The Portevin-Le Chatelier Effect in hydrogenated nickel
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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 was studied in detail by B o n i s z e w s k i and Smith 1 and l a t e r by Wilcox and Smith. 2'3 B a s e d upon this work and work by Keh et al. 4 on F e - N a l l o y s , it j s thought that solute diffusion to mobile d i s l o c a t i o n s i s r e s p o n s i b l e for the s e r r a t e d yielding. Since it is the r e s u l t of s i m u l t a n e o u s aging and s t r a i n i n g it is thought that the a c t i v a t i o n e n e r g y of the p r o c e s s is u s u a l l y the s a m e as that for the diffusion of the locking solute. T h e r e f o r e , for h y d r o g e n a t e d n i c k e l , hydrogen diffusion to the d i s l o c a t i o n s is thought to be rate c o n t r o l l i n g . As w e l l , r e c e n t w o r k by Keh et al. 4 on F e - N a l l o y s has e m p h a s i z e d the following: 1) The a c t i v a t i o n e n e r g y for the i n i t i a t i o n of the s e r r a t i o n s is equal to that for the solute diffusion. 2) The i n i t i a t i o n t e m p e r a t u r e for the s e r r a t i o n s (Lower C r i t i c a l T e m p e r a t u r e ) is s t r o n g l y c o n c e n t r a tion dependent. 3) The t e m p e r a t u r e at which the s e r r a t i o n s d i s a p p e a r is m i l d l y c o n c e n t r a t i o n dependent. 4) The a c t i v a t i o n e n e r g y for the d i s a p p e a r a n c e of the s e r r a t i o n s equals (QD + AE) where QD is the a c tivation e n e r g y for solute diffusion and AE iS the binding e n e r g y of the solute to the d i s l o c a t i o n s . This paper e x a m i n e s the p o s t u l a t e s made by Keh et al. on F e - N alloys in r e l a t i o n to h y d r o g e n a t e d n i c k e l s a m p l e s , and c o m p a r e s the p r e s e n t w o r k with that of B o n i s z e w s k i and Smith. 1 EXPERIMENTAL F o r this w o r k 99.95 wt pct + Ni c o n t a i n i n g 0.010 wt pct C, 0.003 wt pct Cu, and 0.04 wt pct Fe was used. The final w i r e s w e r e a n n e a l e d in vacuo (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 c o n s t a n t g r a i n d i a m e t e r of ~0.07 m m . S a m p l e s 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, s T h i s y i e l d s g r e a t e r homog e n e i t y of hydrogen than the cathodic h y d r o g e n a t i o n e m p l o y e d by B o n i s z e w s k i and Smith. ~ F o u r hydrogen 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. METALLURGICALTRANSACTIONS
l e v e l s w e r e used, n a m e l y , 0.04, 0.08, 0.13, and 0.21 at. pct H. 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 sile m a c h i n e b u i l t to the design of Adams. 6 All s a m ples w e r e 1 m m d i a m , 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 four s t r a i n r a t e s (~
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