Stability of the directionally solidified eutectics NiAl-Cr and NiAl-Mo
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b e c a u s e the i n t e r f a c e is c o n s t r a i n e d to lie in a specific crystallographic plane. T h i s r e p o r t d e t a i l s the c o a r s e n i n g of both c y l i n d r i c a l and faceted f i b e r s in two e u t e c t i c alloys with the s a m e m a t r i x and with high m e l t i n g t e m p e r a t u r e s . The s y s t e m NiA1-Cr has about 0.34 v o l u m e f r a c t i o n of c y l i n d r i c a l c h r o m i u m f i b e r s i n a m a t r i x of NiA1.7'8 The second alloy c o n s i s t s of a l e s s e r v o l u m e f r a c t i o n (0.09) of faceted m o l y b d e n u m f i b e r s in the s a m e m a t r i x , NiAlfl N e i t h e r alloy i s p r e d i c t e d to s p h e r o i d i z e b e c a u s e of the high v o l u m e f r a c t i o n of f i b e r s in the NiA1-Cr case and the p r e s e n c e of facets on the m o l y b d e n u m f i b e r s .6 The two alloys provide the m e a n s for c o m p a r i n g c o a r s e n i n g of c y l i n d r i c a l f i b e r s and faceted f i b e r s in the s a m e matrix. E X P E R I M E N T A L PROCEDURE The alloys of eutectic c o m p o s i t i o n7-9 w e r e f i r s t v a c u u m - m e l t e d and cast into copper m o l d s u n d e r a r g o n . The c a s t ingots were r e m e l t e d u n d e r a r g o n in c r u c i b l e s of h i g h - p u r i t y A1203 i n s i d e a v e r t i c a l B r i d g e m a n - t y p e d i r e c t i o n a l s o l i d i f i c a t i o n a p p a r a t u s . The c r u c i b l e r e s t e d on a w a t e r - c o o l e d copper b a s e which provided the n e c e s s a r y t e m p e r a t u r e g r a d i e n t . The m e l t was l o w e r e d f r o m the hot zone of the d i r e c t i o n a l s o l i d i f i c a t i o n a p p a r a t u s at 0.6 c m / h or 5 cm//h i n the case of one of the NiA1-Mo ingots. W a f e r s about 2 m m thick were cut t r a n s v e r s e l y f r o m the 2.2 cm diam ingots. Two w a f e r s f r o m each ingot, with the s a m e i n t e r f i b e r spacing, w e r e q u a r t e r e d to give n e a r l y i d e n t i c a l s a m p l e s . The c o a r s e n i n g t e m p e r a t u r e used in the study was 1400~ which is 96 pct and 93 pct of the m e l t i n g t e m p e r a t u r e s of the e u t e c t i c s NiA1-Cr and NiA1-Mo, r e s p e c t i v e l y . The s a m p l e s w e r e placed i n a l u m i n a c r u c i b l e s and heated to 1400~ + 5~ in an a r g o n a t m o s p h e r e for t i m e s up to 330 h. The a n n e a l e d s a m p l e s w e r e polished to r e v e a l the m i c r o s t r u c t u r e . P h o t o m i c r o g r a p h s w e r e taken at X1000 and e n l a r g e d to f a c i l i t a t e f i b e r size and d i s t r i b u t i o n m e a s u r e m e n t s . Some s a m p l e s w e r e sectioned p a r a l l e l to the f i b e r and s e l e c t i v e l y etched to r e m o v e the m a t r i x 7 for e x a m i n a t i o n by 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 y . VOLUME 4, JANUARY 1973-33
E X P E R I M E N T A L RESULTS
The c o a r s e n i n g of the N i A 1 - C r e u t e c t i c i s i l l u s t r a t e
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