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The kinetics of interfaces in alloy solidification pose a classic free boundary problem. This paper introduces an approach that amalgamates the distinctive characteristics of sharp and diffuse interface models. The motion of the diffuse interface is governed by the phase-field equation featuring a traveling wave function [I. Steinbach, Modell. Simul. Mater. Sci. Eng. 17(7), 073001 (2009)]. To emulate solute rejection in the sharp interface model, the concept of the middle-obstacle and the casting operation are employed. Moreover, the undercooling along the interface normal is flattened to minimize the impact of bulk undercooling in the interface and the associated effects, such as stretching and arc-length diffusion. Notably, the results for interface kinetics under both equilibrium and non-equilibrium conditions closely approximate their analytical solutions, all achieved with artificially wide interfaces and comparatively low computational cost.

A new diffuse interface model has been proposed in this study for simulating binary alloy solidification under universal cooling conditions, involving both equilibrium and non-equilibrium solute partitioning. Starting from the Gibbs-Thomson equation, which is the classical theory that describes the dynamics of a sharp interface, the phase-field equation is derived using a traveling wave solution that represents a diffuse interface. To tackle the spurious effects caused by the variation of liquid concentration within the diffuse interface with artificial width, a middle obstacle is introduced to sharpen the diffuse interface and an invariant liquid concentration can be found for determining a constant undercooling in the interface normal direction. For slow solidification under equilibrium conditions, the convergence performance of the dendrite tip shows superior invulnerability to the width effect of the diffuse interface. For rapid solidification under non-equilibrium conditions, the output partition coefficients obtained from the steady-state concentration profiles agree with the input velocity-dependent function. The proposed model is promising to be an indispensable tool for the development of advanced alloy materials through the microstructure control of solidification under a wide range of cooling conditions.

In order to control the grain structure of multi-crystalline (mc) silicon during directional solidification, the development process of grain boundaries (GBs) with respect to the temperature gradient should be understood. A phase-field model incorporated with anisotropic interface energy and anisotropic attachment kinetic coefficient has produced the faceted shape of a growing silicon crystal, which is in agreement with experimental observation. The growth of coupled silicon grains under various growth velocities has been simulated to see the morphology of the solid-liquid front and the development process of the GBs. It has been found that the direction of GB is governed by either the kinetic rule or the equilibrium rule at the grain groove, depending on the growth velocity and the orientation relationship between grains on two sides. The GB beneath a groove with facet-facet surfaces follows the bisector of the two surfaces, while the direction of a GB stays far from the bisector when the groove has a rough surface. This research provides a numerical approach to predicting grain boundary development and gaining insights from grain structure evolution in mc-silicon, which can be potentially applied for high-efficiency and low-cost solar cells.

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 rates: 10 mm/h, 50 mm/h, and 100 mm/h. 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 0.1at% 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., Comment: 19 pages, 13 figures