Coarsening of tungsten grains in liquid nickel-tungsten matrix
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KANG AND DUK N. YOON
In s i n t e r e d W - N i alloys with 1, 7, and 30 wt pct Ni the t u n g s t e n g r a i n growth in liquid m a t r i x at 1540~ was i n v e s t i g a t e d . The o b s e r v e d g r a i n size d i s t r i b u t i o n s and growth r a t e a r e c o m p a r e d with the t h e o r e t i c a l p r e d i c t i o n s of W a g n e r , Lifshitz and Slyozov, Lay, and A r d e l l . In the 70 pct W-30 pct Ni alloy the t u n g s t e n p a r t i c l e s s e t t l e d to the bottom of the s p e c i m e n s i m m e d i a t e l y upon m e l t i n g of the m a t r i x , but the s p h e r i c a l g r a i n shape is m a i n t a i n e d d u r i n g the i n i t i a l stage of a n n e a l i n g . In these s p e c i m e n s the l i n e a r i n t e r c e p t d i s t r i bution of the g r a i n s a g r e e s with the p r e d i c t i o n of the LSW (Lifshitz, Slyozov, and Wagner) t h e o r y for the r e a c t i o n c o n t r o l l e d growth m e c h a n i s m . On the other hand the growth r a t e a p p e a r s to follow the t 1/~ law p r e d i c t e d for the diffusion c o n t r o l l e d m e c h a n i s m . T h e s e r e s u l t s a r e c o n s i s t e n t with Lay and A r d e l l ' s t h e o r y in which the c o n c e n t r a t i o n g r a d i e n t a r o u n d g r a i n s is i n v e r s e l y p r o p o r t i o n a l to the a v e r a g e g r a i n size in the l i m i t of s m a l l m a t r i x f r a c t i o n . In the alloys with 1 and 7 pct Ni a m e a n i n g f u l c o m p a r i s o n of the o b s e r v e d l i n e a r i n t e r c e p t d i s t r i b u t i o n of the g r a i n s with t h e o r e t i c a l p r e d i c t i o n s is difficult b e c a u s e of g r a i n contact f l a t t e n i n g due to d e n s i f i c a t i o n . The g r a i n growth is l a r g e r with l e s s m a t r i x f r a c t i o n in the s p e c i m e n s and this r e s u l t p r o v i d e s an evidence for the diffusion cont r o l l e d g r a i n growth d u r i n g the liquid p h a s e s i n t e r i n g of this alloy. c o a r s e nH i n g of solid g rE a i n s d u r i n g liquid phase T s i n t e r i n g has b e e n studied f a i r l y e x t e n s i v e l y s i n c e W a g n e r 1 and Lifshitz and Slyozov ~ obtained a c o m plete t h e o r e t i c a l solution for a s y s t e m of d i s p e r s e d g r a i n s . The LSW (Lifshitz, Slyozov, and W a g n e r ) t h e o r y i s b a s e d on the growth k i n e t i c s which is cont r o l l e d e i t h e r by the diffusion of solute a t o m s through the m a t r i x or by the solution and p r e c i p i t a t i o n r e a c tion at the g r a i n s u r f a c e s . It p r e d i c t s that a f t e r a suff i c i e n t a n n e a l i n g t r e a t m e n t the g r a i n size d i s t r i b u t i o n will a p p r o a c h a s t a t i o n a r y c h a r a c t e r i s t i c function which is d e t e r m i n e d by the p r e d o m i n a n t c o n t r o l l i n g m e c h a n i s m for the growth k i n e t i c s . F u r t h e r m o r e , the t h e o r y shows that the a v e r a g e p a r t i c l e size r i n c r
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