Directional solidification experiments were carried out with Al—Pb alloys under the effect of a direct current(DC).The experimental results show that the DC causes a migration of the minority phase droplets(MPDs) from the middle part to the surface region of the sample.Samples with either a finely dispersed microstructure or a shell/core structure were obtained by solidifying the alloy under the effect of the properly selected DC densities.A model was developed to describe the microstructure evolution in an immiscible alloy directionally solidified under the effect of the DC.The microstructure formation in the Al—Pb alloys was calculated.The numerical results are in favorable agreement with the experimental ones.They demonstrate that the DC affects the microstructure formation mainly through changing the spatial motions of the MPDs and the temperature field of the melt in front of the solid/liquid interface.The formation mechanisms of the finely dispersed microstructure as well as the shell/core structure were sufficiently clarified.
A model was developed to describe the microstracture evolution in a directionally solidified ternary monotectic alloy.The directional solidification experiments were carried out on Al-3Pb-lSn(wt%) alloys by using a Bridgman apparatus.The microstracture evolution in the directionally solidified sample was calculated.The numerical results agree well with the experimental ones.It is demonstrated that the nucleation of the minority phase droplets occur at two different positions.One corresponds to the liquid-liquid decomposition,which occurs in front of the solidification interface.The other is at the liquid/solid interface.The nucleation rate of the minority phase droplets at the liquid/solid interface is significantly higher than at the position in front of the solidification interface.The characteristic of the nucleation process leads to a bimodal size distribution of the minority particles in the directionally solidified sample.
