The Portevin-Le Chatelier Effect in hydrogenated nickel alloys
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P o r t e v i n - L e C h a t e l i e r Effect is o b s e r v e d in a l loys containing sufficient solute to lock d i s l o c a t i o n s when the s t r a i n r a t e , ~, a p p r o x i m a t e l y equals the product of the mobile dislocation d e n s i t y , p , and the solute diffusivity, D. ~ If the effect is to be o b s e r v e d , the solute m u s t lock d i s l o c a t i o n s and the t e s t i n g t e m p e r a t u r e m u s t be suff i c i e n t to m o b i l i z e the solute but not so high that the solute is e v a p o r a t e d f r o m the d i s l o c a t i o n s t h e m s e l v e s . If this is so, then hydrogen should cause the P o r t e v i n Le C h a t e l i e r Effect in n i c k e l alloys when test conditions a r e favorable and d i s l o c a t i o n - l o c k i n g e x i s t s . The P o r t e v i n - L e C h a t e l i e r Effect in h y d r o g e n a t e d n i c k e l has b e e n studied in detail ~-4 and it is thought that solute diffusion to mobile d i s l o c a t i o n s is r e s p o n s i b l e for s e r r a t e d yielding. In this p a p e r the study of s e r r a t e d yielding in h y d r o g e n a t e d nickel 4 is expanded to cover h y d r o g e n a t e d n i c k e l - c o b a l t , n i c k e l c o p p e r , and n i c k e l - z i n c a l l o y s . EXPERIMENTAL By a method d e s c r i b e d in an e a r l i e r paper S 99.95 pct + nickel shot, 99.98 pct + oxygen free c o p p e r , 99.54 pct + cobalt, and 99.95 pct + zinc w e r e used to make alloy w i r e s . The w i r e s w e r e a n n e a l e d in v a c u o (10 -5 t o r r ) at v a r i o u s t e m p e r a t u r e s to yield a cons t a n t g r a i n d i a m e t e r of a p p r o x i m a t e l y 0.07 m m . Samples w e r e h y d r o g e n a t e d t h e r m a l l y in an autoclave at v a r i o u s p r e s s u r e s a r o u n d 7 atm and v a r i o u s t e m p e r a t u r e s (approximately 500~ below the p r e v i o u s a n n e a l i n g t e m p e r a t u r e s f i A c o n s t a n t hydrogen conc e n t r a t i o n of 0.08 at. pct H was used for most of the work. T h i s was d e t e r m i n e d by e q u i l i b r a t i n g , q u e n c h ing, and a n a l y s i s using an exhalograph. All m e c h a n i c a l t e s t s w e r e done u s i n g a " h a r d " t e n s i l e m a c h i n e b u i l t to the design of Adams. 7 All s a m p l e s w e r e 1 m m diam with a c o n s t a n t gage length of 38 m m . L o a d - t i m e c u r v e s at ~ = 1.37 x 10 -4 s e c - ' w e r e a u t o g r a p h i c a l l y r e c o r d e d . Suitable c o r r e c t i o n s J. S. BLAKEMORE,formerly International Nickel Research Fellow, Department of Metallurgy, University of Newcastle, N.S.W., Australia, is Research Scientist, Australian Atomic Energy Commission, Sutherland, N.S.W. Manuscript submitted March 14, 1969. METALLURGICALT R A N S A C T I O N S
for m a c h i n e e l a s t i c i t y w e r e made on the s t r e s s - s t r a i n legends which a r e i n s e r t e d in all f i g u r e s in this p a p e r . The low t e m p e r a t u r e s w e r e obtained by using s u
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