Phase-field modeling of isothermal dendritic coarsening in ternary alloys

The process of isothermal dendritic coarsening in a Ni-Al-Nb ternary system is simulated by using the phase-field method. The coarsening behaviors and coarsening mechanisms with different solid fractions f_S are investigated in detail. Simulated results show that, as f_S increases from 64% to 88%, the coarsened morphology of the liquid phase varies from connected plate to rod-like shape, which is similar to the lamellar to rod transition in eutectics due to the interface energy. Simulated results also indicate that the dendritic isothermal coarsening mechanisms are dominated by remelting of the third arms, coalescence and smoothing in the case of low solid fraction, while in the case of high solid fraction, coalescence, smoothing, Rayleigh instability, rounding and shrinking away of small liquid droplets are the main mechanisms of dendritic coarsening. The increase of liquid diffusion coefficient D^L will also trigger the Rayleigh instability to accelerate the morphology transition from plate to cylindrical for the liquid phase.