Intergranular brittleness studies in Tungsten Using Auger Spectroscopy
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The p r e s e n t i n v e s t i g a t i o n was a i m e d at u n d e r s t a n d i n g the p r o b l e m of b r i t t l e n e s s in powder m e t a l l u r g y t u n g s t e n by u t i l i z i n g Auger E l e c t r o n E m i s s i o n Spectroscopy a s a tool for d i r e c t c h e m i cal a n a l y s i s of f r a c t u r e s u r f a c e s . S u b s t a n t i a l s e g r e g a t i o n of p h o s p h o r u s to the g r a i n b o u n d a r i e s was o b s e r v e d in t u n g s t e n and the c o n c e n t r a t i o n of phosphorus can be r e l a t e d to the d u c t i l e - b r i t tle t r a n s i t i o n t e m p e r a t u r e s . The amount of s e g r e g a t i o n was dependent upon the g r a i n s i z e - - t h e l a r g e r the g r a i n s i z e , the g r e a t e r the c o n c e n t r a t i o n of phosphorus at the g r a i n b o u n d a r y and the associated embrittlement.
A m a j o r l i m i t a t i o n in the u s e f u l n e s s of t u n g s t e n i s i t s t e n d e n c y to be b r i t t l e . This difficulty i s e s p e c i a l l y acute when it i s in the p o l y c r y s t a l l i n e form. I n s p e c t i o n of the f r a c t u r e s u r f a c e s r e v e a l s that p o l y c r y s t a l l i n e t u n g s t e n often f r a c t u r e s along the g r a i n b o u n d a r i e s . It has been postulated I that a m a j o r cause of g r a i n b o u n d a r y f r a c t u r e is due to the s e g r e g a t i o n of oxygen to the g r a i n b o u n d a r i e s r e s u l t i n g in e m b r i t t l e m e n t . However, t h e r e is no d i r e c t evidence that oxygen i s r e s p o n s i b l e for the b r i t t l e n e s s and t h e r e f o r e in an e f fort to e x a m i n e this h y p o t h e s i s d i f f e r e n t g r a d e s of t u n g sten w e r e p r e p a r e d by L a m p M e t a l s and C o m p o n e n t s d i v i s i o n of the G e n e r a l E l e c t r i c Co., with v a r i o u s a m o u n t s of c a r b o n additions i n the powder stage. The added c a r b o n was expected to i n t e r a c t with oxygen p r e s e n t in the m a t e r i a l and thus c o n t r o l the b r i t t l e n e s s . The m e a s u r e d d u c t i l e - b r i t t l e t r a n s i t i o n t e m p e r a t u r e s (DBTT), however, b o r e no r e l a t i o n to the additions of c a r b o n , thus i n d i c a t i n g that e i t h e r c a r b o n did not r e a c t as expected or oxygen i s not the c a u s e of the o b s e r v e d b r i t t l e n e s s in t u n g s t e n . The f r e q u e n t o c c u r r e n c e of i n t e r c r y s t a l l i n e f r a c t u r e s in t h e s e m a t e r i a l s suggested the usage of Auger E l e c t r o n E m i s s i o n A n a l y s i s for d i r e c t d e t e r m i n a t i o n of s e g r e g a n t s at the g r a i n b o u n d a r i e s . This technique has proved its e f f e c t i v e n e s s in d i r e c t l y identifying c e r t a i n s e g r e g a n t s in the g r a i n b o u n d a r i e s of t e m p e r e m b r i t tled low alloy s t e e l s 2's and, t h e r e f o r e , it might be u s e ful in study of e m
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