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the t h e r m a l l y a c t i v a t e d o r r a t e t h e o r y of d e f o r m a tion, f l u c t u a t i o n s in l a t t i c e v i b r a t i o n a l e n e r g y a s s i s t the l o c a l net s t r e s s in moving an i m p e d e d s e g m e n t of d i s l o c a t i o n p a s t an o b s t a c l e . The t h e r m a l l y a c t i v a t e d e v e n t s o c c u r with a s u c c e s s f r e q u e n c y given b y the p r o d u c t of s o m e a t t e m p t f r e q u e n c y and a B o l t z m a n n p r o b a b i l i t y t e r m , e x p ( - A G / k T ) , w h e r e AG i s the f r e e e n e r g y d i f f e r e n c e b e t w e e n a b a s e and a c t i v a t e d s t a t e , k i s B o l t z m a n n ' s c o n s t a n t , and T is the a b s o l u t e t e m perature. The p l a s t i c s h e a r s t r a i n - r a t e i s d e t e r m i n e d b y the d e n s i t y of a c t i v a b l e d i s l o c a t i o n s e g m e n t s , the f r e quency of s u c c e s s f u l a c t i v a t i o n e v e n t s , the B u r g e r s v e c t o r of the d i s l o c a t i o n s , and the a v e r a g e a r e a swept b y a d i s l o c a t i o n s e g m e n t b e f o r e it is s t o p p e d b y o t h e r obstacles. For a single thermally activated process the s t r a i n - r a t e c a n b e written* = Yo exp ( -

AG(~'))

kT

[1]

w h e r e the p r e - e x p o n e n t t a l t e r m Y0 is the s t r a i n - r a t e which would o c c u r tf the p r o b a b i l i t y of s u c c e s s of the a c t i v a t i o n e v e n t s w e r e unity. F r o m s t u d y of the t e m p e r a t u r e and r a t e d e p e n d e n c e of p l a s t i c d e f o r m a t i o n of m e t a l s it is p o s s i b l e to d e t e r m t n e AG(~-) and f r o m that function gain i n s i g h t into the dislocation-obstacle Interaction process. This method when s u c c e s s f u l l y a p p l i e d , e n a b l e s the p r e d i c t i o n of flow s t r e s s and r a t e d e p e n d e n c e of the flow s t r e s s f o r a given m a t e r i a l condition f r o m l i m i t e d e x p e r i m e n t a l data. Thermal activation analysis has been applied previo u s l y to c o p p e r , z's a l u m i n u m , a-6 s i l v e r , 5 and m a g n e s i u m . 7,s In the p r e s e n t p a p e r r e s u l t s a r e r e p o r t e d for p u r e gold s i n g l e c r y s t a l s and a m o d i f i c a t i o n of e a r l i e r t e c h n i q u e s f o r o b t a i n i n g the a c t i v a t i o n p a r a m e t e r s i s discussed.

ROBERT B. HERRING, formerly of the Department of Materials Science, Northwestern University, Evanston, Illinois, is currently a Senior Engineer, Basic Technology Laboratory, Motorola Semiconductor Products Division, Phoenix, Arizona 85005. M. MESHII is Professor, Department of Materials Science, Northwestern University, Evanston, Illinois 60201. Manuscript submitted June 14, 1972. METALLURGICAL TRANSACTIONS

EXPERIMENTAL PROCEDURE A gold single crystal approximately 0.00125 x 0.006 x 0.25 m long was grown in a high purity graphite mold by a single pass induction zone melting technique from sta