The microstrain behavior of a bcc Fe-Si-Ti alloy
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v e r y e a r l y s t a g e s of y i e l d i n g in s i n g l e p h a s e m e t a l s and a l l o y s have b e e n s t u d i e d i n t e n s i v e l y with the a i m of d e t e r m i n i n g the r a t e - c o n t r o l l i n g m e c h a n i s m s of p l a s t i c flow and f o r c h a r a c t e r i z i n g the i n i t i a l d i s l o c a t i o n glide p r o c e s s e s . In p a r t i c u l a r , the m i c r o s t r a i n technique has found e x t e n s i v e u s e in inv e s t i g a t i o n s of the v e r y s t r o n g t e m p e r a t u r e d e p e n d e n c e of the flow s t r e s s in bcc m e t a l s , e s p e c i a l l y i r o n . In c o n t r a s t to t h i s , r e l a t i v e l y few i n v e s t i g a t i o n s of s t r e n g t h e n i n g m e c h a n i s m s and r a t e - c o n t r o l l i n g p r o c e s s e s in d i s p e r s i o n and p r e c i p i t a t i o n h a r d e n e d a l l o y s y s t e m s have u t i l i z e d the m i c r o s t r a i n technique, a l though it w a s f i r s t e m p l o y e d by R o b e r t s , C a r r u t h e r s and A v e r b a c h , 1 in an i n v e s t i g a t i o n of the d i s p e r s i o n s t r e n g t h e n i n g of s t e e l . The c o n v e n t i o n a l p a r a m e t e r s d e t e r m i n e d f r o m m i c r o s t r a i n e x p e r i m e n t s a r e t h e e l a s t i c l i m i t , erE, and the a n e l a s t i c l i m i t , crA. T h e s e p a r a m e t e r s have b e e n defined by B r o w n e a s the s t r e s s n e c e s s a r y f o r the o n s e t of m e c h a n i c a l h y s t e r e s i s and the s t r e s s n e c e s s a r y f o r the p r o d u c t i o n of an i r r e v e r s i b l e p l a s t i c s t r a i n , r e s p e c t i v e l y . Thus, at s t r e s s l e v e l s g r e a t e r than a E but l e s s than aA c l o s e d m e c h a n i c a l h y s t e r e s i s loops a r e g e n e r a t e d . B r o w n 2 i n t e r p r e t s the l a t t i c e r e s i s t a n c e d u r i n g t h i s a n e l a s t i c d i s l o c a t i o n m o v e m e n t a s b e i n g due to the e x i s t e n c e of a f r i c t i o n a l s t r e s s , af, which c a u s e s the d i s l o c a t i o n s to d i s s i p a t e energy during movement. This irreversible energy l o s s , W, is given by: w--
w h e r e ep is d i s l o c a t i o n - p r o d u c e d s t r a i n . McMahon" h a s e m p h a s i z e d that m o b i l e , unpinned d i s l o c a t i o n s m u s t be p r e s e n t in a c r y s t a l f o r h y s t e r e s i s e f f e c t s to o c c u r . A b a c k s t r e s s of s u f f i c i e n t m a g n i t u d e to o v e r c o m e the f r i c t i o n s t r e s s m u s t be g e n e r a t e d in o r d e r to p r o v i d e the d r i v i n g f o r c e to r e t u r n the d i s l o c a t i o n s to t h e i r o r i g i n a l p o s i t i o n s and so g e n e r a t e a G. W. WALTER and I. O. SMITH are Research Fellow and Senior Lecturer in Physical Metallurgy, respectively, Department of Mining and Metallurgical Engineering, University of Queensland, St. Lucia, 4067, Australia. Manuscript submitted June 30
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