Pattern formation mechanism of directionally-solidified MoSi₂/Mo₅Si₃ eutectic by phase-field simulation

A phase-field study has been conducted to obtain an understanding of the formation mechanism of the script lamellar pattern of MoSi₂/Mo₅Si₃ eutectic composite, which is a promising candidate for high-temperature structural application. The spacing of the lamellar pattern in the simulation results shows good agreement with that of experimental observations and analytical solutions under three growth velocities: 10 mm h⁻¹, 50 mm h⁻¹, and 100 mm h⁻¹. The discontinuity of Mo₅Si₃ rods, in contrast to the regular eutectic with a continuous pattern, is claimed to be caused by the instability of the solid-liquid interface. In this study, the implementation of Mo₅Si₃ nucleation over the solid-liquid interface has been proposed and successfully reproduced the characteristic of discontinuity. A highly random and intersected lamellar pattern similar to that observed in the ternary MoSi₂/Mo₅Si₃ eutectic alloyed with 0.1 at% Co has been obtained in simulation owing to the increase in the frequency of nucleation. In addition, it has been demonstrated that the inclination of the Mo₅Si₃ rod can be reproduced by taking into account the strong relaxation of lattice strain energy, which is generally considered to be negligible in eutectic reaction, as the result of the formation of ledge-terrace structure.