Morphology of cellular precipitation of tin from lead-tin bicrystals

Lead-tin alloy bicrystals misoriented by specified tilts from the [100] axis have been grown and some features of the cellular (discontinuous) precipitation of tin from the alloy have been investigated. In this process cells originate at the bicrystal grain boundary. The habit plane is found to be the set {111} of lead. The crystallographic relations between the tin plate and the lead matrix are found to be [001]sn∥〈110〉pb and {310}sn∥{111}pb. We suppose that this habit and orientation relationship is that of minimum interfacial energy. The tin plate is first observed along the grain boundary in a position such that one side of it lies on the habit plane of an adjacent grain, but owing to the tilt angle, the opposite side is not on a habit plane of the other grain. Then the interfacial energies of the two sides of the plate are different and this provides a driving force which will lead the boundary to move around the plate to replace the higher energy interface by the lower. This replacive motion not only transfers the plate into the matrix but also enables it to branch, which is essential to the formation of a cell. The choice of the habit plane from one of the adjacent grains by the tin plate, as it grows along the boundary, is found to depend on the local grain boundary orientation, which in turn determines the cell orientation. © 1967.