High-angle grain-boundary premelting transition: A molecular-dynamics study

We studied the structure and self-diffusion in the fcc [001] (310) $\Sigma =5\mathrm{}$ coincidence high-angle tilt grain boundary by molecular-dynamics simulation. The calculations have been performed at various temperatures and fixed reduced number density, $\rho =0.984\mathrm{}$, using Lennard-Jones (12-6) pair-interaction potential. The temperature dependence of the local thermodynamic properties in a small region around the grain boundary clearly points out that a structural transition occurs well below the melting point ($T_{\mathrm{trans}}<0.5\mathrm{}{T}_m$, with $T_m$ the melting-point temperature). This transition appears to be very smooth and indicates the on-set of a partially disordered structure inside the boundary which goes continuously to the liquid at the melting temperature. The thickness of the region affected by the transition grows very slowly with increasing temperature and finally, due to the finite size of the simulated system, causes the migration of the grain boundary. In our simulation this happens for $T\sim 0.9T_m$. The self-diffusion coefficient, $D_{\mathrm{GB}}$, in the grain boundary has a value comparable to, although definitely lower than, that of the quenched liquid at the corresponding temperature and density. Like the structural properties, $D_{\mathrm{GB}}$ goes continuously to the liquid value at $T=T_m$.