Phase-field-crystal study of grain boundary premelting and shearing in bcc iron

TL;DR

This study uses the phase-field-crystal method to analyze grain boundary premelting and shearing in bcc iron, revealing how premelted layer width varies with temperature and misorientation, and identifying coupling modes during shear.

Contribution

It provides a comprehensive PFC-based analysis of grain boundary behaviors in bcc iron, including thermodynamic disjoining potentials and shear response, with quantitative comparisons to MD and analytical models.

Findings

Premelted layer width diverges near melting for certain misorientations.

Disjoining potential can be repulsive or attractive depending on misorientation.

Coupling modes during shear transition are quantitatively predicted and match previous theories.

Abstract

We use the phase-field-crystal (PFC) method to investigate the equilibrium premelting and nonequilibrium shearing behaviors of $[001]$ symmetric tilt grain boundaries (GBs) at high homologous temperature over the complete range of misorientation $0<\theta<90^\circ$ in classical models of bcc Fe. We characterize the dependence of the premelted layer width $W$ as a function of temperature and misorientation and compute the thermodynamic disjoining potential whose derivative with respect to $W$ represents the structural force between crystal-melt interfaces due to the spatial overlap of density waves. The disjoining potential is also computed by molecular dynamics (MD) simulations, for quantitative comparison with PFC simulations, and coarse-grained amplitude equations (AE) derived from PFC that provide additional analytical insights. We find that, for GBs over an intermediate range of misorientation ($\theta_{\rm min}<\theta<\theta_{\rm max}$), $W$ diverges as the melting temperature is approached from below, corresponding to a purely repulsive disjoining potential, while for GBs outside this range ($\theta<\theta_{\rm min}$ or $\theta_{\rm max}<\theta<90^\circ$), $W$ remains finite at the melting point, with its value corresponding to a shallow attractive minimum of the disjoining potential. In response to a shear stress parallel to the GB plane, GBs in PFC simulations exhibit coupled motion normal to this plane, with a discontinuous change of the coupling factor as a function of $\theta$ that reflects a transition between two coupling modes, and/or sliding (shearing of the two grains). The coupling factor for the two coupling modes is in excellent quantitative agreement with previous theoretical predictions [J. W. Cahn, Y. Mishin, and A. Suzuki, Acta Mater. 54, 4953 (2006)].