Composition dependence of density in NiTi and CoTi
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Composition Dependence
Table ;. Calculated and Mea~dred Densities in NiTi Phase
Composition At. Pct Ni 50 51 52 53 54 55
Calculated Densities,g per cu cm Substitutional Vacancy 6.516 6.554 6.593 6.632 6.672 6.711
6.516 6.475 6.436 6.402 6.369 6.340
Measured DensitiesCorrected* g per cu cm 6.443 • 0.001 6.490 6.540 6.589 6.649 6.678
*Measured at 200~ then corrected to the values at 20~ by using the linear coefficient of thermal expansion as 10.4 X l0 "6 per ~
of Density in NiTi and CoTi Table II. Calculated and Measured Densities in CoTi Phase
T . S U Z U K I A N D K. M A S U M O T O T H E t y p e of d e f e c t s t r u c t u r e s p r e s e n t i n t h e o r d e r e d N i T i a n d C o T i p h a s e s a r e of i n t e r e s t f o r t h e f o l l o w i n g reasons: 1)the martensitic transformation temperature i n N i T i i s c o n s i d e r e d to b e a f f e c t e d b y t h e s t r u c t u r e d e f e c t , 1 2) a q u e n c h - h a r d e n i n g m e c h a n i s m i n n i c k e l r i c h N i T i i s c l o s e l y r e l a t e d to t h e e x i s t e n c e of p o i n t d e f e c t s , 2 a n d 3) t h e c o m p o s i t i o n d e p e n d e n c e of s o m e mechanical properties in cobalt-rich CoTi is similar t o t h a t of w a t e r - q u e n c h e d n i c k e l - r i c h N i T i . S o m e o b s e r v a t i o n s on t h e d e n s i t y a n d t h e l a t t i c e p a r a m e t e r in equiatomic iron group titanides which may give inform a t i o n on t h e d e f e c t s t r u c t u r e h a v e b e e n r e p o r t e d . / ' 3 - 5 However, it is difficult to estimate the defect structure from these data because the martensitic transformation which takes place near room temperature was not taken into account and no significant relation between lattice parameter and composition was observed for NiTi. In CoTi phase, only the lattice parameter data m e a s u r e d b y Sfiiwe a n d S h i m o m u r a 4 a r e a v a i l a b l e . I n t h e p r e s e n t p a p e r , w e d e s c r i b e t h e v a r i a t i o n of density with composition over the nonstoichiometric compositions for the phases NiTi and CoTi and the t y p e of p o i n t d e f e c t w h i c h i s d e r i v e d f r o m t h e r e s u l t s on the density measurements. Materials were prepared by nonconsumable arc melting. Sponge titanium, electrolytic nickel, and cob a l t of 9 9 . 9 9 p c t p u r i t y w e r e u s e d a s s t a r t i n g m a t e r ials~ Ten ingots having compositions ranging from 48 to 57 a t . p c t N i f o r N i T i a n d n i n e i n g o t s f r o m 48 to 56 a t . p c t C o f o r C o T i i n s t e p s of 1 p c t w e r e m e l t e d i n t h i s w a y . A l l i n g o t s w e r e h o m o g e n i z e d a t 1000 ~ C f o r 100 h r a t a p r e s s u r e of 10 -s m m m e r c u r y a n d t h e n w a t e r q u e n c h e d . T h e s p e c i m e n s of a p p r o x i m a t e l y 5 m m b y 5 m m b y 10 m m w e r e f i n i s h e d b y s u r f a c e g r i n d i n g after spark machining. Prior to the density measurem e n t s , t h e h o m o g e n e i t y r a n g e
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