Femtosecond laser ablation ofCuxZr1−xbulk metallic glasses: A molecular dynamics study

Molecular-dynamics simulations combined with a two-temperature model are used to study laser ablation in ${\text{Cu}}_{x}{\text{Zr}}_{1\ensuremath{-}x}$ $(x=0.33,0.50,0.67)$ metallic glasses as well as crystalline ${\text{CuZr}}_{2}$ in the ${\text{C11}}_{b}$ $({\text{MoSi}}_{2})$ structure. Ablation thresholds are found to be $430\ifmmode\pm\else\textpm\fi{}10,450\ifmmode\pm\else\textpm\fi{}10,510\ifmmode\pm\else\textpm\fi{}10$, and $470\ifmmode\pm\else\textpm\fi{}10$ ${\mathrm{J}/\mathrm{m}}_{2}$ for a-${\text{Cu}}_{2}\text{Zr}$, a-CuZr, ${\text{a-CuZr}}_{2}$, and ${\text{c-CuZr}}_{2}$, respectively. The larger threshold in amorphous ${\text{CuZr}}_{2}$ results from a weaker electron-phonon coupling and thus longer electron-ion equilibration time. We observe that the velocity of the pressure waves in the amorphous samples is not affected by the fluence, in contrast to the crystal; this is due to differences in the behavior of the shear modulus with increasing pressure. The heat-affected zone in the different systems is characterized in terms of the melting depth as well as inelastic deformations. The melting depth is found to be smaller in the crystal than in the amorphous targets because of its higher melting temperature. The inelastic deformations are investigated in terms of the von Mises shear strain invariant ${\ensuremath{\eta}}^{\text{Mises}}$; the homogeneous nucleation of shear transformation zones is observed in the glass as reported in previous theoretical and experimental studies. The coalescence of the shear transformation zones is also found at higher fluence.