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Z. A. MUNIR

The effects of v a r i o u s t r a n s i t i o n m e t a l a d d i t i o n s on the s i n t e r i n g of a w e l l - c h a r a c t e r i z e d , fine t u n g s t e n powder w e r e a n a l y z e d u s i n g both i s o t h e r m a l and c o n s t a n t h e a t i n g r a t e exp e r i m e n t s in the t e m p e r a t u r e r a n g e 900 to 1400°C. A p p r o x i m a t e l y four a t o m i c m o n o l a y e r s of p a l l a d i u m on the t u n g s t e n powder s u r f a c e w e r e found to be the o p t i m a l e n h a n c e r , followed by n i c k e l , cobalt, p l a t i n u m , and i r o n . The a d d i t i o n of Cu to the t u n g s t e n had no a p p r e c i a b l e effect on the s i n t e r i n g k i n e t i c s . S i n t e r i n g e n h a n c e m e n t by t h e s e t r a n s i t i o n m e t a l s is r e l a t e d to t h e i r p e r i o d i c c h a r t p o s i t i o n (i.e., e l e c t r o n s t r u c t u r e ) . An o v e r a l l n o n - A r r h e n i u s s h r i n k a g e t e m p e r a t u r e d e p e n d e n c e i s a t t r i b u t e d to g r a i n growth in the a c t i v a t o r - t r e a t e d s p e c i m e n s . The a c t i v a t i o n e n e r g y f o r t u n g s t e n d e n s i f i c a t i o n was d e t e r m i n e d to be 430 to 450 k J / m o l , which is in g e n e r a l a g r e e m e n t with a g r a i n b o u n d a r y diffusion p r o c e s s .

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s i n t e r i n g t e m p e r a t u r e for s u c h r e f r a c t o r y m e t a l s a s t u n g s t e n and m o l y b d e n u m can be r e d u c e d by the a d d i t i o n of s m a l l q u a n t i t i e s of s e l e c t e d t r a n s i t i o n metals.I-e° R e c e n t r e p o r t s have a t t e m p t e d to c o r r e l a t e the e n h a n c e d s i n t e r i n g with the s t r u c t u r e of the t r a n s i t i o n m e t a l a d d i t i v e s . 13,~%2° A p r e v i o u s a r t i cle on the a c t i v a t e d s i n t e r i n g of h a f n i u m ~9 showed such an e l e c t r o n c o n c e n t r a t i o n a p p r o a c h to be a v a l i d m e a n s for identifying b e n e f i c i a l s i n t e r i n g a c t i v a t o r s . The e l e c t r o n c o n c e n t r a t i o n concept p r e d i c t s the t r a n s i tion m e t a l s with n e a r l y c o m p l e t e d e l e c t r o n s u b s h e l l s (i.e., Ni, P d and Pt) to be the b e t t e r s i n t e r i n g a c t i v a t o r s . ~3'~4'2° However, s t u d i e s of h a f n i u m a c t i v a t e d s i n t e r i n g show a n o v e r a l l a n o m a l o u s t e m p e r a t u r e s e n s i tivity. In the p r e s e n t study, a n a t t e m p t has b e e n made to document a s i m i l a r t e m p e r a t u r e sensitivity in tungs t e n u s i n g t r a n s i t i o n m e t a l a d d i t i v e s . The a p p r o a c h was to c o m b i n e s h r i n k a g e d e t e r m i n a t i o n s f r o m i s o t h e r m a l s i n t e r i n g e x p e r i m e n t s with s h r i n k a g e r a t e m e a s u r e m e n t s conducted in c o n s t a n t h e a t i n g r a t e exp e r i m e n t s . T h i s c o m b i n a t i o n p r o