Effect of nitrogen on the strength of thorium
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S T U D I E S of the m e c h a n i c a l p r o p e r t i e s of t h o r i u m by Zerwekh and Scott I and of t h o r i u m - c a r b o n a l l o y s by P e t e r s o n and Skaggs z have shown that the flow s t r e s s can be r e s o l v e d into an a t h e r m a l c o m p o n e n t and a t h e r m a l l y activated c o m p o n e n t . The t h e r m a l c o m p o n e n t a p p e a r s to r e s u l t f r o m a s t r o n g , s h o r t r a n g e i n t e r a c t i o n between the g l i s s i l e d i s l o c a t i o n s and individual i n t e r s t i t i a l solute a t o m s . This t h e r m a l l y a c t i v a t e d c o m p o nent in the c a s e of t h o r i u m - c a r b o n alloys d e c r e a s e d r a p i d l y with t e m p e r a t u r e , produced a m a r k e d s t r a i n r a t e effect and was d i r e c t l y p r o p o r t i o n a l to the c a r b o n concentration. In view of the i n t e r e s t i n g b e h a v i o r of t h o r i u m - c a r b o n a l l o y s , a study of the effect of n i t r o g e n on t h o r i u m was u n d e r t a k e n . N i t r o g e n is not as s o l u b l e in t h o r i u m as is c a r b o n but Gerds and Mallett 3 r e p o r t a s o l u b i l i t y l i m i t of 1100 ppm at 1000~ Above the s o l u b i l i t y l i m i t , alpha t h o r i u m is in e q u i l i b r i u m with t h o r i u m m o n o n i t r i d e , which has the NaC1 s t r u c t u r e . Goldhoff 4 e t a l r e p o r t e d that n i t r o g e n i n c r e a s e d the yield s t r e n g t h and u l t i m a t e t e n s i l e s t r e n g t h of t h o r i u m at r o o m t e m p e r a t u r e and also r e p o r t e d a g e - h a r d e n i n g effects in t h o r i u m - n i t r o gen alloys c o n t a i n i n g 0.18 and 0.41 wt pct n i t r o g e n a f t e r quenching f r o m 850~ and aging at 300~ This im, e s t i g a t i o n m e a s u r e d the flow s t r e s s of alloys c o n t a i n i n g up to 600 ppm n i t r o g e n f r o m 4.2 K to 773 K and at a n u m b e r of s t r a i n r a t e s . The h i g h e r n i t r o g e n alloys w e r e studied both a s - q u e n c h e d and in the aged condition. E X P E R I M E N T A L PROCEDURE Specimen P r e p a r a t i o n and C h a r a c t e r i z a t i o n A t h o r i u m b i l l e t p r e p a r e d by the m a g n e s i u m i n t e r m e d i a t e alloy p r o c e s s d e s c r i b e d by P e t e r s o n e l a l s was used to p r e p a r e all the s p e c i m e n s t e s t e d in this i n v e s t i gation. An a n a l y s i s of this m e t a l is given in T a b l e I. A t h o r i u m - n i t r o g e n m a s t e r a l l o y was p r e p a r e d by a r c m e l t i n g 30 g r a m s of t h o r i u m in a p r e d e t e r m i n e d v o l ume of n i t r o g e n gas. Alloys w e r e then p r e p a r e d by a r c - m e l t i n g p o r t i o n s of the t h o r i u m b i l l e t with the a p p r o p r i a t e a m o u n t of m a s t e r alloy. The alloys w e r e r e m e l t e d into an elongated mold and cold swaged into 6 m m diam r o d s . D. T. PETERSON is Senior Metallurgist, Ames Laboratory, ERDA and Professor
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