The high temperature thermodynamic properties of Ni-Ti alloys
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INr e c e n t y e a r s ,
i n t e r e s t in the N i - T i b i n a r y s y s t e m has b e e n g e n e r a t e d by a n u m b e r of f a c t o r s . E x t e n s i v e work h a s b e e n r e p o r t e d on the m e m o r y effects and c o r r o s i o n r e s i s t a n c e of the e q u i a t o m i c c o m p o s i t i o n . I n t e r e s t in N i - T i a l l o y s at high t e m p e r a t u r e s r e s u l t s f r o m the expanding u s e of n i c k e l - b a s e d s u p e r a l l o y s . P r e v i o u s t h e r m o d y n a m i c data for the N i - T i s y s t e m is l i m i t e d . K u b a s c h e w s k i I d e t e r m i n e d the heat of f o r m a tion for v a r i o u s N i - T i a l l o y s at 298 K by a c a l o r i m e t r i c t e c h n i q u e . C h e r k a s o v , A v e r i n , and S a m a r i n z were able to obtain a value of ~ i in N i - T i f r o m e q u i l i b r i u m oxygen c o n c e n t r a t i o n s in v a r i o u s N i - T i - C r m e l t s . T h e i r r e s u l t s i n d i c a t e a l a r g e n e g a t i v e deviation in t i t a n i u m a c t i v i t y at 1600 ~ C. The N i - T i phase d i a g r a m by H a n s e n and Anderko, 3 E l l i o t t , 4 and Shunk 5 i s shown in Fig. 1, f r o m which it can be s e e n that the i n t e r m e t a l l i c compounds NiTi and NisTi m e l t c o n g r u e n t l y , w h e r e a s NiTi2 d e c o m p o s e s by a p e r i t e c t i c r e a c t i o n . The r e l a t i v e s t a b i l i t i e s of the compounds a r e i n d i c a t e d by t h e i r high m e l t i n g p o i n t s . The p r o p e r t i e s of the liquid phase u s u a l l y c o r r e l a t e with such stable i n t e r m e t a l l i c compounds.
99.99 pct p u r i t y w e r e used a s charge m a t e r i a l s , and the total alloy charge weights w e r e a p p r o x i m a t e l y one g r a m . The t i t a n i u m m a s s peak at 48 a . m . u , and the n i c k e l m a s s peak at 58 a . m . u , w e r e used for all i n t e n sity m e a s u r e m e n t s . R e s o l u t i o n of t h e s e two peaks was b e s t when an i o n i z a t i o n potential of 15 V and a t r a p c u r r e n t of 0.50 /~amp w e r e used. I n t e n s i t y m e a s u r e m e n t s w e r e made in a n u p - a n d - d o w n m a n n e r between 1475 ~ and 1725~ following a slow h e a t i n g to 1600~ The p r e s e n t a u t h o r s b e l i e v e that t h e r m a l cycling a s opposed to d e c r e a s i n g t e m p e r a t u r e m e a s u r e m e n t s r e sult in l e s s b i a s e d i n t e n s i t y data. Empty cell b a k e - o u t s w e r e p e r f o r m e d b e f o r e e v e r y t h i r d r u n and c h e m i c a l c l e a n i n g of the cell and f u r n a c e a s s e m b l y was p e r f o r m e d b e f o r e e v e r y e x p e r i m e n t . The cell a s s e m b l y was r e p e a t e d l y m i s a l i g n e d d u r i n g an e x p e r i m e n t to d e t e r m i n e b a c k g r o u n d i n t e n s i t i e s . C o m p a r i s o n of this v a l u e with the s h u t t e r e d i n t e n s i t y r e v e a l e d no change in e i t h e r b a c k g r o u n d
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