Cyclic stress induced surface nanocrystallization adjacent to indentation edge of Zr-based bulk metallic glass at room temperature

Crystallization behaviors of bulk metallic glass (BMG) have been obtaining widespread concern, especially under service conditions including elevated temperature, electrical pulse, mechanical stress and high pressure. In this paper, cyclic stress was considered as typical service condition and applied on a Zr55Cu30Al10Ni5 BMG specimen with prefabricated double V-notches. Through indentation tests on fatigue-fractured specimen along with the connection line between notch tips, diversities in indentation serrated flow behaviors and morphological anisotropies were directly observed. Through transmission electron microscopy (TEM) characterization and selected area electron diffraction (SAED) analysis of thinned BMG specimen adjacent to indentation edge, direct experimental evidence of local crystallization at room temperature (RT) was verified. Meanwhile, the distance-dependent crystallization behavior was investigated in terms of gradually increased quantity of diffraction spots and precipitated nanocrystalline adjacent to indentation edge. Furthermore, high resolution transmission electron microscopy (HRTEM) images and SAED patterns at identical indentation microregion of tensile-fractured BMG specimen did not exhibit obvious nanocrystalline and diffraction spots, which indicated that the cyclic stress rather than static stress induced surface crystallization adjacent to indentation edge at RT.