Transient electron properties and phase change in femtosecond laser processing of NiTi alloy: Multiscale simulation and experimental investigation

NiTi alloy serves as an important biomedical material, and its functional surface processed by femtosecond laser has clinical application prospects. The knowledge of transient properties and phase change is the footstone of the quantitative prediction of femtosecond laser processing of NiTi alloy, but is not adequately studied. This work aims to address the data accessibility of electron temperature-dependent properties of NiTi alloy and the investigation of femtosecond laser-induced phase change based on these data. The electron heat capacity and electron-phonon coupling factor of NiTi alloy are obtained by the density functional theory and density functional perturbation theory with ab initio accuracy, which makes the multiscale simulation feasible. The atomistic motion during phase change is captured by snapshots in the molecular dynamics coupled two-temperature model (MD-TTM). To overcome the effect of the Langevin thermostat on pressure wave propagation, a temporally partial-applied MD-TTM method is proposed, which predicts a reversible solid-solid phase change within tens of picoseconds under the melting region. The optical response of ejected atoms in phase change is experimentally observed by transient reflectivity microscopy. It is found that the experimental reflectivity drop and simulated atom ejection have the same time range. The agreement between experimental and simulated ablation thresholds proves the validation of the proposed temporally partial-applied MD-TTM method. The simulated atomistic structure change after femtosecond laser processing is supported by the experimental characterization of the surface amorphous and subsurface crystal structures. The reported data and results contribute to further quantitative investigation and application of NiTi alloy processed by femtosecond laser.