An Fe-Cr-Nb pseudobinary eutectic alloy
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ing point was d e t e r m i n e d f r o m cooling c u r v e s c a r r i e d out with a P t / P t - R d t h e r m o c o u p l e s i t u a t e d in a slowly cooled s a m p l e of the 99.9+ pct e u t e c t i c alloy. The 99.6 pct a s - c a s t alloy b a r s were p l a c e d in a mold, which was m a d e f r o m a l u m i n a c e m e n t , i n s i d e a g r a p h i t e s u s c e p t o r . The d r i e d m o l d e n s e m b l e was p l a c e d in a s i l i c a tube which was flushed with 99.95 pct a r g o n . The alloy was d i r e c t i o n a l l y solidified by p a s s i n g an induction coil o v e r the s i l i c a tube. The s o l i d i f i c a t i o n r a t e of the alloy was a s s u m e d to be equal to the velocity of the induction coil. V a r i o u s r a t e s between 1.15 • 10 -3 cm p e r sec (4.14 cm p e r hr) and 5.78 • 10 -3 cm p e r sec (20.82 cm p e r hr) were i m p o s e d on the b a r s . The h i g h e r p u r i t y alloy b a r s w e r e m a c h i n e d to fit into 1.11 cm b o r e sheaths of h i g h - p u r i t y r e c r y s t a l l i z e d a l u m i n a . T h e s e w e r e p l a c e d i n s i d e loosely fitting g r a p h i t e sheaths and packed with a l u m i n a powder and c e m e n t . The alloy was d i r e c t i o n a l l y solidified by l o w e r i n g the c o m p o s i t e m o l d t h r o u g h a t h r e e - t u r n i n duction coil u n d e r a n a r g o n a t m o s p h e r e . The r a t e s of s o l i d i f i c a t i o n were between 0.27 • 10 -3 cm p e r sec (1.0 cm p e r hr) and 5.55 • 10 -3 cm p e r sec (20 cm p e r hr). S t a n d a r d m e t a l l o g r a p h i c t e c h n i q u e s were u s e d to e x a m i n e the morphology of each b a r . The a l l o y s were s u i t a b l y e l e c t r o e t c h e d in a s a t u r a t e d oxalic acid s o l u tion for 20 sec at 6 v. The v o l u m e f r a c t i o n of the m i n o r p h a s e was e s t i m a t e d in a " Q u a n t i m e t " q u a n t i t a t i v e t e l e v i s i o n m i c r o s c o p e on etched s a m p l e s and from m e a s u r e m e n t s on s c a n n i n g e l e c t r o n m i c r o g r a p h s of deeply etched s p e c i m e n s . S p e c i m e n s were p r e p a r e d for e x a m i n a t i o n in a " S t e r e o s c a n " S c a n n i n g E l e c t r o n M i c r o s c o p e (SEM) by p r e f e r e n t i a l l y r e m o v i n g the m a t r i x p h a s e by a 5 m i n e l e c t r o e t c h in s a t u r a t e d oxalic acid solution at 20~ 6 v p o t e n t i a l , and a c u r r e n t d e n sity of 0.85 to 0.90 amp p e r sq cm. T e n s i l e s p e c i m e n s were p r e p a r e d f r o m the d i r e c tionally solidified b a r s by c o n v e n t i o n a l m a c h i n i n g and g r i n d i n g m e t h o d s . They had a gage length of 1.4 cm and a d i a m e t e r of 0.5 cm. The d i m e n s i o n s of each s p e c i m e n were m e a s u r e d to ~:0.005 cm b e f o r e and after t e s t i n g . The t e n s i l e s p e c i m e n s were s t r a i n e d at v a r i o u s t e m p e r a t u r e s between 20 ~ and 1000~ in a T F KM m o d e l I n s t r o n T
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