Kinetics of Ti 3 Al grain boundary precipitation in Ti-Al binary and Ti-Al-X ternary alloys and correlation with mechani
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T H E m o s t widely used t i t a n i u m - b a s e alloys c o n t a i n 8.5 at. pct (5 wt pct) or m o r e a l u m i n u m . E x a m p l e s of such alloys a r e : T i - 5 A1-2.5 Sn, T i - 6 A1-4 V, T i - 6 A16 V-2 Sn(-0.7 Cu-0.7 Fe), and T i - 8 AI-1 Mo-1 V.* An *ThesedesignationsexpressalloycomposiUonin weightpercentand are the most commonlyusedmeansof specifyingtitanium-basealloysof U.S.producers. e m b r i t t l i n g p h e n o m e n o n a s s o c i a t e d with the f i r s t i n t e r m e d i a t e phase in the T i - A l s y s t e m has been r e c o g n i z e d for some t i m e . 1 It is widely a g r e e d that this f i r s t i n t e r m e d i a t e phase is o r d e r e d hcp with DO19 s t r u c t u r e and is b a s e d upon Ti3A1. z-8 X - r a y and e l e c t r o n diffraction evidence of the o c c u r r e n c e of TisA1 in the c o m m e r c i a l a l l o y s T i - 8 AI-1 Mo-1 V and T i - 6 A1-4 V has been r e p o r t e d . 9-11 Solubility of a l u m i n u m in t i t a n i u m at 550~ was r e p o r t e d to be 11 at. pct (6.5 wt pct) by Kornilov et a l . , s 12 at. pct (7 wt pct) by B l a c k b u r n , 7 but only 7.5 at. pct (4.4 wt pct) by C r o s s l e y . 8 The t e r m TisA1 i s u s e d for all p r e c i p i tates reported herein. In addition to l o s s of t o u g h n e s s , t h e r e a r e s u s p i c i o n s that the p r e s e n c e of i n c r e a s i n g a m o u n t s of TisA1 in t i t a n i u m a l l o y s is a s s o c i a t e d with i n c r e a s i n g s u s c e p t i b i l i t y to s t r e s s c o r r o s i o n . 12,~3 Very r e c e n t l y , M a r g o l i n and P o r t i s c h 14 r e p o r t e d that Ti-A1 b i n a r y a l l o y s cont a i n i n g f r o m 16 to 20 at. pct (9.5 to 12.5 wt pct) A1 were highly s u s c e p t i b l e to hydrogen pickup d u r i n g e t c h i n g , and that Ti3A1 played a key role in t h i s phenomenon. Since the f o r m a t i o n of TiaA1 in dilute b i n a r y a l l o y s can be s u p p r e s s e d by q u e n c h i n g f r o m above the + Ti3A1 phase field, t h r e e q u e s t i o n s a r e of keen i n t e r e s t . What a r e the k i n e t i c s of TisA1 f o r m a t i o n , how a r e the k i n e t i c s i n f l u e n c e d by t e r n a r y a l l o y i n g a d d i t i o n s , and how a r e m e c h a n i c a l p r o p e r t i e s affected by the p r e c i p i t a t i o n r e a c t i o n ? T h e s e t h r e e q u e s t i o n s were the s u b j e c t s of t h i s i n v e s t i g a t i o n . K i n e t i c s s t u d i e s were made on a l l o y s s e l e c t e d so that the total c o n t e n t s of s u b s t i t u t i o n a l , high a - s o l u F. A. CROSSLEY, formerly with IIT Research Institute, Chicago, I11., is Staff Scientist, Materials Sciences Laboratory, Lockheed Missiles & Space Co., Palo Alto, Calif. Manuscript submitted September 29, 1969. METALLURGICALTRANSACTIONS
bility a d d i t i o n s , i . e . , a l u m i n u m , t i n , and z i r c o n i u m , were at l e v e l s of 12, 13.5, and 15 at. pct. Other t e r n a r y a d d i t i o n s studied were the i s o m o r p
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