Molecular dynamics simulations of homogeneous and heterogeneous melting scenarios in metals: volume scaling and concentration of defects

This work investigates the connection between heterogeneous melting at equilibrium and homogeneous melting at the limit of superheating. A phenomenological relationship connecting the volumes attained by metallic species at melting under equilibrium and superheating conditions is pointed out. Numerical simulations have been used to tentatively rationalize such a relationship within the framework of defect-mediated melting scenarios. A detailed mechanistic analysis of the heterogeneous and homogeneous melting processes in metals was carried out. A semicrystal was used to study the heterogeneous nucleation of the molten phase at a free surface. A crystalline bulk was instead used to simulate the homogeneous melting process at temperatures significantly higher than the equilibrium melting point. In both cases melting was attained by slowly heating the crystalline lattice. Static order parameters were employed to quantify the degree of disorder during the gradual temperature rise and detect the sudden collapse of the crystalline structures. The numerical findings suggest that both heterogeneous and homogeneous melting processes are mediated by atoms with defective coordination. Their concentration in the crystalline regions close to the surface at the equilibrium melting point was found to be approximately the same as in the bulk system at the limit of superheating.