Phase transformations in Ti- 3AI- 8V- 6Cr- 4Zr- 4Mo
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T H E continuing s e a r c h for h i g h e r strength, deep hardenable titanium alloys has recently prompted r e n e w e d i n t e r e s t i n m e t a s t a b l e b e t a T i a l l o y s . ~-6 A t equivalent strength-to-weight r a t i o s t h e s e alloys gene r a l l y p o s s e s s s u p e r i o r fracture toughness and s t r e s ~ c o r r o s i o n r e s i s t a n c e w h e n c o m p a r e d t o a-/3 T f a l l o y s . %8 F u r t h e r m o r e , m e t a s t a b l e b e t a T i a l l o y s c a n be r e a d i l y w o r k e d / f o r m e d i n t h e /3 c o n d i t i o n p r i o r t o s t r e n g t h e n i n g by a g i n g . 1'9'~° S e l e c t i o n of a n a g i n g c y c l e s h o u l d o f c o u r s e be b a s e d u p o n a k n o w l e d g e o f the time-temperature-transfornaation b e h a v i o r of t h e p a r t i c u l a r alloy u n d e r consideration and how t h e s e t r a n s formations influence the mechanical properties of this alloy. T h e p u r p o s e of t h e p r e s e n t i n v e s t i g a t i o n w a s t o e s tablish this t i m e - t e m p e r a t u r e - t r a n s f o r m a t i o n behavior for a typical, commercial metastable f l - T i alloy, Ti3A1- 8 V - 6 C r - 4 Z r - 4 M o . T h i s a l l o y , s o m e t i m e s r e f e r r e d t o a s R M I 38644, w a s s e l e c t e d s i n c e p r e v i o u s e x a m i n a t i o n s h a d s h o w n t h a t i t c o u l d be h e a t t r e a t e d t o s t r e n g t h s i n e x c e s s of 1300 M P a 11-14 a n d t h a t a t t h e s e levels it was i m m u n e t o s t r e s s c o r r o s i o n c r a c k i n g in 3.5 p c t N a C 1 .~5 I n a d d i t i o n , p r e l i m i n a r y s t u d i e s sugg e s t e d that a t h e r m a l ~6,17 a n d i s o t h e r m a l ¢o, ~7,18 /3 + fi' p h a s e s e p a r a t i o n , x2 T i C r 2 , ~9'2° a n d b o t h B u r g e r s ( T y p e 1)e~ a n d n o n - B u r g e r s ( T y p e 2) a ~2 m a y a l l be o b s e r v e d i n R M I 38644 d e p e n d i n g upon t h e e x a c t a g i n g c o n d i t i o n s s p e c i f i e d . A m o r e d e t a i l e d e x a m i n a t i o n of t h e a g i n g k i n e t i c s of t h i s a l l o y t h e r e f o r e a p p e a r e d w a r r a n t e d a n d is r e p o r t e d h e r e i n .
b a t h . All s a m p l e s were finally air cooled f r o m t h e a g i n g temperature. T h e r e p o r t e d Rockwell C h a r d n e s s measurements a r e the a v e r a g e o f f i v e r e a d i n g s t a k e n o n a p l a n e p e r p e n d i c u l a r t o the o r i g i n a l r o l l i n g d i r e c t i o n . O p t i c a l metallography involved s t a n d a r d polishing p r o c e d u r e s w i t h f i n a l e t c h i n g b e i n g a c c o m p l i s h e d u s i n g e i t h e r 15 m l HNO3, 5 m l H F , 15 m l l a c t i c a c i d , 65 m l HzO o r 1 t o 3 m l H F , 2 t o 6 m l HNO3, HzO t o 1000 m l . Thin f o i l s for transmission electron microscopy were initially w a f e r e d f r o m t h e hardness/optical m e t a l l o g r a p h y s a m p l e s a n d h a n d g r o u n d t o 0.1 m m . F i n a l t h i n n i n g w a s p e r
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