Phase-field simulation of the Si precipitation process in Mg2Si under an applied stress

The growth of Si precipitates in Mg2Si matrix under an external stress has been simulated by the phase-field method. The temporal evolution of a microstructure was determined by solving the time-dependent Cahn-Allen equations for the concentration and Ginzburg-Landau equations for the order parameters for precipitates. We observed that the external stress accelerates the Si precipitate growth, which is mainly due to reduction of the free energy of the Si precipitates under an external stress. Under a tensile stress, the Si precipitates preferentially align along the stress direction. Increasing the tensile stress accelerates the growth and decreases the aspect ratio of the Si precipitates. Finally, the Si precipitates aggregate to form a lath structure for all cases. Under a compressive stress, the growth rate is larger and the aspect ratio of the Si precipitates is smaller than those in the case of tensile stress at the same absolute value of the applied stress. We found the obtained microstructure contains uniformly dispersed Si lenticular precipitates in the Mg2Si matrix, which is suitable for thermoelectric materials because the phonons are frequently scattered at the interface between the Si precipitates and the Mg2Si matrix. Our present simulation results suggest that thermoelectric properties of this type of material will be significantly improved compared to the same material without stress.