Heat treatments for improved mechanical properties of CuNiFe spinodal alloys
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I. INTRODUCTION WHEN an alloy which undergoes spinodal decomposition z'2 is aged inside the miscibility gap, the well known modulated two phase microstructure results. The age hardening response produced by this microstructure has also received some attention, most notably by Carpenter 3 (AuPt), Schwartz et al, 4 (CuNiSn), and Dahlgren, s-7 Butler and Thomas, 8 and Livak and Thomas, 9 (all CuNiFe). Dahlgren, s Butler and Thomas, 8 and Livak and Thomas 9 have shown that for relatively long aging periods the yield strength is proportional to the difference in lattice parameter ha, and therefore composition, between the two decomposed phases, and is n e a r l y independent of both the s p i n o d a l wavelength )~ and the r e l a t i v e v o l u m e f r a c t i o n s of the two p h a s e s . T h e s e r e s u l t s axe c o n s i s t e n t with the h a r d e n i n g m o d e l p r o p o s e d by D a h l g r e n , G which a p p l i e s to the s p i n o d a l m i c r o s t r u c t u r e when it can be d e s c r i b e d a s a l t e r n a t ing C u - r i c h and N i - r i c h c o h e r e n t p l a t e s . T h i s is a v e r y w e l l defined m i c r o s t r u c t u r e which lends i t s e l f to a s t r a i g h t f o r w a r d i n t e r p r e t a t i o n ; h o w e v e r , in the e a r l y s t a g e s of s p i n o d a l d e c o m p o s i t i o n this is c e r t a i n l y not the c a s e . F o r the e a r l y s t a g e s of s p i n o d a l d e c o m p o s i tion the y i e l d s t r e n g t h is not only s e n s i t i v e to ~Xa but a l s o to ~ and the shape of the c o m p o s i t i o n p r o f i l e , z~ A g e h a r d e n e d C u N i F e s p i n o d a l m a t e r i a l s have b e e n o b s e r v e d to f a i l g e n e r a l l y in a b r i t t l e i n t e r g r a n u l a r fashion, y e t they show f a i r l y good t e n s i l e e l o n g a t i o n s , tz T h i s b e h a v i o r has b e e n a t t r i b u t e d to e i t h e r s o l u t e segregation, or a discontinuous coarsening reaction which o c c u r s at the g r a i n b o u n d a r i e s , 3 The p r o b l e m of g r a i n b o u n d a r y e m b r i t t l e m e n t h a s b e e n o v e r c o m e in the c a s e of CuNiSn a l l o y s by t h e r m o m e c h a n i c a l l y t r e a t ing the m a t e r i a l . '4 H o w e v e r , it i s hoped that the g r a i n b o u n d a r y e m b r i t t l e m e n t in C u N i F e a l l o y s can be s u p p r e s s e d by a t h e r m a l t r e a t m e n t only. T h e p r e s e n t w o r k has a i m e d to d e s i g n nonconvent i o n a l heat t r e a t m e n t s that allow the d e v e l o p m e n t of the s p i n o d a l m i c r o s t r u c t u r e such that t h e r e is a l a r g e d i f f e r e n c e in l a t t i c e p a r a m e t e r b e t w e e n the d e c o m p o s e d p h a s e s , and which w i l l r e s u l t in a r e d u c t i o n o r e l i m i n a K. G. KUBARYCH and M. OKADA are Graduate Students, and G. THOMAS is Professor, Department of Materials Science and Engineering, University of California, Berkeley, CA 94720. Manuscript submit
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