Effect of extrinsic grain-boundary defects on grain-boundary sliding resistance

Abstract

A bstract A computer simulation was performed to examine the role of extrinsic grainboundary ( GB) defects in the sliding of the S 11, 101 {131} symmetric tilt boundary in aluminium by means of an embedded-atom method potential. The sliding resistance of the equilibrium GB was determined f rom a computation of the dependence of the boundary energy on in-plane translation of one grain relative to the other. Low-energy f aulted structures were f ound that correspond to unstable intermediate states and the presence of stable partial GB dislocations. Extrinsic dislocations and disconnections were found to lower signi® cantly the sliding stress relative to the equilibrium GB. §1. Introduction Grain-boundary (GB) sliding, which involves the relative translation of two adjacent grains by shear parallel to the boundary plane, is an important mode of deformation in superplasticit y. A signi® cant fraction of the total strain measured during superplastic deformation is contributed by GB sliding. In some cases GB sliding is responsible for a substantial portion of the superplastic strain (Langdon 1994).GB sliding is known to occur by the motion of GB dislocations rather than by the simultaneous shear of the entire boundary (Kegg et al. 1973, Sutton and Ballu 1995). Some of the GB dislocations were observed to originate from GB sources, but most were produced by interaction of lattice dislocations with the GB. There have been numerous studies of the interaction of lattice dislocations with GBs (Pond and Smith 1977, Darby et al. 1978, Clark and Smith 1979, Kokawa et al. 1983, King and Chen 1984. GBs appear to be e cient sinks for lattice dislocations since signi® cant elastic energy reductions occur during dissociation of lattice dislocations into GB dislocations (Kokawa et al. 1983). A GB dislocation is expected to be glissile if its Burgers vector is parallel to the boundary plane. However, Ishida (1970) and King and Smith (1980) have pointed out that, even if the Burgers vector is parallel to the boundary plane, a GB dislocation generally is accompanied by a geometrically necessary step. In this case, glide of the GB dislocation may require movement of the step associated with it. A s shown by Mori and Tangri (1979), the probability of GB sliding by simple shear is small since there are few GB dislocations without intrinsic steps. This probability was found to decrease as the reciprocal density R of the coincident site lattice (CSL) increased.