A comparison of the effect of neutron irradiation on high purity vanadium and vanadium oxygen alloys
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T H E changes in the yield s t r e s s at low t e m p e r a t u r e s produced by n e u t r o n i r r a d i a t i o n appear to be a v e r y s e n s i t i v e function of the i m p u r i t y i n t e r s t i t i a l content. 1-3 (In all c a s e s , the yield s t r e s s r e f e r s to the r e s o l v e d yield s t r e s s . ) If the yield s t r e s s T2; c o n s i s t s of t w o c o m p o n e n t s , the effective s t r e s s r* and the long r a n g e i n t e r n a l s t r e s s ri, then it is p o s s i b l e to e x a m i n e the effect of the n e u t r o n damage on both c o m p o n e n t s of Ty. The effective s t r e s s of y i e l d i n g of the n e u t r o n - i r r a d i a ted s a m p l e s is defined as r~RR, and the effective s t r e s s of yielding for the n o n - i r r a d i a t e d s a m p l e s is defined as T*. The d i f f e r e n c e of r~RR - T* is defined as AT*. It has b e e n e x p e r i m e n t a l l y o b s e r v e d that aT* at low t e m p e r a t u r e s i n c r e a s e s as the i n t e r s t i t i a l content d e c r e a s e s . The r e a s o n for the i n c r e a s e in AT* as the i n t e r s t i t i a l content d e c r e a s e s is not a p p a r e n t . One p o s s i b l e e x p l a n a t i o n is that if it is a s s u m e d that T* at 77 K i n c r e a s e s clue to a s m a l l i n c r e a s e of the i n t e r s t i t i a l c o n c e n t r a t i o n and T~RR i n c r e a s e s by only a s m a l l a m o u n t due to the s a m e s m a l l i n c r e a s e in the i n t e r s t i t i a l c o n c e n t r a t i o n , the net r e s u l t then would be a d e c r e a s e in ~ r * as the i n t e r s t i t i a l content i n c r e a s e s (Fig. 1). Previous transmission electron microscopy investigations of n e u t r o n - i r r a d i a t e d i n t e r m e d i a t e p u r i t y v a nadium r e v e a l e d a high d e n s i t y of defects of a u n i f o r m s i z e , about 130,~ in d i a m e t e r . 4 In a r e c e n t i n v e s t i g a t i o n of n e u t r o n - i r r a d i a t e d v a n a d i u m - o x y g e n alloys, Bajaj and W e c h s l e r 5 o b s e r v e d a d e c r e a s e in a v e r a g e defect s i z e of 138 to 7 8 ~ due to an i n c r e a s e in oxygen c o n c e n t r a t i o n f r o m 300 to 1500 at. ppm. They also observed a n a p p r o x i m a t e fivefold i n c r e a s e in defect d e n s i t y (i.e.,
f r o m 5 to 25 • Is defect c l u s t e r s per cm 3) for the s a m e i n c r e a s e in oxygen c o n c e n t r a t i o n . Loomis and G e r b e r 6 found, in the case of n e u t r o n i r r a d i a t e d Nb, that the d e n s i t y of defects d e c r e a s e s as the i m p u r i t y i n t e r s t i t i a l content d e c r e a s e s for the s a m e n e u t r o n fluence. "ffney were able to r e l a t e the change in ry at 300 K to n e u t r o n i r r a d i a t i o n s with the defect d e n sity. Tucker and W e c h s l e r , v who also i n v e s t i g a t e d n i o b i u m , also found a r e l a t i o n s h i p b e t w e e n the defect of d e n s i t y and the i n c r e a s e in the yield s t r e s
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