Effect of gas bubbles on recrystallization of tungsten
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HIGHp u r i t y t u n g s t e n r e c r y s t a l l i z e s
at 1200 ~ to 1300~ ~-4 C o m m e r c i a l powder m e t a l l u r g y t u n g s t e n r e q u i r e s somewhat higher t e m p e r a t u r e s , 5 but the r e c r y s t a l l i z a t i o n t e m p e r a t u r e s for h e a v i l y d e f o r m e d , doped, powder m e t a l l u r g y t u n g s t e n m a y exceed 2000~ s-8 M o r e o v e r , the r e c r y s t a l l i z e d g r a i n s in such doped, s i n t e r e d t u n g s t e n a r e n a r r o w and e l o n gated, with t h e i r long axes in the working d i r e c t i o n . This i s a d e s i r a b l e s t r u c t u r e for i n c a n d e s c e n t l a m p f i l a m e n t s b e c a u s e it p r o v i d e s a good r e s i s t a n c e to s a g g i n g at e l e v a t e d t e m p e r a t u r e s , allowing extended operating lifetimes. The dopants used in powder m e t a l l u r g y t u n g s t e n a r e u s u a l l y s m a l l a m o u n t s of p o t a s s i u m , a l u m i n u m , and s i l i c o n added in the f o r m of oxides. The m e c h a n i s m b y which t h e s e dopants e x e r t t h e i r influence on r e c r y s t a l l i z a t i o n and g r a i n growth has b e e n the s u b j e c t of much s p e c u l a t i o n . M e i j e r i n g and Rieck 9 have p o s t u l a t e d that the dopants f o r m s k i n s or " t u b e s " of s e c o n d p h a s e a r o u n d the e l o n g a t e d f i b e r s in the a s d r a w n t u n g s t e n ; r e c r y s t a l l i z a t i o n and g r a i n growth a r e p e r m i t t e d only through b r e a k s in the s k i n s . M a n n e r k o s k i ~~has p r o p o s e d t h a t the dopants a r e d i s solved and s e g r e g a t e d in s o l u t i o n at the f i b e r and r e c r y s t a l l i z e d g r a i n b o u n d a r i e s , thus r e t a r d i n g t h e i r m o v e m e n t . Swalin and G e i s l e r s have s u g g e s t e d that v o l a t i l e dopants s t a b i l i z e s i n t e r i n g p o r o s i t y in powder m e t a l l u r g y t u n g s t e n , and that the r e t a i n e d p o r o s i t y i n h i b i t s r e c r y s t a l l i z a t i o n . W a l t e r a has found s t r i n g s of fine p a r t i c l e s in r o l l e d doped t u n g s t e n sheet, s o m e of which he identified as m u l l i t e p a r t i c l e s . He a s s u m e d that t h e s e c o n t r o l l e d the r e c r y s t a l l i z a t i o n p r o c e s s . Koo 1~ h a s q u e s t i o n e d W a l t e r ' s c o n c l u s i o n and contends that m a n y of the fine p a r t i c l e s a r e r e a l l y gas b u b b l e s f o r m e d f r o m e v a p o r a t e d dopants. Koo "~ and Das and Radcliffe ~2 have o b s e r v e d such b u b b l e s in doped tungsten. The q u e s t i o n of whether gas b u b b l e s a r e the c a u s e of the p e c u l i a r r e c r y s t a l l i z a t i o n b e h a v i o r of doped powder m e t a l l u r g y t u n g s t e n is o b v i o u s l y i m p o r t a n t and is now b e i n g i n v e s t i g a t e d b y Moon and Koo. ~ K. FARRELL, A. C. SCHAFFHAUSER,and J. T. HOUSTON are Research Staff
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