Phase-field crystal study of grain-boundary premelting

TL;DR

This study uses the phase-field crystal model to analyze grain-boundary premelting, revealing different behaviors for high- and low-angle boundaries and providing insights into the disjoining potential and wetting conditions.

Contribution

It offers a detailed phase-field crystal analysis of grain-boundary premelting, highlighting the qualitative differences between high- and low-angle boundaries and refining wetting condition estimates.

Findings

High-angle boundaries are uniformly wetted with diverging premelted layers.

Low-angle boundaries feature liquid pools around dislocations separated by solid bridges.

The disjoining potential switches from repulsive to attractive depending on boundary angle.

Abstract

We study the phenomenon of grain-boundary premelting for temperatures below the melting point in the phase-field crystal model of a pure material with hexagonal ordering in two dimensions. We investigate the structures of symmetric tilt boundaries as a function of misorientation for two different inclinations and compute in the grand canonical ensemble the disjoining potential V(w) that governs the fundamental interaction between crystal-melt interfaces as a function of the premelted layer width w. The results reveal qualitatively different behaviors for high-angle grain boundaries that are uniformly wetted, with w diverging logarithmically as the melting point is approached from below, and low-angle boundaries that are punctuated by liquid pools surrounding dislocations, separated by solid bridges. This qualitative difference between high and low angle boundaries is reflected in the w-dependence of the disjoining potential that is purely repulsive (V'(w)<0 for all w) above a critical misorientation, but switches from repulsive at small w to attractive at large w for low angles. In the latter case, V(w) has a minimum that corresponds to a premelted boundary of finite width at the melting point. Furthermore, we find that the standard wetting condition (the grain boundary energy is equal to twice the solid-liquid free energy) gives a much too low estimate of the critical misorientation when a low-temperature value of the grain boundary energy is used. In contrast, a reasonable estimate is obtained if the grain boundary energy is extrapolated to the melting point, taking into account both the elastic softening of the material at high temperature and local melting around dislocations.