Temperature dependence of yield stress in fe-1.67 at. pct Cu and fe-0.5 at. pct Au
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A s u p e r s a t u r a t e d s o l i d solution of 1.67 at. pct Cu in Fe has a lower t e m p e r a t u r e dependence of the yield s t r e s s ( p r e s t r a i n e d 0.2 pct at 368~ than pure i r o n over the t e m p e r a t u r e r a n g e 77 ~ to 298~ No change in t e m p e r a t u r e dependence is o b s e r v e d on aging to f o r m copper c l u s t e r s (1 hr at 475~ but after p r e c i p i t a t i o n of e the t e m p e r a t u r e dependerfce is the s a m e a s in pure i r o n . The a c t i v a t i o n volume at 77~ i s unaffected by t r e a t m e n t in the alloy and the amount of d e f o r m a t i o n . The value is e s s e n t i a l l y the s a m e a s in i r o n . It was suggested that copper c l u s t e r s p r e s e n t after aging 1 hr as well as d i s s o l v e d copper a s s i s t f o r m a t i o n of double kinks at 77~ r e d u c i n g the s t r e s s r e q u i r e d to s u r m o u n t the P e i e r l s b a r r i e r . Addition of 0.5 at. pct Au a l s o r e d u c e d the t e m p e r a t u r e dependence of i r o n .
T H E t e m p e r a t u r e dependence of the flow s t r e s s of F e - C u alloys is of i n t e r e s t for a n u m b e r of r e a s o n s . Many s u b s t i t u t i o n a l a l l o y i n g e l e m e n t s c a u s e alloy softening in iron at low t e m p e r a t u r e s . 1 - 5 I r o n with 0.8 at. pct Cu was r e p o r t e d to have the s a m e y i e l d s t r e n g t h a s unalloyed i r o n at 77~ The effect of gold h a s not b e e n p r e v i o u s l y r e p o r t e d . Alloys of i r o n with copper solute a r e p r e c i p i t a t i o n h a r d e n i n g . The a m o u n t of d i s solved copper m a y thus be v a r i e d by heat t r e a t m e n t . The aging o c c u r s in two s t a g e s . 7-9 The f i r s t p r e c i p i t a t e i s probably bcc c o p p e r - r i c h c l u s t e r s while the second p r e c i p i t a t e is fcc copper (e-phase), the stable s e c o n d phase. Both p r e c i p i t a t e s give s u b s t a n t i a l h a r d e n i n g at r o o m t e m p e r a t u r e but the s i t u a t i o n is not c l e a r at 77~ 7 In the p r e s e n t i n v e s t i g a t i o n , the t e m p e r a t u r e dependence of the yield s t r e s s f r o m 298 ~ to 77~ of an F e 1.67 at. pct Cu alloy a f t e r p r e s t r a i n i n g 0.2 pct at 365~ was studied in the s u p e r - s a t u r a t e d solid s o l u t i o n condition as well a s a f t e r v a r i o u s a m o u n t s of aging. To r e m o v e c a r b o n and n i t r o g e n , s a m p l e s were t r e a t e d in wet H2 and a l s o H2 r e a c t e d with ZrH2. The s t r a i n r a t e s e n s i t i v i t y was d e t e r m i n e d e n a b l i n g c a l c u l a t i o n of the a c t i v a t i o n volume and a c t i v a t i o n enthalpy. An F e - 0 . 5 at. pct Au alloy was a l s o studied. E X P E R I M E N T A L PROCEDURE The F e - l . 6 7 at. pct Cu alloy was s u p p l i e d by the U.S. Steel Corp. Bain L a b o r a t o r y a s a plate 1 by 3 in. in c r o s s - s e c t i o n . The c a r b o n content was 70 ppm b y weight and the n i
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