Embrittlement of titanium alloys by long time, high temperature exposure
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N T E R E S T in the a p p l i c a t i o n of t i t a n i u m a l l o y s c o n t a i n i n g l a r g e a d d i t i o n s of a l u m i n u m a n d tin h a s b e e n r e n e w e d to d e v e l o p a l i g h t - w e i g h t s u p e r i o r c r e e p r e s i s t a n c e m a t e r i a l for the 7 0 0 ° to 1 0 0 0 ° F t e m p e r a t u r e r a n g e . H o w e v e r , c o m m e r c i a l a l l o y s of this type a r e k n o w n to e x h i b i t a loss in r o o m t e m p e r a t u r e t e n s i l e ductility a f t e r long t i m e c r e e p e x p o s u r e s in the 8 0 0 ° t o 1 2 0 0 ° F temperature range. The tensile embrittlement h a s b e e n s h o w nI to b e p a r t i a l l y attributable to the s h a l l o w o x y g e n e n r i c h e d diffusion l a y e r f o r m e d o n the s u r f a c e of t i t a n i u m a l l o y s in this t e m p e r a t u r e r a n g e . H o w e v e r , the d e g r e e of e m b r i t t l e m e n t for the d i f f e r e n t a l l o y s is s u g g e s t e d to a r i s e f r o m d i f f e r e n c e s in t o u g h n e s s o r i n t e r n a l metallurgical stability as it a f f e c t s toughness. T h e p r e s e n t study i n v o l v e d a n investigation of the k i n e t i c s of the i n t e r n a l e m b r i t t l e m e n t p r o c e s s f o r two c o m m e r c i a l high t e m p e r a t u r e titanium alloys, Ti-6A1-2Sn-4Zr-2Mo (Ti-6242) and Ti-5A1-6Sn-2Zr1Mo-0.25Si (Ti-5621S). THEORETICAL BACKGROUND C r o s s l e y a n d C a r e w2 i n i t i a l l y r e p o r t e d e m b r i t t l e m e r i t of the T i - 8 wt p c t A1 a l l o y i n 1 9 5 7 . T h i s b e h a v i o r w a s c o n f i r m e d by S o l t i s 3 a n d a t t r i b u t e d t o the o r d e r e d Tin A1 p h a s e i n t h e c o m m e r c i a l T i - 8 A 1 - 1 M o - 1 V a l l o y . A l t h o u g h t h e r e i s a l a c k o f a g r e e m e n t on t h e T i - A 1 p h a s e d i a g r a m ,4-7 t h e e x i s t e n c e o f the o r d e r e d Ti3A1 p h a s e is generally acknowledged. The l a c k o f a g r e e m e n t f o r t h e p h a s e b o u n d a r y l o c a t i o n i n the b i n a r y Ti-A1 s y s t e m p r e s e n t s complexities when considering t h e t e r n a r y Ti-A1-Sn r e l a t e d a l l o y s . The T i - S n syst e m 8 i s k n o w n t o e x h i b i t a Tin Sn p h a s e w i t h the DO19 s t r u c t u r e s i m i l a r to t h a t f o r T i a A 1 . T h e r e f o r e , it is reasonable t o a s s u m e that t h e ternary Ti-AI-Sn p h a s e d i a g r a m c o n s i s t s of a c o n t i n u o u s s o l i d s o l u t i o n b e t w e e n Ti3A1 a n d T i 3 S n a t ~ 1 0 0 0 ° F . H o w e v e r , the l o c a t i o n of the Ti3X p h a s e b o u n d a r i e s a n d the t e r m i n a l a s o l i d solubility boundary are unknown. Nevertheless, t h e assumption of forming o r d e r e d phase domains, s i m i l a r t o that of the T i - 8 wt p c t A1 a l l o y , d o e s a l l o w a c r i t e C. E. SHAMBLEN is Metallurgist, Aircraft Engine Group, General Electric C o , Cincinnati, Ohio Manuscript submitted May 18, 1970. METALI_I.'.R(,I('AL
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