On the measurements of superplasticity in an Al-Cu alloy
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r e c e n t y e a r s m a n y p a p e r s have b e e n devoted to the p h e n o m e n o n of s u p e r p l a s t i c i t y . N u m e r o u s authors have m e a s u r e d u n u s u a l l y l a r g e v a l u e s of t e n s i l e elongation at high t e m p e r a t u r e s for a specific c l a s s of alloys, x-6 The l a r g e r e l o n g a t i o n v a l u e s have b e e n i n d i c a t e d to be a s s o ciated with a s t r a i n r a t e s e n s i t i v e p a r a m e t e r rn e x p r e s s e d in the p h e n o m e n o l o g i c a l equation O = g~mE n
in which e is the flow s t r e s s , K is a c o n s t a n t for a given t e m p e r a t u r e , g is s t r a i n r a t e , E is s t r a i n , and n is the s t r a i n h a r d e n i n g p a r a m e t e r . The p a r a m e t e r m is u s u ally d e t e r m i n e d b y the s t r a i n r a t e change m e t h o d . l - a This e n t a i l s , in p r i n c i p l e , d e f o r m i n g a s p e c i m e n at one s t r a i n r a t e ~1 and noting the s t r e s s (a~), then changing the s t r a i n r a t e to ~ and m e a s u r i n g the second flow s t r e s s , ~2. U t i l i z i n g this method and a s s u m i n g that s u p e r p l a s t i c m a t e r i a l s a r e v i s c o u s (n = 05, rn is c a l c u lated f r o m the following equation log e~ log E.2 E1 Values of rn were also determined using a variation of the above technique,3but there was a considerable difference among the values of m. The techniques of determining rn of superplastic alloys by the strain rate change method were reviewed by Hedworth and Stowell7 and were foundto be inconsistent and unsatisfactory. A major difficulty as recognized by other authors ~'7 arises because superplastic materials are not viscous and the flow stress is a function both of strain and strain rate; such procedures then result in values of m which are strain dependent and not accurately definable. Another problem with accurate measurements of rn is due to the effect of grain growth during testing. The extremely fine grain size materials (less than 1 to I0 G. RAI, formerly Research Assistant, Department of Metallurgy and Materials Science, MIT, is now with the Research Department, Hindustan Steel Limited, Bihar, India. N. J. GRANT is Director, Center for Materials Science and Engineering and Professor, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass. 02139. Manuscript submitted July 20, 1973. METALLURGICAL TRANSACTIONSA
pm) a r e s u b j e c t to g r a i n growth at the s u p e r p l a s t i c t e s t t e m p e r a t u r e s u n l e s s they a r e s t a b i l i z e d above the t e s t ing t e m p e r a t u r e . 1'z'4 At low s t r a i n r a t e s and high t e m p e r a t u r e s the flow s t r e s s is s t r o n g l y dependent on the g r a i n sizeX-4; thus the flow s t r e s s d e r i v e d in such e x p e r i m e n t s is r e l a t e d to the r e s u l t a n t g r a i n s i z e of the m a t e r i a l . Since actual d e f o r m a t i o n t i m e is much g r e a t e r at low than a
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