Effect of microstructure on the strength, toughness, and stress-corrosion cracking susceptibility of a metastable beta t
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AND
M. J. BLACKBURN
This p a p e r d e s c r i b e s the influence of m i c t r o s t r u c t u r e on the m e c h a n i c a l p r o p e r t i e s of the a l l o y T i - l l . 5 M o - 6 Z r - 4 . 5 S n . The phase t r a n s f o r m a t i o n s a r e s i m i l a r to those that o c c u r in b i n a r y T i - M o alloys c o n t a i n i n g 10 to 12 pct Mo. Thus, the 13 p h a s e can be r e t a i n e d by quenching f r o m above 1400~ The/3 p h a s e d e f o r m s in a c o m p l e x m a n n e r , i n c l u d i n g m e c h a n i c a l twinning, and is c h a r a c t e r i z e d by low s t r e n g t h , high d u c t i l i t y , and high t o u g h n e s s . The w phase, which a l s o f o r m s on quenching, is s t a b l e at t e m p e r a t u r e s up to 800~ Yield s t r e n g t h s of up to 220 ksi have b e e n m e a s u r e d in 03 + r s t r u c t u r e s , the s t r e n g t h l e v e l b e i n g dependent on the s i z e and v o l u m e f r a c t i o n of the r phase. In c o n t r a s t , ofracture toughness r e a c h e s a m i n i m u m v a l u e of ~ 2 0 ksi ~ when the w p a r t i c l e s i z e _>100A. 03 + (~) s t r u c t u r e s show good c o m b i n a t i o n s of yield s t r e n g t h and f r a c t u r e t o u g h n e s s . U n f o r t u n a t e l y , the b e s t c o m b i n a t i o n s a r e s u s c e p t i b l e to s t r e s s - c o r r o s i o n c r a c k i n g in aqueous s o l u t i o n s c o n t a i n i n g halide ions. T R A N S I T I O N e l e m e n t s that tend to s t a b i l i z e the h i g h t e m p e r a t u r e e q u i l i b r i u m bcc/3 phase of t i t a n i u m can be d i v i d e d into two types: 1) those f o r m i n g / 3 i s o m o r phous s y s t e m s (e.g., T i - M o , T i - V , T i - N b ) a n d 2 ) t h o s e f o r m i n g / 3 eutectoid s y s t e m s (e.g., T i - C u , T i - N i , T i Cr). In the b i n a r y T i - M o s y s t e m , the p r o d u c t s obtained on q u e n c h i n g f r o m the/3 p h a s e f i e l d a r e s u m m a r i z e d in Fig. 1. ~-~ Also included a r e the p h a s e s obtained on a g ing and on p l a s t i c d e f o r m a t i o n of the r e t a i n e d m e t a s t a b l e / 3 p h a s e . It is a p p a r e n t that the b i n a r y T i - M o s y s t e m is quite complex and that s m a l l changes in m o l y b d e n u m content m a y m a r k e d l y change the t r a n s formation behavior. The f i r s t m e t a s t a b l e beta a l l o y to be p r o d u c e d in c o m m e r c i a l q u a n t i t i e s , c a l l e d B120 ?CA, had the n o m inal c o m p o s i t i o n T i - 1 3 V - 1 1 C r - 3 A i . Although p o s s e s s ing both good r o o m t e m p e r a t u r e f o r m a b i l i t y and good h e a t - t r e a t m e n t r e s p o n s e , t h i s a l l o y was difficult to hot r o l l and e m b r i t t l e d at s e r v i c e t e m p e r a t u r e s above 600~ The e m b r i t t l e m e n t was a t t r i b u t e d to the p r e c i p i t a t i o n of the i n t e r m e t a l l i c compound TiCr2. ~~ The a l l o y T i - l l . S M o - 6 Z r - 4 . 5 S n , called Beta HI, was s p e
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