Homogeneous melting of superheated crystals: Molecular dynamics simulations

The homogeneous melting mechanism in a superheated fcc lattice is studied through molecular dynamics simulations, usually for about 20 000 atoms, with the Ercolessi and Adams interaction that represents aluminum. The periodic boundary conditions for the simulation cell suppress the usual surface-initiated melting at T_m=939 K, and the solid-to-liquid transition takes place at the temperature T_s=1.3T_m. By logging the position of each atom at every time step in the simulation, we can follow the melting process in detail at the atomic level. Thermal fluctuations close to T_s create interstitial-vacancy pairs, which occasionally separate into mobile interstitials and almost immobile vacancies. There is an attraction between two interstitials, with a calculated maximum interaction energy of about 0.7 eV. When three to four migrating interstitials have come close enough to form a bound aggregate of point defects, and a few thermally created interstitial-vacancy pairs have been added to the aggregate, such a defect configuration usually continues to grow irreversibly to the liquid state. For 20 000 atoms in the simulation cell, the growth process takes about 10²τ to be completed, where τ is the period of a typical atomic vibration in the solid phase. This melting mechanism involves fewer atoms in its crucial initial phase than has been suggested in other melting models. The elastic shear moduli c₄₄ and c^′=(c₁₁−c₁₂)∕2 were calculated as a function of temperature and were shown to be finite at the onset of melting.