The initial development of a yield point due to carbon in nickel 200 during static strain aging
- PDF / 405,532 Bytes
- 4 Pages / 613 x 788.28 pts Page_size
- 22 Downloads / 192 Views
IN
a p r e v i o u s paper, work on s t a t i c s t r a i n aging in n i c k e l c o n t a i n i n g c a r b o n 2 was r e p o r t e d . T e n s i l e s p e c i m e n s of Nickel 200 were p r e s t r a i n e d 5 pct, aged between 10 and 106 s at t e m p e r a t u r e s b e t w e e n 373 and 473 K, and then they w e r e r e s t r a i n e d . The e x p e r i m e n t a l o b s e r v a t i o n s were o b s e r v e d to s u p p o r t an aging m e c h a n i s m o r i g i n a l l y p r o p o s e d by Rose and Glover 1 and by J e n k i n s and Smith ~ for a u s t e n i t i c s t a i n l e s s s t e e l s . In this m e c h a n i s m , the h a r d e n i n g agent is a s s u m e d to be a c a r b o n - v a c a n c y p a i r which r e o r i e n t s in the s t r e s s field of a d i s l o c a t i o n . The p a i r r o t a t i o n is a c c o m p l i s h e d by the m o v e m e n t of the v a cancy r a t h e r than the c a r b o n atom s i n c e the v a c a n c y has a lower a c t i v a t i o n e n e r g y for motion, about 0.87 eV (20.0 kcal//mole) c o m p a r e d to 1.43 eV (33.0 kcalr / mole) for a c a r b o n atom in n i c k e l . Rose and Glover p r o p o s e d a t h r e e - s t a g e aging p r o c e s s : a) r o t a t i o n of e x i s t i n g p a i r s ; b) f o r m a t i o n of new p a i r s ; and c) m i g-ration of v a c a n c i e s into o r d e r i n g r e g i o n s a r o u n d d i s locations. T h i s p a p e r c o n c e r n s the f i r s t stage in the aging p r o c e s s d u r i n g which v a c a n c i e s c r e a t e d d u r i n g def o r m a t i o n a r e t r a p p e d by i n t e r s t i t i a l c a r b o n a t o m s and the r e s u l t i n g c a r b o n - v a c a n c y dipoles r e o r i e n t into t h e i r lowest e n e r g y c o n f i g u r a t i o n with r e s p e c t to n e a r b y d i s l o c a t i o n s . The r e s u l t is an i n i t i a l c o m ponent of the yield point that f o r m s v e r y quickly. The analogous p r o c e s s of o r d e r i n g of the c a r b o n a t o m s in i r o n in the s t r e s s field of a d i s l o c a t i o n was e x a m i n e d by Schoeck and S e e g e r 4 in 1959. A s s u m i n g the c o n c e n t r a t i o n of solute was s m a l l , they showed that the line e n e r g y of a d i s l o c a t i o n s u r r o u n d e d by a Snoek o r d e r e d a t m o s p h e r e was d e c r e a s e d by a n a m o u n t Uo given by
where b is the B u r g e r s v e c t o r m a g n i t u d e . The yield s t r e s s i n c r e m e n t r e s u l t i n g f r o m the o r d e r i n g of solute a t o m s in the s t r e s s fields of d i s l o c a t i o n s has g e n e r a l l y b e e n i g n o r e d in static s t r a i n aging e x p e r i m e n t s p r o b a b l y b e c a u s e it o c c u r s v e r y quickly at the t e m p e r a t u r e s that a r e n o r m a l l y involved. Snoek o r d e r ing, however, could explain the r a p i d s t a t i c s t r a i n aging p h e n o m e n a o b s e r v e d in t e n s i l e t e s t s on a low c a r bon s t e e l by W i l s o n and R u s s e l l 5 as well as s i m i l
Data Loading...
