High temperature creep cavitation mechanisms in a continuously cast high purity copper
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T H E phenomenon of c r e e p cavitation limiting the application of m e t a l s at high t e m p e r a t u r e s is of cons i d e r a b l e i n t e r e s t in p r e s e n t technology. Although this p r o b l e m has r e c e i v e d much attention since the time of its f i r s t investigation,~ the m e c h a n i s m s of void f o r m a t i o n a r e s t i l l subject to d i s c u s s i o n . Void nucleation is a s s u m e d g e n e r a l l y to r e q u i r e the action of a s h e a r s t r e s s , z It has been suggested that g r a i n boundary sliding, GBS, may nucleate voids at ledges 3-6 that a r e f o r m e d when lattice slip i n t e r s e c t s with a g r a i n boundary, or at other g r a i n boundary d i s c o n t i n u i t i e s . In such a c a s e nucleation may occur when GBS p r o v i d e s a tensile s t r e s s a c r o s s the ledge, and when sliding p r o d u c e s cavity space at the ledge at a f a s t e r r a t e than diffusion can supply atoms to fill it up again. 6 Other suggestions a r e that voids may nucleate in the manner outlined in Ref. 3 due to the i n t e r a c t i o n of GBS with " g r o s s i r r e g u l a r i t i e s " in the boundary that a r e caused, e.g., by p a r t i c l e pinning,7 or at " t r i p l e p o i n t s " of high angle g r a i n boundaries with a low angle grain boundary (dislocation boundary), a At the p r e s e n t time the view is widely accepted that void nucleation takes place at nonwetting second phase p a r t i c l e s .2,9-11 T h e o r i e s of void growth during c r e e p a r e based on the idea of vacancy condensation under the action of the s t r e s s , Crn, n o r m a l to the g r a i n boundary, lz The r a t e of growth, ~-, of the void r a d i u s is found 13'14 to depend on s t r e s s and t e m p e r a t u r e as ~" oc Dgb (an - P ) / T
[1 ]
(Dg b = g r a i n boundary diffusivity, P = h y d r o s t a t i c
A. RUKWIED,formerly Physicist,EngineeringMetallurgySection, Metallurgy Division,National Bureau of Standards, U. S. Department of Commerce, Washington,D. C., is now with AEG-Telefunken,Bereich Forschung und Entwicklung, Frankurt(Main)-Niederrad,Germany. Manuscript submitted July 6, 1971. METALLURGICAL TRANSACTIONS
p r e s s u r e s u p e r i m p o s e d on applied s t r e s s , T = a b solute t e m p e r a t u r e ) . Thus, g r a i n boundary diffusion controls the r a t e of void growth. In this a n a l y s i s all voids a r e a s s u m e d to be nucleated instantly at the outset of the c r e e p t e s t . However, it was shown recently15,16 that the void population i n c r e a s e s continuously during c r e e p . T h e r e is no e x p l i c i t theory as yet for the r a t e of growth of the void r a d i u s due to GBS which would p e r m i t a c o m p a r i s o n with Eq. [1]. However, a m e c h a n i s m has been suggested 17 which d e s c r i b e s the f o r m a tion and the growth of c r e e p c a v i t i e s by grain boundary d i s l o c a t i o n s which move by a mixture of c l i m b and glide, and thus a c c o m p l i s h GBS. The r a t e o
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