Phase-field modeling of crystalline nanostructures formed by vapor-liquid-solid growth and liquid metal dealloying

Abstract

Phase-field modeling emerged as an effective method on the research of morphological evolution. In this thesis, we use this method to investigate the nanostructure formation in two projects. In the first project, we use both the 2D and 3D phase-field method to analyze the orientation selection and some relevant aspects of nanowires with the vapor-liquid-solid growth mechanism. Combining the 2D phase-field modeling and theoretical analysis, we predict the orientation of nanowire with anisotropic solid-liquid and solid-vapor interface energy. We also investigate the effect on the orientation selection of the anisotropic interface kinetics. Furthermore, we present the calculation of the morphology of nanowire tips while the nanowire grows with anisotropic solid-liquid and solid-vapor interface energy. In addition to the 2D phase-field simulations, we extend the model to 3D. For the 3D phase-field model, we extract parameters of the anisotropic solid-liquid and solid-vapor γ-plot from the existing experimental measurements and atomistic simulations. The simulations successfully reproduce the morphologies of nanowires and sawtooth sidewalls observed from experiments. We note that the solid-vapor interface is asymmetric even though the solid-vapor interface anisotropy is symmetric. The reason for the symmetry breaking is the asymmetric torque term from the solid-vapor interface. We also investigate the effect of the anisotropic interface kinetics on the orientation selection and tip morphology. In the second project, we use the phase-field method to investigate the nanoporous structure generated from liquid metal dealloying. Liquid metal dealloying is a novel technique that consists of immersing a base alloy into a liquid metal and selectively dissolving one element out, thereby causing the undissolved element to produce nanocomposite and open nanoporous structures with the ultra-high interfacial area for diverse applications. We first present results of combined phase-field modeling and experimental studies that investigate how the structure topology, the kinetics of dealloying, and composition can be controlled by varying the liquid melt composition. We find that two important effects can control the morphology of dealloyed structures. One effect is that reducing the dissolution kinetics causes diffusion-coupled growth at the dealloying front to be unstable. The other effect is that decreasing the solubility of the immiscible element can reduce the coarsening of the existing structures. During the research of liquid metal dealloying, we find that quantitative discrepancies of the interfacial concentration exist between experiment and modeling predictions. The simulations with finite solid-state diffusion reveal that solid-state diffusion enables the solid-liquid interface to relax to local chemical equilibrium. The results also show that even though the solid-state diffusivity is four to five orders of magnitude smaller than the liquid-state diffusivity, it is found to stabilize the diffusion-coupled growth of lamellar structures, thereby favoring the formation of this aligned structure over high-genus topologically connected structures. As a natural extension of our study of the liquid metal dealloying, we use the phase-field model to investigate the transient-eutectic dealloying process, that is relevant for coating applications. Finally, we present the research of the coarsening of the binary alloy system, which provides an understanding of the coarsening of the dealloyed structures. Especially, we investigate the effect of faceted solid-liquid interface and anisotropic interface mobility on coarsening behavior. We present several methods to demonstrate the novel property of the faceted coarsening structure, including the interface energy distribution, scatter plots and stereographic projections of interface normal directions, and the ratio of different sets of facet areas.