Transitions between stress-induced martensites in Cu Al Ni alloys
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the p a s t few y e a r s a l a r g e n u m b e r of p a p e r s have been p u b l i s h e d on a l l o y s y s t e m s which show s t r e s s induced m a r t e n s i t e (SIM) f o r m a t i o n . The m a i n r e a s o n f o r the i n t e r e s t in t h e s e s y s t e m s is t h e i r unusual m e c h a n i c a l p r o p e r t i e s , e s p e c i a l l y the p s e u d o e l a s t i c and s t r a i n - m e m o r y effects. I Cu-AI-Ni is one of the more e x t e n s i v e l y s t u d i e d s y s t e m s owing to the e a s e of a l l o y p r e p a r a t i o n and the s i m p l i c i t y with which the M s t e m p e r a t u r e can be c o n t r o l l e d by v a r y i n g the n i c k e l and a l u m i n u m contents. H o w e v e r the m e c h a n i c a l p r o p e r t i e s of t h e s e a l l o y s a r e c o m p l i c a t e d by the f a c t that two m a r t e n s i t e s can [ o r m h a v i n g d i f f e r e n t c r y s t a l l o g r a p h i c s t r u c t u r e s . In the r e l a t e d Cu-A1 s y s t e m , 2'a the s t r u c t u r e of the t h e r m a l m a r t e n s i t e obtained d e p e n d s on the a l l o y c o m p o s i t i o n . At a l u m i n u m c o n t e n t s b e l o w 12.'7 wt pct the [31 cubic o r d e r e d p a r e n t p h a s e t r a n s f o r m s to/3~ m a r t e n s i t e with a 3R long r a n g e s t a c k i n g o r d e r , 4 w h i l s t at c o m p o s i t i o n s above 13.2 wt p c t A1, 7 ' m a r t e n s i t e with a 2H s t r u c t u r e was obtained, s At i n t e r m e d i a t e c o m p o s i t i o n s a m i x t u r e of the /3[ and Y' s t r u c t u r e s was o b s e r v e d . 3 O i s h i and Brown s found that on d e f o r m a t i o n of a ~1 s i n g l e c r y s t a l s p e c i m e n of Cu-14 wt pct A1-3 wt p c t Ni in the r a n g e b e t w e e n M s and A s , two t y p e s of m a r t e n s i t e w e r e o b s e r v e d . One had a thin p l a t e m o r p h o l o g y and was a l w a y s r e v e r s i b l e d u r i n g unloading, and the o t h e r was in the f o r m of t h i c k b a n d s . It was o b s e r v e d that e a c h kind p r e s e n t e d a d i f f e r e n t h a b i t plane. In a s u b s e q u e n t p a p e r O t s u k a et al 7 o b s e r v e d for the s a m e c o m p o s i t i o n , that at t e m p e r a t u r e s above A f the p s e u d o e l a s t i c b e h a v i o r was due to the f o r m a t i o n of the ~[ t y p e s t r e s s - i n d u c e d m a r t e n s i t e , with a thin p l a t e - l i k e m o r p h o l o g y , w h i l s t for d e f o r m a t i o n in a r a n g e n e a r M s the f o r m a t i o n of a t h i c k p l a t e m o r p h o l o g y was due to the y ' SIM. The y ' m a r t e n s i t e i s a l s o o b t a i n e d when this a l l o y i s cooled b e l o w i t s M s t e m p e r a t u r e . In the p r e s e n t w o r k the t r a n s i t i o n f r o m ~[ to 7 ' was C. RODRIGUEZ, formerly Post Doctoral Fellow, Department of Metallurgy, University of British Columbia, Vancouver, Canada, is now at Departamento de Metalurgia, Comision Nacional de Energia Atomica, Buenos Aires, Argentin
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