Convection-induced novel oscillating microstructure formation in peritectic systems

Abstract

Critical experiments have been carried out to examine the formation of banded microstructures in the peritectic Sn–Cd system in both diffusive and convective regimes. It is shown that in directional solidification studies by a conventional method in 3–6 mm diameter tubes, a new oscillatory structure forms which is induced by convection. In this oscillatory mode the two phases form an interconnected network which appear as bands only on specific cross-sections. A successive sectioning has revealed that this microstructure consists of a large tree-like structure of the primary phase that is surrounded by the peritectic phase. A new experimental technique is developed to obtain diffusive growth by directionally solidifying in very thin samples. As the diameter of the sample was reduced, convection effects were reduced and the oscillatory structure was found to disappear for compositions in the hyperperitectic region. Only in the hypoperitectic composition range, where band formation is predicted by the diffusive model, discrete bands have been observed to form in thin samples of diameter 0.6 mm or less.