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h r were followed by a n n e a l s of up to 48 hr at 750~ and above, 100 hr at 650~ and 225 hr at 550~ M e t a l l o g r a p h i c and X - r a y t e c h n i q u e s were used to obtain a s e r i e s of i s o t h e r m a l s e c t i o n s down to 350~ The r e s u l t s of his study c o n f i r m e d the m a i n c o n c l u s i o n s of Vogel and gave the c o m p o s i t i o n s of the t e r n a r y phase field ~ + y + Ph b e t w e e n 1100 ~ and 350~ Our i n i t i a l c a l c u l a t i o n s of e q u i l i b r a t i o n t i m e s in the F e - N i - P s y s t e m , u s i n g m e a s u r e d F e - N i diffusion coe f f i c i e n t s , 7 led u s to b e l i e v e that p r e v i o u s i n v e s t i g a t o r s had not given t h e i r a l l o y s s u f f i c i e n t l y long heat t r e a t m e n t s . P r e l i m i n a r y work, which used an e l e c t r o n m i c r o p r o b e to m e a s u r e both h o m o g e n e i t y and phase c o m p o s i t i o n s , showed that these c a l c u l a t i o n s were c o r r e c t and that l o n g e r heat t r e a t m e n t t i m e s were n e c e s s a r y to approach e q u i l i b r i u m in the t e r n a r y s y s t e m . In o r d e r to obtain the phase b o u n d a r i e s of the F e - N i - P s y s t e m , p a r t i c u l a r l y in the solid state r a n g e below 750~ we have r e d e t e r m i n e d the t e r n a r y phase d i a g r a m from 0 to 100 pct Ni and f r o m 0 to 16.5 wt pct P. We p r e p a r e d over 130 a l l o y s of s e lected c o m p o s i t i o n s . After long heat t r e a t m e n t s , at t e m p e r a t u r e s between 1100~ a n d 550~ the c o e x i s t i n g phases were a n a l y z e d with the e l e c t r o n m i c r o p r o b e . D i r e c t m e a s u r e m e n t s of homogeneity and phase c o m p o s i t i o n s were obtained. The a p p l i c a t i o n of the t e r n a r y data to the d e v e l o p m e n t of the Widmanstf/tten p a t t e r n in i r o n m e t e o r i t e s is d e s c r i b e d in a n o t h e r paper. 8 E X P E R I M E N T A L PROCEDURE Alloy P r e p a r a t i o n The t e r n a r y alloys used in this study were p r e p a r e d by induction m e l t i n g of weighed q u a n t i t i e s of the pure e l e m e n t s . High p u r i t y (99.95 wt pct) F e , nickel with a VOLUME 1, JUNE 1970-1759

c a r b o n content of l e s s than 20 ppm, and r e d p h o s p h o r u s of 99.999 pct p u r i t y were used. The t e r n a r y a l l o y s were induction m e l t e d in an a l u m i n a c r u c i b l e u n d e r a protective a t m o s p h e r e of a r g o n a n d H2. S m a l l i n t e r n a l voids were noted in m o s t s p e c i m e n s upon m e t a l l o g r a p h i c i n v e s t i g a t i o n . Altogether s o m e 130 F e - N i - P a l l o y s were produced. To p r e v e n t p h o s p h o r u s v a p o r i z a t i o n each alloy was placed in an a l u m i n a c r u c i b l e , and s e a l e d u n d e r v a c u u m (-10 -3 t o r r ) in a q u a r t z tube. The alloys w e r e then heat t r e a t e d at t e m p e r a t u r e s between 1000 ~ and l l 0 0 ~ fo