Dynamic strain aging and ductility minima in zirconium
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S T R A I N aging is a w e l l - k n o w n phenomenon that o c c u r s in m a n y m a t e r i a l s , i n c l u d i n g m i l d steel. When a s p e c i m e n i s d e f o r m e d beyond the yield point, unloaded, and then reloaded, a s h a r p yield point i s not o b s e r v e d . However, if the s p e c i m e n i s aged sufficiently in the unloaded condition and t e s t e d again, the yield point r e t u r n s . At those t e m p e r a t u r e s w h e r e it i s n o r m a l l y studied, this phenomenon u s u a l l y o c c u r s over a finite p e r i o d of t i m e . However, s t r a i n aging can be explained in t e r m s of d i s l o c a t i o n p i n n i n g by diffusing i m p u r i t y a t o m s I so that the k i n e t i c s of the p r o c e s s should d e pend l a r g e l y on the diffusion r a t e s of the i m p u r i t y a t o m s . As a r e s u l t s t r a i n aging p h e n o m e n a m a y o c c u r s i m u l t a n e o u s l y with d e f o r m a t i o n if the t e m p e r a t u r e is s u i t a b l y r a i s e d . When this happens, the m e t a l i s said to u n d e r g o d y n a m i c s t r a i n aging. The d y n a m i c a s p e c t s of s t r a i n aging that have been identified a r e a) the a p p e a r a n c e of p l a t e a u s or peaks in flow s t r e s s - t e m p e r a t u r e d i a g r a m s , b) d i s c o n t i n u o u s yielding or the P o r t e v i n - L e C h a t e l i e r p h e n o m e n o n , c) a b n o r m a l l y low s t r a i n r a t e s e n s i t i v i t y , d) a b n o r m a l work h a r d e n i n g r a t e s , and e) ductility m i n i m a . 2,3 Dynamic s t r a i n aging m a n i f e s t s itself, at l e a s t to some d e g r e e , in one o r m o r e of these a s p e c t s in m o s t m e t a l l i c m a t e r i a l s . The study of the v a r i a t i o n of the c o m p r e s s i v e flow s t r e s s of c o l u m b i u m ( n i o b i u m ) with t e m p e r a t u r e by W e s s e l , F r a n c e , and Begley 3 is worth noting. They obs e r v e d that the flow s t r e s s , i n s t e a d of d e c r e a s i n g s t e a d i l y with a r i s e in t e m p e r a t u r e , showed peaks at i n t e r m e d i a t e t e m p e r a t u r e s . Two peaks were obtained at a s t r a i n rate of 10-4 sec-1; one at about 230~ and the o t h e r at 400~ On i n c r e a s i n g the s t r a i n r a t e by a f a c tor of 10, the peaks moved to higher t e m p e r a t u r e s , n a m e l y , about 300 ~ and 450~ r e s p e c t i v e l y . T h e s e peaks, which will be r e f e r r e d to as h a r d e n i n g peaks, a r e t h e r e f o r e s t r a i n r a t e dependent. W e s s e l , F r a n c e , and Begley a t t r i b u t e d the lower t e m p e r a t u r e peak to oxygen a t o m s in solution, and the higher t e m p e r a t u r e peak to n i t r o g e n a n d / o r c a r b o n in solution. S i m i l a r shifts in h a r d e n i n g peak t e m p e r a t u r e s with c h a n g e s in s t r a i n rate have also been r e p o r t e d in m i l d V. RAMACHANDRANand R. E. REED-HILL are former Graduate Student now Post Doctoral W
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