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C. LAIRD,

AND

D. P. POPE

L i m i t e d r e s u l t s on the fatigue of p s e u d o - e l a s t i c m a t e r i a l i n d i c a t e that, a s a c l a s s , t h e s e m a t e r i a l s should have t r u l y outstanding fatigue p r o p e r t i e s . To check this, the m e c h a n i s m s of c y c l i c d e f o r m a t i o n and f r a c t u r e have b e e n s t u d i e d in C u - A l - N i c h o s e n b e c a u s e its t r a n s f o r m a t i o n b e h a v i o r is well u n d e r s t o o d . Since this a l l o y is n o t o r i o u s l y b r i t t l e , p u l s a t i n g c o m p r e s s i o n fatigue t e s t s w e r e c a r r i e d out in p o l y c r y s t a l l i n e m a t e r i a l . The d e t a i l s of the s t r e s s - i n d u c e d m a r t e n s i t e b e h a v i o r w e r e s t u d i e d by in situ v i d e o o b s e r v a t i o n s . The a l l o y was found to undergo c y c l i c h a r d e n i n g and f a i l u r e e v e n t u a l l y o c c u r r e d b y m u l t i p l e n u c l e a tion of c r a c k s at g r a i n b o u n d a r i e s , b y a m e c h a n i s m s i m i l a r in p r i n c i p l e to that which o c c u r s in r e g u l a r m e t a l s c y c l e d at high p l a s t i c s t r a i n s . The C o f f i n - M a n s o n law w a s obeyed.

IN

the p a s t few y e a r s , o v e r one h u n d r e d p a p e r s have b e e n p u b l i s h e d d e a l i n g with s t r e s s - i n d u c e d m a r t e n s i t e (SIM), p a r t i c u l a r l y the type giving r i s e to p s e u d o e l a s t i c i t y and the s h a p e m e m o r y effect (for r e v i e w , s e e D e l a e y et all-3). The bulk of t h e s e p a p e r s have e m p h a s i z e d the t r a n s f o r m a t i o n a l , c r y s t a l l o g r a p h i c and u n i d i r e c t i o n a l d e f o r m a t i o n a s p e c t of t h i s i n t e r e s t i n g c l a s s of m a t e r i a l , mad m a n y a p p l i c a t i o n s , m a i n l y in f a s t e n e r s and h e a t e n g i n e s , have b e e n d e v e l o p e d . A f u r t h e r v e r y p r o m i s i n g avenue for a p p l i c a t i o n was r e v e a l e d when P,a c h i n g e r 4 showed that a s i n g l e c r y s t a l of Cu-A1-Ni a l l o y could w i t h s t a n d o v e r 60,000 a l t e r n a tions of 0.01 " p l a s t i c " s t r a i n without showing any s i g n s of fatigue f a i l u r e . (A c o n v e n t i o n a l d u c t i l e m e t a l shows a l i f e of about 1000 c y c l e s for such a s t r a i n . ) G r e a t e r e n c o u r a g e m e n t was f o r t h c o m i n g when TiNt was found to have t r u l y r e m a r k a b l e fatigue p r o p e r t i e s s and Busch et al 8 l a t e r s u p p o r t e d R a c h i n g e r ' s d i s c o v e r y 4 in Cu-A1Ni. In s p i t e of t h e s e r e s u l t s , no s y s t e m a t i c study of the fatigue p r o p e r t i e s of p s e u d o e l a s t i c a l l o y s h a s b e e n p u b l i s h e d so f a r . R o d r i g u e z and Brown, h o w e v e r , have r e p o r t e d the r e s u l t s of p r e l i m i n a r y w o r k on m o n o c r y s t a l l i n e Cu-A1-Ni (Ref. 7