Molybdenum in Nb-C alloys
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The effects of m o l y b d e n u m alloying additions to n i o b i u m on the c a r b i d e p h a s e s and t h e i r p r e c i p i t a t i o n b e h a v i o r w e r e i n v e s t i g a t e d . The e x p e r i m e n t a l a l l o y s included Nb-0.1C, Nb-15Mo-0.1C, and Nb-30Mo-0.1C. After s e l e c t e d heat t r e a t m e n t s the m i c r o s t r u c t u r a l changes were d e t e r m i n e d by m e t a l l o g r a p h y and the c a r b i d e p h a s e s w e r e e x t r a c t e d and identified by X - r a y diffraction and c h e m i c a l a n a l y s i s . The r e s u l t s a r e e s s e n t i a l l y in a g r e e m e n t with r e c e n t phase d i a g r a m d e t e r m i n a t i o n s . Additions of 30 wt pct Mo a p p e a r s to slightly i n c r e a s e the s o l u b i l i t y of c a r b o n in n i o b i u m at t e m p e r a t u r e s a r o u n d 1650~ The s o l u b i l i t y of m o l y b d e n u m in Nb2C is v e r y s m a l l . D i s c o n t i n u o u s p r e c i p i t a t i o n of/3-Nb~C was found to o c c u r in the Nb-30Mo-0.1C a l l o y d u r i n g a n n e a l i n g at 1200~ The i m p o r t a n t , o v e r a l l effect of m o l y b d e n u m in Nb-C a l l o y s is to d e c r e a s e the r a t e of n i o b i u m c a r b i d e p r e c i p i t a t i o n so that a p p r e c i a b l e c a r b o n s u p e r s a t u r a t i o n can be a c h i e v e d even after c o m p a r a t i v e l y slow f u r n a c e cooling. D I S P E R S E D c a r b i d e p h a s e s a r e known 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 p r o p e r t i e s of n i o b i u m b a s e a l l o y s . ~-9 The d e g r e e of s t r e n g t h i m p r o v e m e n t depends l a r g e l y on the d i s t r i b u t i o n , morphology, and s t a b i l i t y of the d i s p e r s e d c a r b i d e p h a s e s . Among the c a r b i d e p h a s e s c o m m o n l y o b s e r v e d in n i o b i u m a l l o y s , the m o n o c a r b i d e s of z i r c o n i u m and hafnium a r e m o s t a t t r a c t i v e while the n i o b i u m - r i c h b a s e - m e t a l c a r b i d e s a r e ineffective, s i n c e the l a t t e r u s u a l l y e i t h e r o c c u r as c o a r s e p a r t i c l e s or coagulate r a p i d l y . It has r e c e n t l y b e c o m e a p p a r e n t , however, that the d e g r e e of d i s p e r s i o n of the z i r c o n i u m or hafnium c a r b i d e p h a s e s is d e t e r m i n e d to a l a r g e extent by the type and m o r phology of n i o b i u m - r i c h b a s e - m e t a l c a r b i d e s ~~which a r e u s u a l l y p r e s e n t p r i o r to the f o r m a t i o n of the d i s p e r s e d , s t a b l e c a r b i d e p h a s e d u r i n g the r e q u i r e d heat t r e a t m e n t cycle. For the d e v e l o p m e n t of i m p r o v e d c a r b i d e - d i s p e r s i o n s t r e n g t h e n e d n i o b i u m alloys it a p p e a r s , t h e r e f o r e , vital to c o n s i d e r the influence of a l l o y i n g additions on the p r o p e r t i e s of not only the dispersed zirconium- or hafnium-rich monocarbides but
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