Martensitic transformation in binary and ternary alloys based on the AuZn beta prime phase
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T H E / 3 ' phase of the AuZn s y s t e m is located about the e q u i a t o m i c c o m p o s i t i o n and is g e n e r a l l y c o n s i d e r e d to be a "3/2" e l e c t r o n compound. It has the CsCI s t r u c t u r e and r e t a i n s a c o n s i d e r a b l e degree of long r a n g e o r d e r up to the m e l t i n g point. 1 At high t e m p e r a t u r e the m a x i m u m composition r a n g e is f r o m 36.5 to 57 at. pct Zn, while at r o o m t e m p e r a t u r e the phase extends a p p r o x i m a t e l y 2 at. pet on e i t h e r side of the equiatomic c o m p o s i t i o n , see Fig. 1. E a r l i e r work 2 has shown that at low t e m p e r a t u r e s , /3' AuZn a l l o y s containing g r e a t e r than 48 at. pet Zn a r e Unstable and u n d e r g o a m a r t e n s i t i c t r a n s f o r m a t i o n , the m a r t e n s i t e s t a r t t e m p e r a t u r e , Ms, i n c r e a s i n g with i n c r e a s i n g zinc c o n c e n t r a t i o n . In r e c e n t s t u d i e s of b i n a r y and t e r n a r y / 3 ' - p h a s e a l loys, m a r t e n s i t e t r a n s f o r m a t i o n t e m p e r a t u r e s have been d i s c u s s e d in t e r m s of f a c t o r s such as o r d e r i n g , 3'4 e l e c t r o n c o n c e n t r a t i o n , s atomic s i z e , 5 ionic s i z e , s e l e c t r o n e g a t i v i t y , 4 and q u e n c h e d - i n v a c a n c i e s . 6'~ In o r d e r to e x a m i n e the s i g n i f i c a n c e of s o m e of these f a c t o r s in /3' AuZn alloys, the effects on the m a r t e n s i t e t r a n s f o r m a t i o n of s m a l l additions of s e v e r a l t h i r d e l e m e n t s have b e e n studied. As a p r e l i m i n a r y to the work on t e r n a r y alloys and in o r d e r to r e e x a m i n e some m i n o r a n o m a l i e s a p p a r e n t in the e a r l i e r study, 2 the m a r t e n s i t i e t r a n s f o r m a t i o n in the b i n a r y s y s t e m has also been r e i n v e s t i g a t e d .
p a r t i a l p r e s s u r e of h e l i u m . After being m i x e d by vigo r o u s shaking, the a l l o y s , each of which weighed 4 to 5 g, were w a t e r quenched and then r e s e a l e d in Vycor t u b e s u n d e r a p a r t i a l p r e s s u r e of helium and homog e n i z e d at 640~ for 4 days. In the case of AuZnMg a l loys, a s i m i l a r p r o c e d u r e was followed except that the s a m p l e s were held in s p e c t r o g r a p h i c a l l y p u r e g r a p h i t e pots contained within s e a l e d Vycor tubes. The weight l o s s e s involved in the p r e p a r a t i o n of the m a t e r i a l s w e r e very s m a l l , and hence, it h a s been a s s u m e d that they all have the c o m p o s i t i o n s intended.
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Most of the work has been done on t e r n a r y alloys with a c o n s t a n t e l e c t r o n to atom r a t i o of 1.50, with some a d d i t i o n a l m e a s u r e m e n t s on a l l o y s in which the zinc c o n c e n t r a t i o n was m a i n t a i n e d c o n s t a n t at 50 at. pct. The solute additions made were Cu
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