The effect of precipitation hardening on dynamic strain aging and jerky flow
- PDF / 684,041 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 12 Downloads / 229 Views
MosT e x p e r i m e n t e r s
who have s t u d i e d j e r k y flow have u t i l i z e d s o l i d s o l u t i o n a l l o y s , 1 and r e l a t i v e l y l i t t l e is known about the phenomenon in p r e c i p i t a t i o n h a r d ened s y s t e m s . F u r t h e r m o r e , the i n f o r m a t i o n which d o e s e x i s t d o e s not f o r m a c o n s i s t e n t p a t t e r n . F o r e x a m p l e , A d a m s z has s t u d i e d t w o - p h a s e a-/3 b r a s s and h a s shown that an i n c r e a s e in the v o l u m e f r a c t i o n of/3 i n c r e a s e s the p r e s t r a i n n e c e s s a r y f o r the i n i t i a t i o n of j e r k y flow (i.e., the " c r i t i c a l " s t r a i n ec). He s u g g e s t s that a h a r d e r s e c o n d p h a s e d e l a y s the o n s e t of j e r k y flow b e c a u s e of g r e a t e r d i f f i c u l t y in f o r m i n g the b a n d s of inhomogenous d e f o r m a t i o n c h a r a c t e r i s t i c of j e r k y flow. S i m i l a r e f f e c t s upon aging c o m m e r c i a l A 1 - M g - S i and A1-Cu a l l o y s have been r e p o r t e d , a-s On the o t h e r hand, it has b e e n shown that p r e c i p i t a t i o n of y ' in a C o - b a s e s u p e r a l l o y d e c r e a s e s s .8,7 M a t s u u r a et al s have shown that aging an A1-8.6 p c t Mg a l l o y at 423 K i n c r e a s e s Ec, while aging at 473 K d e c r e a s e s % . In both c a s e s M a t s u u r a et al found that aging i n c r e a s e d the s t r e s s at which j e r k y flow was i n i t i a t e d . In an A 1 - M g - S i a l l o y , M c C o r m i c k 9 has shown that, depending upon t e s t t e m p e r a t u r e and s t r a i n r a t e , % can e i t h e r be i n c r e a s e d o r d e c r e a s e d in f u r n a c e c o o l e d s a m p l e s c o m p a r e d with quenched s a m p l e s . M o s t r e c e n t w o r k e r s a c c e p t C o t t r e l l ' s p r o p o s a l that point d e f e c t s which s i g n i f i c a n t l y enhance diffusion a r e c r e a t e d by the c r i t i c a l s t r a i n , 1~ and a s s u m e that c h a n g e s in t h i s p r o c e s s can p r o d u c e s i g n i f i c a n t e f f e c t s on Ec.a'6'7,9 H o w e v e r , on t h i s b a s i s it i s difficult to u n d e r s t a n d the m u l t i p l i c i t y of r e s u l t s which have b e e n obtained, e v e n allowing for the fact t h a t the s o l u t e c o n c e n t r a t i o n in the m a t r i x c h a n g e s c o n c u r r e n t l y with p r e c i p i t a t i o n and that p r e c i p i t a t e s m a y r e t a r d the d e v e l o p m e n t of L i i d e r s ' b a n d s . R e c e n t l y , an a l t e r n a t i v e m o d e l f o r d y n a m i c s t r a i n aging and j e r k y flow has b e e n p r o p o s e d ; n it s u g g e s t s that j e r k y flow o c c u r s only when c o n d i t i o n s conducive to u n s t a b l e d e f o r m a t i o n (e.g., n e g a t i v e s t r a i n - r a t e s e n s i t i v i t y ) a r e p r e s e n t in the m a t e r i a l , and that, in g e n e r a l , the m e c h a n i s m of d y n a m i c s t r a i
Data Loading...
