Serrated yielding in a 5005 Al alloy
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AL:5005
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Serrated Yielding in a 5005 AI Alloy
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K. MUKHERJEE, T. MAY, C. D'ANTONIO, AND R. J. MACIAG
S E R R A T E D y i e l d i n g in a l u m i n u m - b a s e dilute a l l o y s has been studied by v a r i o u s i n v e s t i g a t o r s . ~-11 The p u r pose of this c o m m u n i c a t i o n i s to r e p o r t our r e s u l t s for a s i m i l a r study on a c o m m e r c i a l 5005 A1 alloy. The t e m p e r a t t i r e and s t r a i n r a t e dependence of the p l a s t i c s t r a i n Co, for the o n s e t of s e r r a t e d yielding, was studied. The s a m p l e s were p r e p a r e d f r o m a c o m m e r c i a l 5005 alloy with a n o m i n a l c h e m i c a l composition of A1-0.8 at. pct Mg. Flat t e n s i l e s s a m p l e s with 1 in. gage length and 1/8 in. t h i c k n e s s were m a c h i n e d f r o m a s i n g l e , thoroughly h o m o g e n i z e d ingot. After m a c h i n ing the s a m p l e s were a n n e a l e d at 343 ~ + 10~ for 2 h r and f u r n a c e cooled in air. The a v e r a g e g r a i n d i a m e t e r a f t e r a n n e a l i n g was 0.045 m m . An I n s t r o n t e s t i n g m a chine was used for this study. D e t a i l s of e x p e r i m e n t a l p r o c e d u r e have been r e p o r t e d e l s e w h e r e . 1 Although both type ! and II s e r r a t i o n s ~ were obs e r v e d , only the type I s e r r a t i o n s in the t e m p e r a t u r e r a n g e of 40 ~ to - 4 0 ~ were studied in detail. A m o d i fied C o t t r e U equation ~'la of the f o r m :
~c =A ~'~ E~ exp (-AH/kT)
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1.0 0.8
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I 3.5
3.3
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4-1
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X 103(~ -I Fig. i - - P l a s t i c s t r a i n ~0 f o r t h e o n s e t of P - L e f f e c t at t h r e e different strain rates vs reciprocal absolute temperature.
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AL; 5005 Slope:1,82_+0.02 /
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was u s e d for analyzing the data. Where ec and ~ a r e the c r i t i c a l s t r a i n r a t e and g r a i n size, r e s p e c t i v e l y , A, /3, and n a r e m a t e r i a l c o n s t a n t s , l'~a and AH is the a c t i v a t i o n e n e r g y a s s o c i a t e d with the p r o c e s s . Acc o r d i n g to Eq. [1], for a set of c o n s t a n t g r a i n size s a m p l e s , the slope of the In eo vs 1/T plot, at a constant s t r a i n r a t e , is (/,H/nk), a l s o the slope of the plot of in 4 c vs In co, at a c o n s t a n t t e m p e r a t u r e , is n. T h e s e two plots a r e shown in F i g s . 1 and 2, r e s p e c tively. F r o m the slopes of t h e s e two plots the a c t i v a tion e n e r g y AH can be c a l c u l a t e d . F u r t h e r , the a c t i v a tion e n e r g y AH can be obtained d i r e c t l y f r o m the slope of a plot of In gc vs 1/T at a c o n s t a n t c r i t i c a l s t r a i n ~o. Fig. 3 shows such a plot. The e x p e r i m e n t a l v a l u e s of the activation e n e r g y and the exponent n a r e s u m m a r i z e d in Table I. Within the f r a m e w o r k of the C o t t r e l l mode
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