Yield point behavior in extruded aluminum rod
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A. PLUMTREE
The m i c r o s t r u c t u r e of a l u m i n u m , extruded u n d e r i n d u s t r i a l conditions at 250~ has b e e n i n v e s t i g a t e d in r e l a t i o n to the p u r i t y of the b i l l e t . E l e c t r o n m i c r o s c o p y was used to o b s e r v e the s u b s t r u c t u r e and Kikuchi d i f f r a c t i o n t e c h n i q u e s were used to m e a s u r e b o u n d a r y angles and thus d i s t i n g u i s h b e t w e e n r e c r y s t a l l i z a t i o n and r e p o l y g o n i z a t i o n for s a m p l e s of two diff e r e n t p u r i t i e s (99.7 pet A1 and 99.99 pct A1) extruded u n d e r identical conditions. High t e n s i l e flow s t r e s s e s of about 8000 to 9000 psi (55 to 62 MN/m 2) w e r e o b s e r v e d in s p e c i m e n s taken f r o m the f i r s t s e c t i o n s of the high p u r i t y e x t r u s i o n . These high s t r e n g t h l e v e l s were a t t r i b u t e d to the p r e s e n c e of fine m i c r o s t r u c t u r e . When s m a l l r e c r y s t a l l i z e d g r a i n s (0.5 to 2.0/~m diam) w e r e p r e s e n t a yield drop was o b s e r v e d . This p h e n o m e n o n is a s s o c i a t e d with the condition where n e a r l y all the d i s l o c a t i o n s a r e l i k e l y to be i m m o b i l e . The a b s e n c e of a yield point in the 99.7 pct p u r i t y a l u m i n u m extruded under the s a m e conditions as the 99.99 pct p u r i t y a l u m i n u m is due to the e x i s t e n c e of fine s u b g r a i n s i n s t e a d of the fine r e c r y s t a l l i z e d s t r u c t u r e . A s m a l l yield point in 99.7 pct a l u m i n u m was induced by s u b s e quent heat t r e a t m e n t r e s u l t i n g in the f o r m a t i o n of s m a l l r e c r y s t a l l i z e d g r a i n s of s i m i l a r c h a r a c t e r to those in the higher p u r i t y e x t r u s i o n .
A YIELD point at the o n s e t of p l a s t i c d e f o r m a t i o n has b e e n o b s e r v e d in m a n y m e t a l s . ~ This p h e n o m e n o n has b e e n the s u b j e c t of e x t e n s i v e i n v e s t i g a t i o n and it is gene r a l l y accepted that, for a v a r i e t y of m e c h a n i s m s occ u r r i n g in different m e t a l s and alloys, 2 the effect is g e n e r a l in n a t u r e . The p r i n c i p a l m e c h a n i s m s , to which this b e h a v i o r is a t t r i b u t e d , a r e unpinning of i m m o b i l e d i s l o c a t i o n s s and s t r e s s - s e n s i t i v i t y of d i s l o c a t i o n v e locity. 4 Another m e c h a n i s m is the g e n e r a t i o n of mobile d i s l o c a t i o n s f r o m g r a i n b o u n d a r y s o u r c e s , s Much of the l i t e r a t u r e devoted to the study of yield points has b e e n c o n c e r n e d with bcc s y s t e m s ; notably i r o n containing c a r b o n and n i t r o g e n i n t e r s t i t i a l s . 6'7 The effect is m o s t e a s i l y o b s e r v e d in this alloy b e c a u s e of the m a g n i t u d e of the yield drop (Fig. 9) which is c o m m o n l y o b s e r v e d to be ~ 14,000
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