Electron microscope study of the effect of deformation on precipitation and recrystallization in copper-titanium alloys

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l e a d i n g at about 70 pct of s t r a i n to the m i s o r i e n t a f i o n of 2 to 6 deg. 17'18 A n o t h e r i m p o r t a n t s t r u c t u r a l f e a t u r e of the d e f o r m e d m e t a l i s d e f o r m a t i o n band (also c a l l e d t r a n s i t i o n band) s e p a r a t i n g a d i f f e r e n t p a r t s of a c r y s t a l that has u n d e r g o n e d i f f e r e n t v a l u e s of r o t a t i o n d u r i n g d e f o r m a t i o n . 19'2~ It was a l s o d e m o n s t r a t e d x9-2~ that r e c r y s t a l l i z a t i o n n u c l e i a r e f o r m e d in t h o s e r e g i o n s with l a r g e l o c a l m i s o r i e n t a t i o n s , i.e. at t r a n s i t i o n bands and g r a i n b o u n d a r i e s . C o p p e r - t i t a n i u m a l l o y s w e r e c h o s e n f o r the p r e s e n t s t u d y , s i n c e v e r y l i t t l e i s known about the influence of d e f o r m a t i o n on the m e c h a n i s m s of p r e c i p i t a t i o n and r e c r y s t a l l i z a t i o n in t h e s e alloys.3 B e c a u s e of the c o m p l e x i t y of the p r o b l e m , only two c h a r a c t e r i s t i c s t r a i n s a r e going to be d i s c u s s e d h e r e ; one of t h e m c o r r e s p o n d i n g to the i n i t i a l s t a g e s of s u b g r a i n r e f i n e m e n t and d e f o r m a t i o n band f o r m a t i o n , and the o t h e r r e p r e s e n t i n g w e l l d e v e l o p e d d e f o r m a t i o n bands and s u b g r a i n b o u n d a r i e s . P a r t i c u l a r a t t e n t i o n was p a i d to the p r o b l e m of the influence of d e f o r m a t i o n b a n d s , s u b g r a i n and g r a i n b o u n d a r i e s on the i n i t i a l s t a g e s of r e c r y s t a l l i z a t i o n and p r e c i p i t a t i o n , i.e. on the n u c l e a t i o n of r e c r y s t a l l i z e d g r a i n s , continuous and d i s c o n t i n u o u s p r e c i p i t a t i o n , in r e l a t i o n with the a g ing t e m p e r a t u r e and the c o m p o s i t i o n . E X P E R I M E N T A L PROCEDURE A l l the a l l o y s have been c a s t in an induction v a c u u m f u r n a c e f r o m high p u r i t y e l e m e n t s . T h e c o m p o s i t i o n of the a l l o y s u s e d in the p r e s e n t study is shown in

Table I. Composition of Alloys

Alloy

Ti, Wt Pet

Ti, At. Pct

A B

2.42 4.29

3A8 5.61 VOLUME 8A, MAY 1977 751

Table I. The samples for hardness and transmission electron microscopy (cut from the cast ingots after homogenizing annealing) were quenched from 880~ f r o m the argon atmosphere directly into the ice-water bath, using a specially constructed furnace. The samples were rolled directly after quenching up to the deformation of 30 and 70 pct and then aged at 400 and 600~ Thin foils were obtained by polishing using

the window method in an electrolyte consisting of 30 pct nitric acid and 70 pct methanol at -50~ A Philips 301 EM operating at 100 Kv, fitted with double tilt x - y or tilt rotation goniometer stages was used in the present study. The dark field method was applied for studying th