Rapid grain growth of titanium
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Table I. Chemical Composition of Commercial Purity Titanium (A40), In Wt Pct
C
Fe
N
o
H
0.016
0.112
0.0084
0.228
0.0075
MIKIO SUGANOis Assistant Professor of Engineering,Yamagata University, Yonezawa, Yamagata 992, Japan. CHARLES M. GILMORE is Associate Professor of Engineeringand Applied Science, George WashingtonUniversity, Washington,DC 20052. Manuscript submitted October 24, 1978. 1400-VOLUME 10A, SEPTEMBER 1979
Fig. 1-Example of a specimen obtained by rapid grain growth annealing. The grains are differentiated by thermal etching which occurred during the anneal treatment. The upper figure shows the outline of the grains. The scale is in centimeters.
t e s t s were conducted. S p e c i m e n s were i n i t i a l l y a n n e a l ed for two h at 840~ and then a i r cooled to r o o m t e m p e r a t u r e . The s p e c i m e n s were then held at a s e r i e s of m i n i m u m t e m p e r a t u r e s f r o m 872 to 912~ with the t h e r m a l g r a d i e n t , one of these s p e c i m e n s was held at 882~ which is a p p r o x i m a t e l y the /3 to a e q u i l i b r i u m t r a n s f o r m a t i o n t e m p e r a t u r e . A l l of the s p e c i m e n s were then a n n e a l e d at 920~ for two h. The g r a i n s i z e s r e s u l t i n g f r o m this s e r i e s of t e s t s is shown in F i g . 2. T h i s s e r i e s of t e s t s shows that holding at a t e m p e r a t u r e at or just above the i3 t o a t r a n s f o r m a t i o n t e m p e r a t u r e is c r i t i c a l to obtaining l a r g e g r a i n s . It is p r o p o s e d that the heat t r e a t m e n t p r o c e d u r e p r e s e n t e d above, that r e s u l t e d in the m a x i m u m final g r a i n s i z e s , produced the following m e t a l l u r g i c a l changes. 1. Two hour a n n e a l at 840~ T h i s produced a well annealed polycrystalline microstructure. 2. Two hour hold at 882~ This process nucleated only a few H - g r a i n s . Higher t e m p e r a t u r e s p r o d u c e d m o r e /3-grains which r e s u l t e d in s m a l l e r final g r a i n s i z e s . A l s o the 7 rain hold t i m e d i s c u s s e d p r e v i o u s l y produced much l a r g e r g r a i n s than a two h hold. 3. A i r cool f r o m 882~ to r o o m t e m p e r a t u r e . The /3-grains that f o r m e d at 882~ t r a n s f o r m e d d u r i n g a i r cooling to m a r t e n s i t e ( a ' ) by a d i f f u s i o n l e s s t r a n s f o r m a t i o n %7 that obeyed the B u r g e r s ' r e l a t i o n s h i p , s Only a few m a r t e n s i t e g r a i n s e x i s t e d a f t e r the a i r cool b e c a u s e only the ~3-grains t r a n s f o r m to a ' d u r i n g cooling and only a few ~3-grains f o r m e d at 882~ b e cause this t e m p e r a t u r e was v e r y close to the e q u i l i brium temperature. 9 4. Two hour a n n e a l at 920~ The ;3-phase f o r m s at this t e m p e r a t u r e by a n u c l e a t i o n and growth p r o c e s s . The a ' - m a r t e n s i t e g r a i n s acted as p r e f e r e n t i a l n u c l e a tion s i t e s b e c a u s e of th
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