On the relationship between the mechanical and the thermal instabilities of crystalline lattices

Numerical simulations have been employed to explore the melting of pure face-centered-cubic (fcc) metals under superheating conditions. For each chemical species considered, simulations were carried out on two different configurations. The first configuration consisted of two perfect semi-crystals rotated in order to obtain a high-angle twist boundary. The presence of an interface between the semi-crystals permitted to estimate the thermodynamic melting point for the tight-binding interatomic potential employed in calculations. The second configuration consisted, instead, of a perfect crystalline bulk. In this case, the crystalline lattice was slowly heated and the solid brought within the metastable region of existence beyond the thermodynamic melting point. The static long-range order parameter was employed to quantify the degree of disorder during the gradual heating and reveal the sudden collapse of crystalline structures at the equilibrium melting point or at the limit of superheating. The effects of gradual heating on system volume and elastic properties were also accurately monitored. The volume at which the continuous elastic softening should induce the mechanical failure of the crystalline structure was found to approximately correspond to the volume attained by the molten metal at the equilibrium melting temperature. In addition, a relationship was found between the volume reached by crystals at equilibrium melting and at the limit of superheating. Calculations revealed that melting occurs via a proliferation of defects in the bulk of crystalline lattices. Such defects, mainly vacancy-interstitial pairs, tend to form extended string-like clusters. Their density at equilibrium melting as well as at the limit of superheating was approximately the same for all the fcc metals considered.