PHASE AND MICROSTRUCTURE SELECTION INDIRECTIONALLY SOLIDIFIED PERITECTICALLOYS UNDER CONVECTION CONDITION

Abstract

For the phase and microstructure selection during directional solidification of peritectic alloys,the solute distribution ahead of the solid-liquid interface plays a fundamental role.To study the solute distribution under convection condition,the convection factor(Δ) was introduced into the boundary layer model and Bower-Brody-Flemings model and the solute distribution ahead of the planar and cellular interfaces under convection condition during directional solidification were obtained. Based on these solute distributions,the nucleation and constitutional undercooling criterion and the assumption that the maximum interface growth temperature of phase growth is more stability during directional solidification,the nucleation conditions of new phase ahead of the growth interface under convection condition were analyzed and the phase and microstructure selection map in directionally solidified peritectic alloys under convection condition was developed.Compared with the Hunziker's model that is suit for the phase and microstructure selection of peritectic alloys under diffusive condition and the Karma's model that is used to explain the banded structure formation under convection condition,this model can return to the Hunziker's model under diffusive condition and include the Karma's model.Additionally,this model can predict the mixed banded structure and the coupled growth between the cellular primary phase and planar peritectic phase that can form under convection condition,which these models cannot display.In order to estimate the validity of this model,directional solidification experiments of Sn-1.6%Cd(mass fraction) peritectic alloy were carried out under different convection conditions.The results show that the experimental results are in agreement with the calculated results.That is,this model can explain the complex directional solidification microstructures of peritectic alloys appropriately.