Multi-scale model of the dynamic fracture of molten and solid metals

Abstract

A multi-scale model of the tensile fracture of metal melts is developed based on a combination of molecular dynamics (MD) simulations and continuum description of kinetics and dynamics of voids; the model considerably extends the time and length scales of MD. Nucleation of voids due to thermal fluctuations is taken into account. Growth of a void in melts is well described by the Rayleigh-Plesset equation, while in the case of a solid metal we propose a dislocation-based model of the void growth. Based on the MD simulations, we investigate the nucleation rates in the uniform monocrystalline metals and metal melts, dynamics of pre-existing voids and compare them with the continuum model (equations of nucleation and growth). Using of the literature data on the surface tension and viscosity of melts allows us to get a correspondence between the continuum description and MD. With the use of the model, we calculated the strength of the uniform melts of Al, Cu, Ni, Pb, Fe and Ti within a wide range of strain rates (from 103-104 to 109-1011 s-1) and temperatures (from melting temperature to 70-80% of critical temperature). Calculations show that the tensile strength of homogeneous melts decreases slowly with the strain rate decrease. As a result, within the range of strain rates of 106-108 s-1, a homogeneous nucleation mode can be realized, in which the dynamic strength of a melt can be comparable to, or even higher than the strength of a solid metal.