Surface amorphization of bulk NiTi induced by laser radiation

The surface of medical grade Ni (50.8 at.%)Ti is treated with pulsed femtosecond laser radiation causing the formation of an amorphous layer < 200 nm thick. The chemical composition matches that of the original material. The formation of the amorphous layer on NiTi is briefly discussed considering both the thermodynamic and the kinetic perspective including calculated stable and metastable phase diagrams as well as theory for rapid solidification of undercooled melts. Solidification kinetics of NiTi alloy exhibit a tendency for transition to the amorphous state by complete solute and disorder trapping. The observed phenomenon has potential to enhance the properties governed by the surface. Specifically, the homogeneity of the amorphous region may enhance corrosion resistance and/or the resistance against crack initiation. The alloy's pseudoelastic and shape memory bulk properties are expected to remain unaffected by such thin layers, because the original bulk microstructure is conserved.