Ab Initio Nonequilibrium Thermodynamic and Transport Properties of Ultrafast Laser Irradiated 316L Stainless Steel

Abstract : We present calculations of transient behavior of thermodynamic and transport coefficients on the timescale of electron-phonon relaxation upon ultrashort laser excitation of ferrous alloys. Their role defining energy deposition and primary microscopic material response to the laser irradiation is outlined. Nonequilib-rium thermodynamic properties of 316L stainless steel are determined from first-principles calculations. Taking into account the complexity of multi-metallic materials, the density functional theory is first applied to describe the electronic density of states of an alloy stainless steel matrix as a function of electronic heating. An increase of the localization degree of the charge density was found to be responsible for the modification of the electronic structure upon electronic heating, with consequences on chemical potential, electronic capacity and pressure. It is shown that the electronic temperature dependence of stainless steel thermo-dynamic properties are consistent with the behavior observed for pure γ-Fe, outlining the role of the main constituent in the same atomic arrangement. Assuming that similar behaviors extend to the transport properties, the transient electron-phonon coupling, optical properties and thermal conductivities of γ-Fe are derived based on density functional perturbation theory and ab initio molecular dynamics and extrapolated for steel. The insertion of accurate transport coefficients allows to improve current models and to achieve more realistic description of femtosecond pulse laser processing. Effects of fast temperature variation driving phase transitions and strong thermal stresses induced by the laser pulse are finally presented by combining first principle results to a nonequilibrium hydrodynamic approach.