The chromium-platinum constitution diagram
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a l l o y s have b e e n s t u d i e d in a n u m b e r of p r e v i o u s i n v e s t i g a t i o n s 1-8 b u t the r e s u l t s of t h e s e s t u d i e s a r e i n c o m p l e t e and s o m e t i m e s c o n t r a d i c t o r y . One can only conclude that the d a t a a r e s t i l l i n s u f f i c i e n t to e s t a b l i s h the e x a c t c o n s t i t u t i o n of t h i s s y s t e m . 7 T h e r e a r e p r o b a b l y a n u m b e r of r e a s o n s for the unc e r t a i n t i e s which e x i s t in the p r e v i o u s d a t a but, in g e n e r a l , t h e s e u n c e r t a i n t i e s can be a t t r i b u t e d to s e v e r e e x p e r i m e n t a l p r o b l e m s in a l l o y p r e p a r a t i o n and h e a t t r e a t m e n t . Alloy c o m p o s i t i o n s a r e often u n c e r t a i n due to u n p r e d i c t a b l e l o s s e s r e s u l t i n g f r o m the high v a p o r p r e s s u r e of c h r o m i u m at o r n e a r i t s m e l t i n g t e m p e r a t u r e . C o n t a m i n a t i o n of the a l l o y s s o m e t i m e s o c c u r s b y c h e m i c a l r e a c t i o n s with r e f r a c t o r y c e r a m i c s such a s alumina crucibles or thermocouple tubes. Contamination of the a l l o y s m a y a l s o o c c u r f r o m t r a c e s of o x y gen o r n i t r o g e n in a n o m i n a l l y " p u r e " i n e r t a t m o sphere during melting or annealing. If t h e r m o c o u p l e s a r e u s e d for t e m p e r a t u r e m e a s u r e m e n t t h e y a r e s u b j e c t to c o n t a m i n a t i o n s i n c e c h r o m i u m v a p o r can p e n e t r a t e r e f r a c t o r y tubing to r e a c t with the hot t h e r m o c o u p l e and p r o d u c e s e r i o u s e r r o r s in the o b s e r v e d t e m p e r a t u r e s . The u s e of o p t i c a l p y r o m e t r y m a y a v o i d s o m e of the p r o b l e m s of c o n t a m i n a t i o n but it d o e s not e l i m i n a t e the e r r o r s , s i n c e e v a p o r a t i o n of c h r o m i u m a t high t e m p e r a t u r e s r e s u l t s in the r a p i d f o r m a t i o n of m e t a l l i c c h r o m i u m f i l m s on the sight windows. The f i l m s m a y be r e m o v e d b y v a r i o u s c l e a n ing d e v i c e s but will r e a c c u m u l a t e to a s i g n i f i c a n t extent even d u r i n g the r e l a t i v e l y b r i e f t i m e n e c e s s a r y for a temperature measurement. Two-color pyrometers may be u s e f u l in the p r e s e n c e of such f i l m s but only if n e i t h e r of the two o p t i c a l w a v e l e n g t h s e m p l o y e d is s e l e c t i v e l y a b s o r b e d by t h e f i l m . T h i s l a t t e r condition is u n l i k e l y to a p p l y and, a t any r a t e , cannot u s u a l l y be v e r i f i e d . Thus, it i s v e r y d i f f i c u l t to a v o i d e r r o r s when w o r k i n g with c h r o m i u m a l l o y s . In view of t h e s e d i f f i c u l t i e s , it is p r o b a b l y not s u r p r i s i n g that the m e l t i n g p o i n t d a t a o b t a i n e d b y MfJller 1 a r e i n c o
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