Thermomigration of carbon in metals
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An investigation of thermomigration in interstitial solid solutions of carbon in /~-Ti, V, ~-Fe, Co, Ni, and Pd has been carried out using a radioactive tracer technique. A substantial solute redistribution occurred towards the hotter regions in ~-iron and vanadium and towards the colder regions in palladium whilst for Co, Ni and 13-Ti very little segregation was observed. The measured heats of transport were (in kcal per g-atom): (~-Fe, -14.1 ~ 1.5; V,-4.9 ~: 1.0; Pd, +8.4 ~- 1.5; Co, +1.5 • 1.5; Ni, - 1 . 6 =~1.4; f3-Ti, - 1 . 0 :~ 1.0. An examination of the results obtained together with all of the published data for interstitial solute thermomigration suggests that electronic effects could be of major importance in determining the redistributions observed.
W H E N an i n i t i a l l y homogeneous m u l t i c o m p o n e n t m a t e r i a l is placed in a t e m p e r a t u r e g r a d i e n t a c o n c e n t r a t i o n g r a d i e n t is f r e q u e n t l y e s t a b l i s h e d . T h i s is called " t h e r m o m i g r a t i o n . " Despite both the t e c h n o logical and t h e o r e t i c a l i m p o r t a n c e of t h e r m o m i g r a tion, 1 v e r y few r e l i a b l e q u a n t i t a t i v e i n v e s t i g a t i o n s have been c a r r i e d out i n solid m e t a l l i c s y s t e m s and t h e r e is b a r e l y a r u d i m e n t a r y u n d e r s t a n d i n g of the d r i v i n g f o r c e s for the phenomenon. T h i s i s r e f l e c t e d in the c u r r e n t t h e o r e t i c a l a p p r o a c h e s to the s u b j e c t and at the p r e s e n t t i m e t h e r e does not exist a model capable of p r e d i c t i n g , even qualitatively, the effect of t h e r m o m i g r a t i o n in solids. It was to provide a d d i tional q u a n t i t a t i v e data with which to c o m p a r e the c o n s e q u e n c e s of s u b s e q u e n t t h e o r e t i c a l models that the work embodied in this c o m m u n i c a t i o n was u n d e r t a k e n . The s i m p l e s t m e t a l l i c s y s t e m for the p u r p o s e s of t h e o r e t i c a l c o m p a r i s o n is the i n t e r s t i t i a l solid s o l u tion. Solute mobility i s high a t quite m o d e r a t e t e m p e r a t u r e s and effectively only the solute moves, the solvent l a t t i c e s e r v i n g a s a fixed f r a m e of r e f e r e n c e . Thus c o m p l i c a t i o n s which a r i s e when both s p e c i e s in a b i n a r y s y s t e m move a r e avoided. T h e b a s i c m a c r o scopic equations g o v e r n i n g t h e r m o m i g r a t i o n i n such s y s t e m s a r e obtained f r o m i r r e v e r s i b l e t h e r m o d y n a m ics and have been d i s c u s s e d by s e v e r a l a u t h o r s . 1- a The steady state solute c o n c e n t r a t i o n g r a d i e n t i s r e lated to the i m p r e s s e d t e m p e r a t u r e g r a d i e n t through the e x p r e s s i o n V In Ci = (Q~*/k) V ( 1 / T )
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w h e r e C/ i s the solute c o n c e n t r a t i o n , k is B o l t z m a n n ' s constant and T the a b s o l u t e t e m
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