Runaway lattice-mismatched interface in an atomistic simulation of femtosecond laser irradiation of Ag film-Cu substrate system

The atomic mixing and structural transformations in a Ag film-Cu substrate system irradiated by a femtosecond laser pulse are investigated in a simulation performed with a model that couples the classical molecular dynamics method with a continuum-level description of the laser excitation and subsequent relaxation of the conduction-band electrons. The higher strength of the electron-phonon coupling in Cu compared to Ag results in preferential sub-surface heating and melting of the Cu substrate. The melting is followed by fast cooling and rapid resolidification occurring under conditions of strong undercooling below the equilibrium melting temperatures of Cu and Ag. The rapid resolidification results in a complex structure of the interfacial region, where the lattice-mismatched interface is separated from the Ag-Cu mixing region by an intermediate pseudomorphic bcc Cu layer that grows epitaxially on the (001) face of the fcc Ag film during the final stage of the resolidification process. The new lattice-mismatched interface has a three-dimensional structure consisting of a periodic array of stacking fault pyramids outlined by stair-rod partial dislocations. The intermediate bcc layer and the stacking fault pyramid structure of the mismatched interface are likely to present a strong barrier for dislocation propagation, resulting in the effective hardening of the layered structure treated by the laser irradiation. The concentration profiles in the atomic mixing region are substantially wider compared to the width of the equilibrium Cu-Ag interface and have a pronounced asymmetric shape that reflects the preferential melting of the Cu substrate.