Coating tungsten composites for improved low temperature ductility
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G. Z U K A S
At t e m p e r a t u r e s below about 375 K, l i q u i d - p h a s e s i n t e r e d t u n g s t e n b a s e c o m p o s i t e s fail by c l e a v a g e of the tungsten s p h e r o i d s . A l l o y i n g a d d i t i o n s o r h e a t t r e a t m e n t s have not i m p r o v e d the d u c t i l i t y s i g n i f i c a n t l y . The d u c t i l i z i n g p r o c e s s d e s c r i b e d h e r e c o n s i s t s in c o a t i n g the c o m p o s i t e with a d u c t i l e m e t a l followed by h e a t t r e a t m e n t to i n s u r e s u f f i c i e n t diffusion for the c o a t and b a s e to b e c o m e i n t e g r a l . The d u c t i l e c o a t is now the " p i e c e " s u r f a c e , and the i n i t i a t i o n of c l e a v a g e c r a c k s r e q u i r e s much g r e a t e r s t r e s s e s . C o a t s of c o p p e r , nickel, gold and c o b a l t have been u s e d s u c c e s s f u l l y .
F o R c e r t a i n s p e c i a l i z e d a p p l i c a t i o n s , such a s p e n e t r a t i o n p r o j e c t i l e s for u s e a g a i n s t m i l i t a r y a r m o r and t a n k s , m a t e r i a l s of high d e n s i t y having good s t r e n g t h and s o m e d u c t i l i t y a r e r e q u i r e d . Cost and a v a i l a b i l i t y of s u i t a b l e m a t e r i a l s , such a s p l a t i n u m , gold, r h e n i u m and t u n g s t e n - r h e n i u m a l l o y s , p r o h i b i t t h e i r u s e e x c e p t for a few v e r y s p e c i a l i z e d a p p l i c a t i o n s . However, s e v eral manufacturers produce tungsten base composites s p e c i f i c a l l y for this p u r p o s e . Such c o m p o s i t e s fail in a b r i t t l e m a n n e r at a m b i e n t t e m p e r a t u r e s and below t h e r e b y r e d u c i n g t h e i r r e l i a b i l i t y for f i e l d use. The a d d i t i o n of one o r m o r e a l l o y i n g e l e m e n t s , and v a r i ous h e a t - t r e a t m e n t s have b e e n t r i e d in an a t t e m p t to i m p r o v e the low t e m p e r a t u r e d u c t i l i t y of such c o m p o s i t e s , but with l i m i t e d s u c c e s s . This r e p o r t d e s c r i b e s a c o a t i n g and h e a t - t r e a t i n g p r o c e s s which p r o m o t e s low t e m p e r a t u r e d u c t i l i t y in such c o m p o s i t e s . This should not only i m p r o v e the r e l i a b i l i t y of t u n g s t e n b a s e c o m p o s i t e c o m p o n e n t s now in use, but should i n c r e a s e the n u m b e r of a p p l i c a t i o n s .
BACKGROUND Tungsten-base composites contain from 80 pct to 97 pct tungsten with the remainder being generally nickel and copper or iron. Numerous alloying additions or substitutions have been tried but these generally do not change the alloy characteristics. The elemental powders are mixed and liquid-phase sintered at temperatures from 1675 to 1775 K, thereby enabling the production of large parts (50 kg or so) of essentially theoretical density. This is in contrast to pure tungsten (M.P. = 3683 K) where present day equipment is incapable of produc
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