f r o m ASTM #2 to # 7 in 4 c y c l e s ) . An a l t e r n a t e technique was u s e d b y M i l l e r , 7 who r e p o r t e d u l t r a f i n e g r a i n s i z e (0.3 to 1.1 ~tm; ASTM #17 to 19) in an F e - N i a l l o y which had b e e n s e v e r e l y c o l d - w o r k e d and then a n n e a l e d in the t w o - p h a s e (a + 7) r a n g e . V a r i a n t s of both t h e s e p r o c e s s i n g t e c h n i q u e s w e r e used in our r e s e a r c h 2 on the a l l o y F e - 1 2 Ni-0.25 Ti, and both l e d to a l l o y s of e x c e p t i o n a l s t r e n g t h and d u c t i l i t y at c r y o g e n i c t e m p e r a t u r e . H o w e v e r , both p r o c e s s i n g t e c h n i q u e s had u n d e s i r a b l e f e a t u r e s . When the c y c l i c a u s t e n i t i z i n g p r o c e s s was used, the g r a i n s i z e s e e m e d to s t a b i l i z e at 5 to 10 ~tm. A f u r t h e r i m p r o v e m e n t in low t e m p e r a t u r e d u c t i l i t y might be o b t a i n a b l e with f i n e r g r a i n s i z e . A m o r e r e f i n e d s t r u c t u r e can b e a c h i e v e d with m e c h a n i c a l working, but m e c h a n i c a l w o r k i n g i s often an i m p r a c t i c a l o r undes i r a b l e s t e p in final a l l o y p r o c e s s i n g . We t h e r e f o r e sought an a l t e r n a t e t h e r m a l t r e a t m e n t which would accomplish a grain refinement comparable to that obtainable with mechanical work. S. JIN, J. W. MORRIS, Jr., and V. F. ZACKAY are Assistant Research Engineer, Associate Professor, and Professor, respectively, Department of Materials Science and Engineering, University of Californ
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