Effect of solidification parameters on the growth geometry of MC carbide in IN- 100 dendritic monocrystals
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I. INTRODUCTION C a r b i d e s of the MC-type, which f o r m d u r i n g s o l i d i f i cation of n i c k e l - b a s e s u p e r a l l o y s , c o n t r i b u t e to s t r e n g t h e n i n g of g r a i n b o u n d a r i e s at e l e v a t e d t e m p e r a t u r e s . T h i s s t r e n g t h e n i n g r o l e s e e m s to depend s i g n i f i c a n t l y on c a r b i d e g e o m e t r y . Also, fatigue life in alloys, such as MAR-M200 was found to be g r e a t l y affected by the s i z e of p r e e x i s t i n g c r a c k s in MC-type c a r b i d e s . 1 The o c c u r r e n c e of such c r a c k s p r i o r to t e s t i n g a g a i n depends on c a r b i d e shape and s i z e . Mat r i x slip and c r a c k i n i t i a t i o n often o c c u r at p r e c r a c k e d MC c a r b i d e s and, to only a l e s s e r extent, at m i c r o pores.2 The p u r p o s e of the i n v e s t i g a t i o n r e p o r t e d h e r e i n was to c o r r e l a t e s o l i d i f i c a t i o n p a r a m e t e r s , such as growth r a t e and t h e r m a l g r a d i e n t at the s o l i d - l i q u i d i n t e r f a c e , with type, shape, size and c o m p o s i t i o n of the MC c a r b i d e in IN-100 n i c k e l - b a s e s u p e r a l l o y d i r e c t i o n a l l y solidified in the f o r m of d e n d r i t i c m o n o c r y s t a l s . 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 m a k e s this c o r r e l a tion p a r t i c u l a r l y easy and, in addition, offers the poss i b i l i t y of d e t e r m i n i n g the t e m p e r a t u r e at which a given type of c a r b i d e a p p e a r s and the t e m p e r a t u r e r a n g e over which it grows.
x 103 K / m , and growth r a t e , R, between 8.3 x 10 -0 and 67 x 10-0 m/s, Fig. 1. The G/R r a t i o s , as deduced f r o m the s a m e f i g u r e , v a r i e d b e t w e e n 6.71 x 107 and 1.93 x 109 K . s / m 2 and cooling r a t e s , (G x R), b e t w e e n 4.98 • 10 "e and 1.07 K / s . The g e o m e t r y of the faceted c a r b i d e e n c o u n t e r e d in these s p e c i m e n s was c h a r a c t e r i z e d by the a v e r a g e c a r b i d e size and the c a r b i d e - 7 m a t r i x i n t e r f a c e length p e r unit a r e a of c a r b i d e c r o s s - s e c t i o n , or specific p e r i m e t e r , both of which w e r e m e a s u r e d by q u a n t i t a tive m e t a l l o g r a p h i c t e c h n i q u e s . %5 That of the C h i n e s e s c r i p t - t y p e c a r b i d e was c h a r a c t e r i z e d by the a v e r a g e s p a c i n g b e t w e e n a d j a c e n t a r m s . F i n a l l y , the volume pct of c a r b i d e was m e a s u r e d by the point counting technique, whose a p p l i c a t i o n was d e s c r i b e d e l s e w h e r e , 6 and v e r i f i e d by c o m p u t e r i z e d image a n a l y s i s . III. RESULTS AND DISCUSSION The m i c r o s t r u c t u r e of a u n i d i r e c t i o n a l l y solidified ingot in the v i c i n i t y of the quenched i n t e r f a c e is i l l u s t r a t e d in F i g . 2. T h i s figure exhibits on the left side a l o
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