Grain size and grain growth in an equiaxed alpha-beta titanium alloy

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/ i r e c e n t p u b l i c a t i o n d i s c u s s e s a technique for d e t e r m i n i n g the ;3 g r a i n s i z e in an e q u i a x e d ~-;3 Ti a l l o y . The m e t h o d d e v i s e d b y G r e e n f i e l d and M a r o l i n ~ ( h e r e a f t e r c a l l e d the G.M. method) involved a s h o r t t i m e h e a t t r e a t m e n t of the a l l o y at a t e m p e r a t u r e j u s t b e l o w the/3 t r a n s u s which did not a l t e r the/3 g r a i n s i z e but p e r m i t t e d the fl g r a i n b o u n d a r i e s to be d e l i n e a t e d a f t e r etching. Although the a u t h o r s r e p o r t e d no g r a i n growth d u r i n g the t i m e p e r i o d s used, a q u e s t i o n s was r a i s e d a s to the p o s s i b i l i t y that g r a i n g r o w t h could have o c c u r r e d p r i o r to the 30 s to 1 m i n h e a t t r e a t m e n t t i m e used. The p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n to m e a s u r e ~-;3 g r a i n s i z e s b y s e v e r a l t e c h n i q u e s and to e l u c i d a t e the c h a r a c t e r i s t i c s of g r a i n growth in a two p h a s e s t r u c t u r e which has p r e v i o u s l y b e e n w o r k e d in the a-;3 field.

a) Heating to 14K below the ;3 t r a n s u s (1199K) for a p p r o x i m a t e l y 1 min, w a t e r quenching, followed by o p tical microscopy examination after a standard etch t r e a t m e n t . L e s s a was d i s s o l v e d in t h i s p r o c e d u r e than was i n d i c a t e d in the e a r l i e r i n v e s t i g a t i o n . 1 Also, a f t e r the h e a t t r e a t m e n t , the g r a i n b o u n d a r i e s w e r e m o r e r e a d i l y d e l i n e a t e d than in the e a r l i e r work. 1 b) Aging at 764 K for 3 h a f t e r a s o l u t i o n a n n e a l at 1158 K and o p t i c a l m i c r o s c o p y e x a m i n a t i o n a f t e r the standard etching treatment. c) Thin foil e l e c t r o n m i c r o s c o p y with the foil n o r m a l p a r a l l e l to the length of the b a r . Q u a n t i t a t i v e a n a l y s i s of such p a r a m e t e r s a s ~ p a r t i c l e s i z e , s p a c i n g , v o l u m e f r a c t i o n and ;3 g r a i n s i z e w e r e p e r f o r m e d by the l i n e a r i n t e r c e p t method.

E X P E R I M E N T A L PROCEDURE

A. G r a i n Size D e t e r m i n a t i o n f o r G r a i n s in the Size Range of 30 to 50 ~ m

A 12 lb., 9.52 c m (3~ in.) d i a m ingot of a t i t a n i u m a l l o y having a c o m p o s i t i o n T i - 5 . 2 5 A1-5.5 V - 0 . 9 F e - 0 . 5 Cu was p r e p a r e d b y c o n s u m a b l e a r c - m e l t i n g t e c h n i q u e s . C y l i n d r i c a l b a r s of 1.27 c m (89 in.) d i a m w e r e p r o d u c e d f r o m the ingots by e x t r u s i o n in the ;3 f i e l d f r o m 2.95 in. d i a m to s i z e s in the r a n g e 0.960 to 1.062 followed b y swaging at a t e m p e r a t u r e of 1071 K in the ~-;3 field. Sections of the b a r w e r e h e a t t r e a t e d i n v a c u o of 6.7 x 10 -2 N / m s for v a r i o u s t i m e s at 1158 K and quenched into w a t e r