Annealing response of die-cast lead microstructure
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S E V E R A L of the p u b l i s h e d i n v e s t i g a t i o n s 1-~ of s o l i d i f i c a t i o n s e g r e g a t i o n and g r a i n - b o u n d a r y m i g r a t i o n in lead b i c r y s t a l s p r o v i d e s o m e g e n e r a l g u i d e l i n e s to p r e d i c t the d e g r e e of s e g r e g a t i o n of w e l l - d e f i n e d i m p u r i t i e s into g r a i n b o u n d a r i e s and t h e i r role in c o n t r o l ling the m a x i m u m r a t e of g r a i n - b o u n d a r y motion. The m a t e r i a l s studied include l e a d - r i c h alloy b i c r y s t a l s c o n t a i n i n g t y p i c a l l y 2 to 20 p p m of the solute e l e m e n t s Ag, Au, Sb, Sn, Ti, and Zn. " R a n d o m " b o u n d a r i e s in lead b i c r y s t a l s alloyed (0.2 to 2 ppm) with l a r g e s o l u t e a t o m s such as s i l v e r and gold (Goldschmidt r a d i u s 2.88~) exhibit e x p e r i m e n t a l b o u n d a r y m i g r a t i o n v e l o c i t i e s of 0.1 to 5 m m p e r m i n at 300~ tin additions (Goldschmidt r a d i u s 1.58.~ c o m p a r e d to 1.75~, for lead) in the r a n g e 0.1 to 13 ppm, however, show higher v e l o c i t i e s , e.g., 0.5 to 7 m m p e r m i n at 300~ These data s u g g e s t that s e g r e g a t i o n of l a r g e r a t o m s a d j a c e n t to a b o u n d a r y o f f e r s m o r e of a h i n d r a n c e to its diffus i o n a l motion, e.g., for a c o n c e n t r a t i o n of 1 ppm, s i l v e r and gold a r e about 5-fold m o r e efficient than tin to l i m i t v e l o c i t y of m i g r a t i o n . This i n v e s t i g a t i o n was i n i t i a t e d as a p r a c t i c a l e v a l u ation of the d e g r e e to which p u b l i s h e d e x p e r i m e n t a l data and t h e o r e t i c a l models could be used to d e s i g n a g r a i n - c o a r s e n i n g t r e a t m e n t on a c o m m e r c i a l g r a d e of c o r r o d i n g lead. EXPERIMENTAL Table I i n d i c a t e s the c o m p o s i t i o n of the m a t e r i a l used for this study and i n c l u d e s m a x i m u m solid s o l u b i l i t y and d i s t r i b u t i o n coefficient data. M e t a l l o g r a p h i c s t u d i e s w e r e done on s e c t i o n s of e x p e r i m e n t a l s a m p l e s cut f r o m a heavy m e m b e r (3 by 20 rnm) of a d i e - c a s t s a m p l e . Annealing e x p e r i m e n t s w e r e done on s p e c i m e n s which were c a r e f u l l y cut with a j e w e l e r ' s saw to avoid cold d e f o r m a t i o n , which would c o m p r o m i s e the r e s u l t s . Heating was done i n a i r , with a t e m p e r a t u r e (t = ~ T = ~ m e a s u r i n g a c c u r a c y of + 2~ and t i m i n g a c c u r a c y (0 = hr) of + 3 pct. Fig. 1 p r e s e n t s the e x p e r i m e n t a l l y o b s e r v e d g r a i n c o a r s e n i n g data, i n d i c a t i n g an a c t i v a t i o n e n e r g y of 13.6 kcal. The g r a i n - b o u n d a r y m i g r a t i o n r a t e was d e t e r m i n e d b y m e a s u r i n g the r a t e of l a t e r a l b r o a d e n i n g of the c o l u m n a r g r a i n s which c o n s
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