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6. R. E Reed and R. J. Msenault. Scr. Met., 1976, vol. 10, p. 1003 7 D. G. Atteridge, O. A. Chariot, A. B Johnson, Jr., J. R Remark, and R. E. Westerman Conf. 750989, ORNL, p. 234, 1976. 8. M. R. Louthan, G R. Caskey, D E. Rawl, Jr., and C. W. Krapp Conf. 750989, ORNL, p. 117, 1976. 9. A. W Thompson Mater. Sct Eng., 1975, vol 21, p. 41. 10 H K Birnbaum: Private communication, Unwers,ty of Illinois, 1976.

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Fig. 1 - T h e r e s o l v e d y i e l d s t r e s s of n i o b i u m s i n g l e c r y s t a l s a s a f u n c t i o n of t e m p e r a t u r e .

s t r a i n c u r v e s of the t r i t i d e s a m p l e s above 108 K a r e s i m i l a r to that of the u n c h a r g e d s a m p l e s . However, below 108 K, twinning o c c u r r e d a f t e r m a c r o y i e l d i n g in the charged s a m p l e s . The v a r i a t i o n of the y i e l d s t r e s s with t e m p e r a t u r e of the u n c h a r g e d and t r i t i u m charged s a m p l e s is shown in Fig. 1. The yield s t r e s s of the t r i t i u m c h a r g e d s a m p l e s is c o n s i s t e n t l y higher than that of the u n c h a r g e d s a m p l e s . The g e n e r a l i n c r e a s e in the yield s t r e s s is b e l i e v e d to be caused by the 3He p r e s e n t . The high t e m p e r a t u r e yield s t r e s s of n i o bium c h a r g e d with h e l i u m i n c r e a s e s with i n c r e a s i n g h e l i u m c o n c e n t r a t i o n . 7 S i m i l a r s t r e n g t h e n i n g in Type 304L s t a i n l e s s s t e e l was a t t r i b u t e d to h e l i u m bubbles. 8'9 T h e r e is a local p e r t u r b a t i o n at 120 K in the t e m p e r a t u r e dependence of the yield s t r e s s of the t r i t i u m charged s a m p l e s . Chen and A r s e n a u l t t w e r e able to obtain an a c t i v a t i o n e n e r g y of 0.04 eV for the m e c h a n i s m r e s p o n s i b l e for the p e r t u r b a t i o n in the n i o b i u m hydrogen s y t e m . If it i s a s s u m e d that the p e r t u r b a t i o n o b s e r v e d in the n i o b i u m - t r i t i u m s y s t e m is due to a s t r e s s induced r e o r i e n t a t i o n of a t r i t i d e then an a c t i vation e n e r g y can be e s t i m a t e d f r o m the peakshift with the n i o b i u m - h y d r o g e n s y s t e m . The e s t i m a t e d value is 0.062 eV, which is about 1.5 t i m e s l a r g e r than that of hydride r e o r i e n t a t i o n . This i n c r e a s e in a c t i v a t i o n energy is not that expected f r o m a s i m p l e isotope c o n s i d e r a t i o n . 2 B i r n b a u m has e x p e r i m e n t a l l y d e t e r m i n e d the a c t i vation e n e r g y of diffusion of t r i t i u m in niobium, t~ The d i f f e r e n c e in a c t i v a t i o n e n e r g y of diffusion between hydrogen and t r i t i u m is in a g r e e m e n t with the d i f