Microstructure and properties of a Cu-Ni-Cr spinodal alloy
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G. THOMAS
A c o m m e r c i a l Cu-28 p c t N i - 2 . 8 p c t C r a l l o y h a s b e e n h e a t t r e a t e d to p r o d u c e s p i n o d a l s t r u c t u r e s , C o n v e n t i o n a l t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM) and d i f f r a c t i o n have b e e n u s e d to s t u d y the s p i n o d a l l y d e c o m p o s e d p r o d u c t and to m e a s u r e the wavelengths in the s p e c i m e n s . It is found that the c o a r s e n i n g of c o h e r e n t p a r t i c l e s o b e y s the r e l a t i o n X a t ~/3 at 700~ and Xot t 1/4 a t 600~ L o s s of c o h e r e n c y is o b s e r v e d only when a g i n g at 700~ and k ~ 1000.k, when i n t e r f a c i a l d i s l o c a t i o n s with b = a / 2 (110) a r e c r e a t e d a f t e r which the c o a r s e n i n g r a t e i s a c c e l e r a t e d . T e n s i l e t e s t s w e r e p e r f o r m e d to m e a s u r e m e c h a n i c a l p r o p e r t i e s of the a g e d s a m p l e s , and the f r a c t u r e s u r f a c e s w e r e e x a m i n e d by s c a n n i n g e l e c t r o n m i c r o s c o p y . The c o r r e l a t i o n s of m i c r o s t r u c t u r a l c h a n g e s and m e c h a n i cal properties are discussed.
T HE
o b j e c t of this r e s e a r c h is to i n v e s t i g a t e e x p e r i m e n t a l l y the s p i n o d a l c h a r a c t e r i s t i c s of an a l l o y and then c o r r e l a t e the m e c h a n i c a l p r o p e r t i e s with the changes in m i c r o s t r u c t u r e d u r i n g s p i n o d a l d e c o m p o s i t i o n and p a r t i c l e c o a r s e n i n g . C u - N i - C r was c h o s e n p a r t i c u l a r l y for its c o m m e r c i a l i m p o r t a n c e as an a l loy of e x c e l l e n t s t r e n g t h and c o r r o s i o n r e s i s t a n c e in marine environments, t The constitutional studies by M e i j e r i n g e t al 2 have shown that t h e r e is a m i s c i b i l i t y gap in the fcc p h a s e p o r t i o n of the C u - N i - C r s y s t e m , although b i n a r y C u - N i a l l o y s a r e c o m p l e t e l y m i s c i b l e (Fig. 1). B a d i a , K i r b y , and M i h a l i s i n 3 have s t u d i e d the s t r e n g t h e n i n g d u r i n g continuous c o o l i n g of C u - N i - C r a l l o y s containing 20 to 45 pct Ni with up to 4 pct C r . Many of t h e s e e x p e r i m e n t a l o b s e r v a t i o n s w e r e in a c c o r d with the t h e o r e t i c a l c a l c u l a t i o n s of Huston, Calm and H i l l i a r d 4 f o r s p i n o d a l d e c o m p o s i t i o n d u r i n g continuous cooling. F r o m the l i t e r a t u r e , 1 it was concluded that the p r e s ent C u - N i - C r a l l o y would u n d e r g o s p i n o d a l d e c o m p o s i tion when a g e d a t t e m p e r a t u r e s below 760~ with the f o r m a t i o n of C u - r i c h and N i - C r r i c h p h a s e s . T h e s e p h a s e s a r e both fcc, and have l a t t i c e p a r a m e t e r s a f t e r h e a t - t r e a t i n g at 750~ of 3.603 and 3.571~ r e s p e c t i v e l y . 2'3 In this
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