Dynamics of microstructure formation in the two-phase region of peritectic systems

Abstract

Directional solidification experiments have been carried out in the two-phase region of the Sn–Cd peritectic system. A variety of different microstructures have been shown to form under intrinsically non-steady-state growth conditions in both the hyperperitectic and hypoperitectic compositions. The selection of microstructure is shown to be governed by the relative effects of convection, nucleation undercooling, nucleation rate and the growth competition between the two phases. Convection effects have been found to give significantly different microstructures than those predicted by the diffusive growth models. Experiments are thus carried out in samples of different diameters in which convection effects are shown to be reduced as the diameter of the sample is decreased. It is shown that diffusive growth condition is achieved in this alloy system when the sample diameter is <1.0 mm. Experimental studies are then carried out in hypoperitectic alloys in a composition range for which banded microstructures have been predicted by the diffusive growth model. This composition window is established experimentally, and the values of nucleation undercoolings of the primary and peritectic phase have been obtained. The experiments also confirmed the prediction of the model that the banding cycle occurs above and below the peritectic temperature and bands of primary and peritectic phases form by the alternate nucleation and lateral spreading of one phase onto the other. It is also found that when the sample diameter is reduced further, or growth conditions changed, several new dynamical oscillatory microstructures evolve and the formation of these microstructures is shown to be controlled by the competitive growth processes between the nuclei and the parent phase. This competitive growth is shown to be influenced by the distance between the nuclei or the nucleation rate. It is shown that the directional solidification of peritectic alloys provides a means to quantitatively study the complex process of dynamical evolution of two-phase microstructures in which fluid flow, nucleation undercoolings, nucleation rates of the two phases and the competitive growth of the two phases control the evolution of complex two-phase microstructures.