Multiphase-field approach to grain boundary- and triple junction-induced barrierless premelting of polycrystals and size-effect

Grain boundaries (GBs) and their triple junctions (TJs) induced barrierless premelting of the polycrystalline solids and length scales effect are studied using a thermodynamically and physically consistent Ginzburg-Landau type multiphase-field approach at the nanoscale. Distinct dry GB and TJ energies and GB widths are considered in contrast to the earlier phase-field studies, which assumed identical properties for the dry GBs and TJs and are far from reality. The strong effects of the intrinsic length scales, including the dry GB widths and grain size, on the temperatures of barrierless transformations between the solid, premelt (an intermediate stationary state between solid and melt with a degree of disorder) and melt, their nucleation induced by the GB network, kinetics, and complex microstructures evolution are explored, which was still missing in the literature. The thermodynamic processes involved in the melting of polycrystalline solids, which include the jump-like (discontinuous) appearance of premelt or melt pockets in the TJ regions, followed by premelting and melting of the connected GBs, curvature-driven shrinking of the grains surrounded by the melt pools and their jump-like melting at critical temperatures, all occurring at temperatures below the bulk melting point, are revealed. The reverse transformation (solidification) of the molten GB network and the bistability regime, where the temperatures for solid → premelt and premelt → solid transformations differ and yield thermal hysteresis, are studied. The role of the disjoining potential on the jump-like GB premelting and stability of the thermodynamic states is explored.