Creep behavior of a tungsten alloy dispersion strengthened by ZrO 2
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S E V E R A L models have b e e n p r o p o s e d for d e s c r i b i n g s t e a d y - s t a t e c r e e p of m e t a l s at t e m p e r a t u r e s g r e a t e r than 0.5 T m . Bird, Mukherjee, and Dorn t concluded that the s t e a d y - s t a t e c r e e p r a t e of a m e t a l at high t e m p e r a t u r e c o n t r o l l e d by any diffusion m e c h a n i s m is _ AD~b
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and W e e r t m a n s p r o p o s e d a d e t a i l e d model of c r e e p for a m e t a l c o n t a i n i n g a d i s p e r s e d second phase. The model, b a s e d on W e e r t m a n ' s e a r l i e r t h e o r y ~ and the Orowan m e c h a n i s m of bowing of d i s l o c a t i o n s , predicteq l i n e a r s t r e s s d e p e n d e n c y of n = 1 in the low s t r e s s r e gton and an exponent n = 4 in the higher s t r e s s r e g i o n . E x p e r i m e n t s by Ansell and Lenel m on an a l u m i n u m SAP-type a l l o y v e r i f i e d the s t r e s s d e p e n d e n c y for n = 4 but did not find the l i n e a r s t r e s s d e p e n d e n c y b e c a u s e the s t r a i n r a t e a p p r o a c h e d v a l u e s i m m e a s u r a b l y s m a l l . F u r t h e r m o r e , the v a l u e s of s t r a i n r a t e c a l c u lated f r o m the t h e o r y were about 104 l a r g e r than the experimental values. The p r e s e n t work was c a r r i e d out to f u r t h e r t e s t the v a l i d i t y of the A n s e l l - W e e r t m a n model with a n o t h e r dispersoid system, namely tungsten strengthened by a ZrO 2 d i s p e r s o i d .
where ~ is the s t e a d y - s t a t e c r e e p r a t e , D the d i f f u s i v ity, p the s h e a r modulus, b the B u r g e r s v e c t o r , a the applied s h e a r s t r e s s , A and n a r e c o n s t a n t s that d e pend upon the o p e r a t i n g m e c h a n i s m , k is B o l t z m a n n ' s constant, and T is the a b s o l u t e t e m p e r a t u r e . V a r i o u s models provide for the value of n, the s t r e s s d e p e n dency, f r o m 1 to 5.5. The N a b a r r o - H e r r i n g c r e e p , z's a v i s c o u s flow with n = 1, has b e e n well e s t a b l i s h e d o v e r a n a r r o w t e m p e r a t u r e r a n g e . In c a s e s of c r e e p c o n t r o l l e d by d i s l o c a t i o n c l i m b , the p a r a m e t e r n t a k e s a r a n g e of v a l u e s depending upon the c r y s t a l s t r u c t u r e , x MATERIAL AND PROCEDURE F o r fcc m e t a l s , n r a n g e s f r o m 4.2 to 5.5; for bcc, f r o m 4.0 to 4.4; and for hcp, f r o m 3.0 to 5.5. S p e c i m e n s w e r e p r e p a r e d by powder m e t a l l u r g i c a l In s o l i d - s o l u t i o n a l l o y s , v i s c o u s glide motion of d i s methods f r o m b l e n d s of c o m m e r c i a l t u n g s t e n powders locations c o n t r o l s c r e e p at high s t r e s s e s , t Gifkin4 r e and a ZrW~ powder. The c o m m e r c i a l 3/~m t u n g s t e n c e n t l y p r o p o s e d that g r a i n - b o u n d a r y s l i d i n g r e s u l t e d powders t y p i c a l l y contained about 0.11 w t - p c t oxygen. in a
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