Low temperature strengthening in niobium-hydrogen single crystals
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(.) ZONE-~LTEO (b) C~t~3SEO Tr'K ze
SHEAR
STR~JN
Fig. 1--Resolved shear stress-shear strain curves of single
crystals of zone-melted and zone-melted plus vacuum outgassed niobium.
.3"K
0
100
200 TEMPERATURE
.Jr
/ o-Off.:]--.---
300
r
(~
Fig. 3--Temperature dependence of the resolved shear stress of niobium and niobium'hydrogen single crystals. -
I
0 Nb-OOlat %1~
9
, OI
02
.
0
b-'TR~N
9 ND- 0 ( ~ 8 (]1%H
OJ
0.2
Fig. 2--Resolved shear s t r e s s - s h e a r strain curves of single crystals of Nb-H alloys. I n s t r o n m a c h i n e at a s t r a i n r a t e of 8.3 x 10 -4 sec -~ S t r a i n r a t e change e x p e r i m e n t s w e r e p e r f o r m e d by c y c l i n g b e t w e e n s t r a i n r a t e s of 8.3 • 10 -s sec -~ and 8.3 • 10 -4 sec -1 at a c o n s t a n t t e m p e r a t u r e . T e s t t e m p e r a t u r e s above and below r o o m t e m p e r a t u r e w e r e o b t a i n e d by e m p l o y i n g v a r i o u s liquid b a t h s which w e r e c a r e f u l l y e q u i l i b r a t e d and c o n t r o l l e d to •176 Resolved shear s t r e s s - s t r a i n curves were calculated f r o m the l o a d - e x t e n s i o n data by c o m p u t e r . The c r i t i c a l r e s o l v e d s h e a r s t r e s s was t a k e n a s the p r o p o r t i o n a l l i m i t when yield points w e r e a b s e n t and a s the lower yield s t r e s s when d i s t i n c t u p p e r and lower yield points w e r e p r e s e n t . E x a m i n a t i o n of t y p i c a l s t r e s s - s t r a i n c u r v e s , F i g s . 1 and 2, v e r i f i e s that the p a r t i c u l a r choice of yield or flow s t r e s s p a r a m e t e r does not i n f l u e n c e any g e n e r a l t r e n d s in the r e s u l t s o r conclusions derived therefrom.
2) E X P E R I M E N T A L RESULTS 2.1) E f f e c t s of P u r i f i c a t i o n In p r e v i o u s i n v e s t i g a t i o n s the a u t h o r s 14'16'17 and o t h e r s le-2~ have d e m o n s t r a t e d that r e d u c i n g the level of r e s i d u a l i n t e r s t i t i a l s o l u t e s in the bcc m e t a l s d e c r e a s e s both the t h e r m a l and a t h e r m a l c o m p o n e n t s of the yield o r flow s t r e s s , e l i m i n a t e s the i n i t i a l yield p o i n t , and c a u s e s c h a n g e s in the work h a r d e n i n g behavior. Such r e s u l t s for n i o b i u m a r e s u m m a r i z e d in F i g s . 1 and 3. R e s o l v e d s h e a r s t r e s s - s h e a r s t r a i n c u r v e s 1220-VOLUME2, APRIL 1971
for z o n e - m e l t e d n i o b i u m and o u t g a s s e d n i o b i u m tested at 298 ~ and 77~ a r e given in Fig. 1. The yield s t r e s s is r e d u c e d f r o m 2.7 to 0.9 kg p e r sq m m at 298~ and f r o m 35 to 17 kg p e r sq m m with a l a r g e a t t e n d a n t i n c r e a s e in d u c t i l i t y at 77~ The yield point o b s e r v e d in the z o n e - m e l t e d c r y s t a l s at 298~ is e l i m i n a t e d and the t h r e e stages of work h a r d e n i n g b e c o m e much b e t t e r defined by p u r i f i c a t i o n .
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