Anisotropic Behaviour in Cu-Zn-Al SMA Due to the Oriented Growth of Precipitates

The two way shape memory e f f e c t (TWSME) is u s u a l l y induced by t h e p r e s e n c e of e i t h e r s t a b i l i z e d t h i n p l a t e s of m a r t e n s i t e o r p a r t i c u l a r d e f e c t s b r e a k i n g t h e symmetry of t h e p a r e n t phase and making o n l y one o r a few v a r i a n t s f a v o u r a b l e upon t r a n s f o r m a t i o n . G e n e r a l l y , t h e d e f e c t s a r e c r e a t e d i n t h e m a t e r i a l by pe r fo rming a c e r t a i n number o f t r a n s f o r m a t i o n c y c l e s . An a l t e r n a t i v e method t o i nduce t h e TWSME, a v o i d i n g t h e heavy t r a i n i n g p r o c e s s e s , h a s been o b t a i n e d f o r Cu-Zn-A1 s i n g l e c r y s t a l . I t c o n s i s t s on t h e i n t r o d u c t i o n o f small y p r e c i p i t a t e s by a p r o p r i a t e d the rma l t r e a t m e n t s . Then t h e p r e c i p i t a t e s a r e made t o grow a s y m m e t r i c a l l y by t h e a c t i o n o f an a p p l i e d s t r e s s . A n i s o t r o p i c r e s i d u a l s t r e s s e s t h e n f a v o u r t h e appea rance of some s e l e c t e d m a r t e n s i t e v a r i a n t s a t t h e t r a n s f o r m a t i o n c y c l e s .