Superplasticity in a new dispersion strengthened zinc alloy
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IN r e c e n t
y e a r s much r e s e a r c h work has been devoted to identifying the deformation m e c h a n i s m of s u p e r p l a s tic m a t e r i a l s . 1 However, during this p e r i o d v e r y little r e s e a r c h has been a s s o c i a t e d with controlling the onset of s u p e r p l a s t i c i t y . It was felt that v e r y high room t e m p e r a t u r e yield s t r e n g t h s could be obtained in fine grained low melting point alloys if the tendency for sup e r p l a s t i c deformation could be s u p p r e s s e d . In o r d e r to t e s t this hypothesis, r e s e a r c h was initiated to develop a zinc b a s e alloy which exhibits both a high yield strength at room t e m p e r a t u r e and s u p e r p l a s t i c i t y at elevated temperatures. S u p e r p l a s t i c i t y and high strength a r e not compatible p r o p e r t i e s in zinc. F o r example, both a fine grained Zn0.4 A1 alloy2 and c o m m e r c i a l purity zinc 3 exhibited sup e r p l a s t i c s t r a i n s under l o w - t e n s i l e s t r e s s e s (8.1 k g / m m 2) at room t e m p e r a t u r e . Thus the problem was to s u p p r e s s s u p e r p l a s t i c i t y at ambient t e m p e r a t u r e s in o r d e r to r e a l i z e the high yield strength n o r m a l l y afforded by a s m a l l grain d i a m e t e r . If d i s l o c a t i o n s a r e r e s p o n s i b l e for the deformation of fine grained zinc at room t e m p e r a t u r e then a d i s p e r s i o n of s m a l l p r e c i p i t a t e s should be effective in s u p p r e s s i n g s u p e r p l a s t i c i t y and n o r m a l c r e e p by preventing r e c o v e r y . Thus alloys containing elements which i n c r e a s e the r e c r y s t a l l i z a t i o n t e m p e r a t u r e of zinc were s e l e c t e d for this r e s e a r c h . The d i s p e r s i o n of s m a l l second phase p a r t i c l e s also r e d u c e s g r a i n growth at elevated t e m p e r a t u r e s and hinders grain boundary sliding at room temperature. J. D. LEE, formerly Graduate Student in Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada, is now with the Bell Northern Research Laboratories, Ottawa, Canada. P. NIESSEN is Professor of Mechanical Engineering, University of Waterloo. Manuscript submitted May 22, 1972. METALLURGICAL TRANSACTIONS
P e l z e l , 4 in a r e c e n t review of wrought zinc a l l o y s , indicated that the addition of magnesium to zinc produced a m a r k e d i m p r o v e m e n t in the c r e e p s t r e n g t h while F a r g e and W i l l i a m s 5 have shown that s m a l l additions of magnesium to zinc i n c r e a s e d the r e c r y s t a l l i z a t i o n t e m p e r a t u r e . However, it was apparent from the Zn-Mg phase d i a g r a m 6 that the solubility of magnesium in zinc was too low to provide the n e c e s s a r y volume fraction of second phase p a r t i c l e s for g r a i n s i z e stabilization. Because zinc has a low solubility for most e l e m e n t s 6 it was nece s s a r y to d e t e r m i n e if zinc could be s u
