Ductile-brittle transition of thoriated chromium
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The i m p r o v e d ductility in t h o r i a t e d c h r o m i u m may be a s s o c i a t e d with s e v e r a l p o s s i b l e m e c h a n i s m s : 1) p a r t i c l e s may d i s p e r s e s l i p , such that c r i t i c a l s t r e s s or s t r a i n c o n c e n t r a t i o n s for c r a c k n u c l e a t i o n a r e m o r e difficult to achieve; 2) p a r t i c l e s may act as d i s l o c a t i o n s o u r c e s , thus p r o v i d i n g mobile d i s l o c a t i o n s in this n o r m a l l y s o u r c e - p o o r m a t e r i a l , in a m a n n e r s i m i l a r to p r e s t r a i n i n g ; and 3) p a r t i c l e s in g r a i n b o u n d a r i e s may help to t r a n s m i t slip a c r o s s the b o u n d a r i e s , thus r e l i e v i n g s t r e s s c o n c e n t r a t i o n s and i n h i b i t i n g c r a c k n u c l e a tion. C H R O M I U M alloys have been c o n s i d e r e d candidate m a t e r i a l s for high t e m p e r a t u r e s t r u c t u r a l a p p l i c a t i o n s such as components in gas t u r b i n e e n g i n e s . However, one of the f a c t o r s which has made this u n f e a s i b l e is the poor ductility at a m b i e n t t e m p e r a t u r e s . Although pure c h r o m i u m single c r y s t a l s have a t e n s i l e d u c t i l e - t o b r i t t l e t r a n s i t i o n t e m p e r a t u r e (DBTT) of -80~ 1,2 notched i m p a c t t e s t s on p o l y c r y s t a l l i n e c h r o m i u m have r e s u l t e d in a DBTT of 370~ T h e r e have b e e n n u m e r ous s t u d i e s concerne~d with the DBTT of c h r o m i u m and c h r o m i u m a l l o y s , i n c l u d i n g a t t e m p t s to v a r y the m i c r o s t r u c t u r ~ and c o m p o s i t i o n such that the DBTT i s lowe r e d . Although some f e a t u r e s r e g a r d i n g the DBTT a r e not c o m p l e t e l y c l e a r , s e v e r a l g e n e r a l l y a c c e p t e d conc l u s i o n s have been r e a c h e d : a) The DBTT of wrought c h r o m i u m is lower than that of r e c r y s t a l l i z e d m a t e r i a l , s-8 b) B r i t t l e f r a c t u r e is s l i p - i n d u c e d and the c r i t i c a l stage of f r a c t u r e is c r a c k i n i t i a t i o n . 2'9'~~ c) Nitrogen in s o l u t i o n and as n i t r i d e s , e s p e c i a l l y at g r a i n b o u n d a r i e s , i n c r e a s e s b r i t t l e n e s s . ~,6'8''1-16 d) C h r o m i u m is e x t r e m e l y notch-sensitive.l,9'13 e) Solid solution a l l o y i n g of c h r o m i u m with 35 at. pct Re s u b s t a n t i a l l y l o w e r s the DBTT for slow d e f o r m a t i o n rates.9'~7'~8 f) P r e s t r a i n i n g to produce mobile d i s l o c a t i o n s i n c r e a s e s the s u b s e q u e n t ductility of c h r o m i u m . 19-23 g) I n c r e a s i n g the d e f o r m a t i o n rate i n c r e a s e s the DBTT of c h r o m i u m . 9'24-2~ S e v e r a l review p a p e r s 9,17,2a-'a~have d i s c u s s e d t h e s e f a c t o r s in m o r e detail. This r e s e a r c h was c o n c e r n e d with the i n c o r p o r a t i
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