Effect of thermal cycling on the microstructure of a directionally solidified Fe, Cr, Al-TaC eutectic alloy

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Effect of Thermal Cycling on the Microstructure of a Directionally Solidified Fe, Cr, AI-TaC Eutectic Alloy F . H . H A R F AND S. N . T E W A R I

Fig. 3--SEM traetograph of m a t e r i a l g i v e n 15 h homogenization treatment. Typical c l u s t e r s of s m a l l d i m p l e s b e t w e e n m a j o r s u l f i d e s indicated by d . A r r o w s denote pinching in of l a r g e r flattened inclusions. Magnification 300 t i m e s .

Fig. 4 - - S E M fractograph of m a t e r i a l g i v e n 100 h homogenization treatment. Typical smooth d i m p l e s o r smooth a r e a s b e tween s u l f i d e s indicated by d. Magnification 300 t i m e s . t o the c l u s t e r s of s m a l l d i m p l e s f o u n d b e t w e e n m a j o r s u l f i d e s i n t h e 15 h t r e a t e d m a t e r i a l a s s h o w n i n F i g s . 2 a n d 3 . B o t h of t h e s e o b s e r v a t i o n s i n d i c a t e that t h e 100 h t r e a t m e n t h a s l e d t o a c o a r s e n i n g o f t h e s u l fides present. In s u m m a r y , this s t u d y has s h o w n that SEM f r a c t o g r a p h y is v e r y e f f e c t i v e in q u a l i t a t i v e l y c o n f i r m i n g t h e e x i s t e n c e o f the v a r i o u s s t a g e s i n the h o m o g e n i z a t i o n of M n S i n c l u s i o n s i n l o w a l l o y s t e e l . T h e S E M fractographs present convincing evidence of how the a c t u a l sulfide i n c l u s i o n s a r e r e a c t i n g to h o m o g e n i z a tion treatments. 202-VOLUME 8A, JANUARY 1977

D i r e c t i o n a l l y s o l i d i f i e d (DS) e u t e c t i c a l l o y s a r e b e i n g investigated a s the next generation material f o r a i r c r a f t gas t u r b i n e b l a d e s . T h e i r use is e x p e c t e d to r e s u l t in s i g n i f i c a n t e n g i n e p e r f o r m a n c e a d v a n t a g e s due t o a 50 t o 1 0 0 ° C i n c r e a s e i n a l l o w able m e t a l temperatures o v e r superalloys. 1 L a m e l l a r i n t e r m e t a l l i c ( N i 3 C b ) r e i n f o r c e d n i c k e l b a s e a l l o y s2 a n d fibrous monocarbide ( T a C ) reinforced nickel a n d c o b a l t b a s e a l l o y s3 h a v e r e c e i v e d the m o s t i n tensive investigation. S i n c e a i r c r a f t gas turbine c o m p o n e n t s e x p e r i e n c e s e v e r e t h e r m a l cycling, microstructural stability u n d e r c y c l i c t h e r m a l e x p o s u r e is a n i m p o r t a n t c o n s i d e r a t i o n i n s e l e c t i n g DS e u t e c t i c a l l o y s f o r g a s t u r b i n e s e r v i c e . T a C f i b e r s i n c o b a l t b a s e DS e u t e c t i c a l l o y s were o b s e r v e d t o develop s u r f a c e serrations w h i c h i n c r e a s e d with the m a x i m u m cycling temperat u r e a n d t h e n u m b e r o f c y c l e s . 4 T h e f o r m a t i o n of t h e s e s e r r a t i o n s was a t t r i b u t e d to t h e i n c r e a s i n g solubility w i t h t e m p e r a t u r e o f t h e T a C f i b e r s in the m a t r i x . It w a s p r o p o s e d that T a C w h i c h d i s s o l v e d a t t h e h i

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