Superplastic deformation, cavitation, and fracture of microduplex Cu-Ni-Zn alloys
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I. INTRODUCTION A thermomechanical process for developing a microduplex structure in an (~/13 Cu-Ni-Zn alloy (nickel silver) has been described by Ansiuni and BadiaJ In addition to improving room temperature tensile properties the procedure leads to a product which is superplastic at elevated temperatures. Schelleng and R6ynolds2 examined the effects of superplastic tensile deformation under a limited range of conditions on the room temperature tensile properties of a microduplex nickel silver alloy, and also reported that cavitation occurred during superpL~stic deformation. As part of a program of work concerned with identifying systems that are prone to cavitation, and with understanding the factors that lead to cavitation, the superplastic behavior of two microduplex nickel silvers has been investigated over a wide range of deformation conditions. CaviLationin the alloys has been studied using metallographic and density techniques, and the effect of cavitation on fracture behavior has been examined.
f u r n a c e attached to an I n s t r o n m a c h i n e which was capable of c r o s s h e a d v e l o c i t i e s in the r a n g e 8 • 10 -5 to 8 • 10 -1 m m s -1. S p e c i m e n s were allowed to e q u i l i b r a t e at the Lest t e m p e r a t u r e for 60 m i n before s t r a i n ing. T e m p e r a t u r e v a r i a t i o n s of ~1.5 and •176 were m e a s u r e d o v e r the c e n t r a l 30 m m s and 70 m m s r e gions of the f u r n a c e , r e s p e c t i v e l y . S u p e r p l a s t i c b e h a v i o r of the alloys was c h a r a c t e r i z e d both by d e t e r m i n a t i o n of s t r a i n r a t e s e n s i t i v i t y i n d i c e s (rn v a l u e s ) , and by s t r a i n i n g s p e c i m e n s to f r a c t u r e at v a r i o u s c o n s t a n t c r o s s h e a d v e l o c i t i e s for a r a n g e of t e m p e r a t u r e s . S p e c i m e n s were also s t r a i n e d to elongations s h o r t of f a i l u r e to enable the p r o c e s s of c a v i t a t i o n to be studied by m e t a l l o g r a p h i c and d e n s i tometric techniques. 2.3. M e t a l l o g r a p h y
2. EXPERIMENTAL 2.1. Material The alloys, coded IN 836 and IN 629, were supplied in the form of sheets of thicknesses 2.5 mm and 1.2 ram, respectively, by the International Nickel Company. The nominal compositions of the alloys are given in Table I. The quaternary alloy containing manganese was developed by Ward e t al 3 with the aim of improving the cold workability of the ternary alloy. Both alloys had a microduplex a//3 structure in the asreceived condition, and the mechanism of formation of this structure in IN836 has been discussed by Kreye and B r e n n e r . 4 The m a t e r i a l s u n d e r w e n t a s i m i l a r f a b r i c a t i o n route, although IN629 was given an additional " s p r i n g t e m p e r " cold r o l l of 60 pct a f t e r the microduplex anneal. 2.2. M e c h a n i c a l T e s t i n g
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M i c r o s t r u c t u r e s of s p e c i m e n s of IN836 w e r e exa m i n e d u s i n g optical and s c a n n i n g e
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