Microstructure Formation in a Directionally Solidified Immiscible Alloy

Directional solidification experiments were carried out with an Al-Pb alloy. A model was developed to analyze the microstructure evolution in a directionally solidified immiscible alloy by taking into account the common actions of the nucleation and the diffusional growth/shrinkage of the minority phase droplets (MPDs), the spatial phase segregation, and the convections of the melt. The microstructure formation under the practical situations was calculated. The numerical results agree with the experimental ones well. They demonstrate that the convective flow against the solidification direction causes an increase in the nucleation rate, while the convective flow along the solidification direction causes a decrease in the nucleation rate. The convective flow leads to a more nonuniform distribution of the MPDs in the melt. It causes an increase in the size of the largest MPDs and is against the obtaining of the immiscible alloys with a well-dispersed microstructure.