The melting behavior of equiaxed grain boundaries

Experiments were performed on bicrystals of high-purity (99.986% and 99.9987%) tin to investigate the dependence of the melting behaviour of grain boundaries, particularly those of the "equiaxed" type, on applied stress, orientation difference, boundary orientation, growth conditions, and purity. By a comparison with previous work on columnar boundaries it was concluded that there was no structural difference between equiaxed and columnar boundaries having the same geometry. Any difference in behaviour was attributed to the build-up of impurities at the equiaxed boundary due to the zone-refining effect on growth. Calculations on the diffusion of Pb, the major impurity in Sn, show that the impurity content does not decrease appreciably in the duration of the test, and consequently could be considered constant from specimen to specimen. Thus, the melting behaviour is a property of the atomic structure of a grain boundary, quite apart from any effect due to impurities. Tilt boundaries exhibit no dependence of melting behaviour on orientation difference above 16°, whereas twist boundaries do. A peak of minimum disorder appears at 41°. The dependence is predictable from coincidence plots.Applied Science, Faculty ofMining Engineering, Keevil Institute ofGraduat