Effect of low-pressure oxygen on the creep properties of W-25 pct Re
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R. L. WAGNER
Th e c r e e p p r o p e r t i e s of W-25 p c t Re w e r e m e a s u r e d in l o w - p r e s s u r e oxygen to s i m u l a t e i t s i n t e r a c t i o n with oxygen d e r i v e d f r o m the d e c o m p o s i t i o n of oxides at high t e m p e r a t u r e s . Th e r e s u l t s a r e c o m p a r e d with the c r e e p p r o p e r t i e s of the alloy in a v a c u u m of 10 -7 t o r r . B e t w e e n 1650 ~ and 2000~ and s t r e s s e s of 1000 and 2000 p s i , the c r e e p r a t e s w e r e l o w e r in 10 -~ t o r r Oz thart in v a c u u m by f a c t o r s as high as 250; h o w e v e r , r u p t u r e s t r a i n s w e r e only 2 to 7 p ct in the 02 e n v i r o n m e n t . S p e c i m e n s e x p o s e d to oxygen b e f o r e c r e e p t e s t i n g in v a c u u m w e r e a l s o s t r o n g e r than s p e c i m e n s t e s t e d in v a c u u m . Th e oxygen content of s p e c i m e n s t e s t e d in oxygen was g e n e r a l l y l o w e r than in the a s - r e c e i v e d m a t e r i a l . T h e l o w e r c r e e p r a t e s of W-25 p ct Re in oxygen a r e a t t r i b u t e d to the p r e s e n c e of a s u r f a c e l a y e r of cr p h a s e . T h i s r e a c t i o n l a y e r r e s u l t s f r o m the f o r m a t i o n of v o l a t i l e tungsten oxides at a h i g h e r r a t e than r h e n i u m o x i d e s .
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high temperature mechanical properties of refractory metals and their alloys are environment sensitive. The creep strength of vanadium alloys at 600~ is significantly higher in vacuum of approximately 10-6 than at 10-9 torr because of their contamination by the residual gases at the higher pressure. The strengthening results from the formation of dispersed carbide and oxide phases. I At 1000~ the creep strength of Nb-i pct Zr is also initially increased by contamination with oxygen at approximately 10-5 torr. Extended testing, however, results in weakeningdue to the formation of a grain-boundary film of NbO.2 Low-pressure oxygen (e.g. 10-5 torr) reduces the creep strength of tungsten above 1650~ because it sublimes as volatile oxides and cavities in the metal grow faster in the oxygen than in vacuum.3 Because of the significant and often pronounced environmental effects on the mechanical behavior of refractory metals at creep temperatures, it is not surprising that numerous investigations have been concerned with studies of the reactions betweenrefractory alloys and low-pressure gases. A review of the principles governing the reactions that occur in vacuum environments shows that pressures of active gas must be below approximately 10-8 torr to minimize contamination.4 C o n s e q u e n t l y , g r e a t e f f o r t and e x p e n s e have b e e n expended to m e a s u r e the c r e e p p r o p e r t i e s in the b e s t a t t a i n a b l e v a c u u m ; h o w e v e r , the r e f e r e n c e s c i t e d above a l s o i n d i c a t e that data c o l l e c t e d u n d e r the a n t i s e p t i c conditions of a high v a c u u m cannot be e x t r a p olated to the m o r e r e
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