Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase
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commercially available creep resistant materials derive their strength from a uniform dispersion of a s e c o n d p h a s e which h i n d e r s the m o v e m e n t of d i s l o c a t i o n s . In c r e e p r e s i s t a n t f e r r i t i c s t e e l s , the d i s p e r s i o n a l m o s t i n v a r i a b l y c o n s i s t s of one o r m o r e typ es of c a r b i d e s , w h e r e a s in high n i c k e l c o n t e n t s u p e r a l l o y s , a l a r g e v o l u m e f r a c t i o n of the i n t e r m e t a l l i c compound Ni3(A1 , Ti) is i n v o l v e d . The c o m p o s i t i o n , m o r p h o l o g y , and c r y s t a l s t r u c t u r e of c a r b i d e s tend to change s l o w l y at e l e v a t e d t e m p e r a t u r e s , and this often r e s u l t s in an i n c r e a s e in c r e e p r a t e . One e x a m p l e of this is found in the c a s e of 0.1 C 1 C r - 0 . 5 Mo s t e e l . The c o m m o n l y u s e d h e a t t r e a t m e n t f o r this s t e e l r e s u l t s in a d i s p e r s i o n of M3C type c a r bide. On long t i m e e x p o s u r e at an e l e v a t e d t e m p e r a t u r e , the c a r b i d e s MTC3 and Mo2C g r a d u a l l y f o r m , and e v e n t u a l l y M6C a p p e a r s . T h e s e c h a n g e s a r e a c c o m p a nied by d e c r e a s e in c r e e p r e s i s t a n c e . 1 A n o t h er p r o b l e m that is f r e q u e n t l y e n c o u n t e r e d in e l e v a t e d t e m p e r a t u r e s t e e l s a r i s e s f r o m t h e i r need f o r oxidation r e s i s t a n c e . C h r o m i u m a d d i t i o n s a r e u s u a l l y e m p l o y e d f o r this p u r p o s e . In s t e e l s , i n c r e a s ing c h r o m i u m ad d i ti o n s f a v o r the f o r m a t i o n of MTC 3 and M2sC 6 type c a r b i d e s . 2 N e i t h e r of t h e s e c a r b i d e s i m p a r t s a p p r e c i a b l e s t r e n g t h e n i n g at e l e v a t e d t e m p e r a t u r e s . 2-4 C o l b e c k and R a i t ~ found a continuous d e c r e a s e in the c r e e p s t r e n g t h of c h r o m i u m - v a n a d i u m s t e e l s as the c h r o m i u m c o n t e n t was i n c r e a s e d f r o m 1 to 8 pct. On the o t h e r hand, w o r t h w h i l e i m p r o v e m e n t s in o x i d a t i o n r e s i s t a n c e at 600~ a r e not o b t a i n e d in f e r r i t i c s t e e l s until about 7 p c t c h r o m i u m is p r e s e n t . ~'4 Such a l l o y s , t h e r e f o r e , h a v e i n h e r e n t l y low c r e e p s t r e n g t h due to the p r e s e n c e of the c a r b i d e MTC3. It s e e m s d e s i r a b l e to e l i m i n a t e c a r b i d e s as the s t r e n g t h e n i n g a g e n t in c r e e p r e s i s t a n t s t e e l s and to M. DILIP BHANDARKAR, formerly with Lawrence Berkeley Laboratory, is now with NASA Langley Research Center, Materials Division, Materials Research Branch, Hampton, VA 23665. M. SHANTHIDAS BHAT is Graduate Student and VICTOR F. ZACKAY and EARL R. PARKER are Professors of Metallurgy, University of California, Lawrence Berkeley Laboratory, Berkeley, CA 94720. Manuscript su
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