Intergranular fracture in an Al-15 Wt Pct Zn alloy
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solids having the rock salt structure. It can be d e m o n strated in AgC1, LiF, NaC1, and MgO. Johnston5 has investigated the role of g r a i n boundaries in c r a c k nucleation by loading bicrystals of MgO in such a way that slip bands originating in the crystal impinged on the boundary. The boundary misorientation was v a r i e d and the probability of c r a c k nucleation at the boundary due t o blocking of the slip band was found to depend on the d e g r e e of misorientation at the boundary. Cleavage c r a c k s appear t o be nucleated in MgO bec a u s e of s t r e s s concentrations resulting from slip band blockage at the g r a i n boundary. The s t r e s s concentration effect of blocked inhomogeneous plastic flow leading t o m i c r o c r a c k formation was f i r s t postulated by Z e n e r1 and analyzed in detail by Stroh.s Both predicted high tensile and s h e a r s t r e s s concentrations a h e a d of a blocked slip band. Presumably the c r a c k nucleation mechanism of blocked inhomogeneous plastic flow leading t o cleavage can apply t o intergranular c r a c k nucleation provided the cohesive strength of the g r a i n boundary is less than the c l e a v a g e strength of the adjacent g r a i n . An important step in describing f r a c t u r e is t o decide whether the critical event in the f r a c t u r e p r o c e s s is the nucleation of a c r a c k or its subsequent growth. For example, if the nucleation stage of fracture is a s s u m e d t o be c r i t i c a l then, once a c r a c k has started t o spread, it will continue expanding until f r a c t u r e is complete provided the external s t r e s s is maintained. One criterion used for deciding which s t a g e is controlling is a comparison of whether the fracture energy for growth is g r e a t e r or less than the free surface energy associated with nucleation.9 One way for growth t o b e c o m e rate controlling is if a c r a c k is nucleated in a brittle second phase particle (presumably of a low s u r f a c e energy) and then r e q u i r e s a h i g h e r energy t o propagate into a surrounding ductile m a t r i x . However, if c r a c k propagation is confined t o a brittle phase, then the isolated dislocation pile-up m o d e l s show17 that nucleation is the c r i t i c a l event provided the f r a c t u r e energy for growth is the s u r f a c e energy appropriate t o the nucleation stage. Generally, the plastic flow involved in m e t a l s precludes this assumption, and a theoretical investigation by Tetelman1° has shown possible contributions t o the energy for c r a c k propagation. This analysis predicts a m a r k e d dependence of the f r a c t u r e energy on c r a c k velocity, VOLUME 2, DECEMBER 1971-3385
w h i c h w a s a t t r i b u t e d t o d i f f e r e n c e s in p l a s t i c f l o w at t h e c r a c k t i p . T h i s w o r k c o m b i n e d w i t h t h a t of H a h n11 s u g g e s t s t h a t t h e r e is a c o n n e c t i o n b e t w e e n s t r e s s s e n s i t i v i t y of d i s l o c a t i o n v e l o c i t y a n
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