Production of chromium base alloys by ball milling in hydrogen iodide
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r e c e n t y e a r s NASA and other o r g a n i z a t i o n s have shown c o n s i d e r a b l e i n t e r e s t in c h r o m i u m and c h r o m i u m base alloys. This i n t e r e s t s t e m m e d from some of c h r o m i u m ' s d e s i r a b l e p r o p e r t i e s : a r e l a t i v e l y high m e l t i n g point (1893~ a high e l a s t i c modulus (282.7 • 103 MN/m 2) and low d e n s i t y (7.2 g/cc). In addition, when c o m p a r e d with other r e f r a c t o r y m e t a l s (W, Mo, Ta, Cb, Re) c h r o m i u m has good oxidation r e s i s t a n c e and equivalent homologous t e m p e r a t u r e t e n s i l e s t r e n g t h (Ref. 15. Unfortunately, c h r o m i u m has a r e l a t i v e l y high d u c t i l e - t o - 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 (DBTT). This DBTT is v e r y s e n s i t i v e to the i m p u r i t y content (nitrogen and carbon, p a r t i c u l a r l y ) of c h r o m i u m and the high r e a c t i v i t y of this m e t a l a g g r a v a t e s the situation. F o r t h e s e r e a s o n s , c h r o m i u m m u s t be p r o c e s s e d with e x t r e m e c a r e in o r d e r to m i n i m i z e its c o n t a m i n a t i o n . Because of these difficulties, the developm.ent of c h r o m i u m b a s e alloys of e n g i n e e r i n g significance has been h i n d e r e d . Ar~maments for and a g a i n s t the use of c h r o m i u m and c h r o m i u m base alloys as e n g i n e e r i n g m a t e r i a l s can be found in s e v e r a l review r e p o r t s (Refs. 2 to 45. These r e v i e w r e p o r t s show that a g r e a t deal of work has been done to i m p r o v e the high t e m p e r a t u r e s t r e n g t h and the DBTT of c h r o m i u m by conventional alloying. However, r e l a t i v e l y little has been done to i m p r o v e t h e s e p r o p e r t i e s of c h r o m i u m by d i s p e r s i o n s t r e n g t h ening. Yet, when c o n s i d e r i n g its d r a m a t i c effects on nickel and n i c k e l - c h r o m i u m alloys, it was felt that d i s p e r s i o n s t r e n g t h e n i n g m a y well offer a good approach for i m p r o v i n g the high t e m p e r a t u r e p r o p e r t i e s of c h r o m i u m . In addition, there were indications that d i s p e r s i o n s t r e n g t h e n i n g m i g h t also i m p r o v e the DBTT of c h r o m i u m (Refs. 5 and 6). D i s p e r s i o n s t r e n g t h e n i n g of m e t a l s can be a c c o m plished by m a n y methods (Ref. 75. Among these m e t h ods, b a l l - m i l l i n g has shown p r o m i s e of providing the r e q u i r e d i n t e r p a r t i c l e s p a c i n g s n e c e s s a r y for effective d i s p e r s i o n s t r e n g t h e n i n g (Ref. 85. In addition, b a l l - m i l l i n g offers the advantages of v e r s a t i l i t y , low cost, and s c a l e - u p potential. In a p r e l i m i n a r y i n v e s t i g a t i o n (Ref. 8) c h r o m i u m t h o r i a m i x t u r e s were m i l l e d to s u b m i c r o n size powd e r s with a m i n i m u m of c o n t a m i n a t i o n by using h
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