Microstructure and mechanical behavior evolution of Ti-based bulk metallic glass induced by sub-T isothermal annealing

In this study, the effects of sub- T _g isothermal annealing on the microstructure, mechanical and serrated flow behaviors of Ti ₃₃Zr ₃₀Cu ₉Ni _5.5Be _22.5 bulk metallic glass (BMG) were investigated. The change in free volume content of the BMG was characterized according to the structural relaxation enthalpy δH _r and the onset structural relaxation temperature T _r, with the results indicating a monotonic decreasing trend of the free volume content with the extension in annealing time. A number of nanocrystals with a size of approximately 4 nm were precipitated on the glassy matrix when the annealing time was extended to 1.0 h, and then the size of these nanocrystals increased with further prolongation of the annealing time. Meanwhile, the room-temperature compressive plasticity of the Ti ₃₃Zr ₃₀Cu ₉Ni _5.5Be _22.5 BMG greatly improved from 1.4% to 4.2%, and the high strength was retained after annealing for 1.0 h, which can be largely attributed to the formation of the nanocrystals with the size of about 4 nm. With the extension of the annealing time, however, the further growth of the nanocrystals resulted in the degeneration of the compressive strength and the plasticity. The serrated rheology analysis revealed that the distribution histogram of the stress drop for the 1.0 h-annealed specimen represented a well characteristic of power-law distribution, whereas the 4.0 h-annealed showed a chaotic dynamics of jerk flow. Furthermore, the 1.0 h-annealed specimen possessed the lower average stress drop (12 MPa), formation energy per unit shear band (254.9 J/m ²) and average shear band velocity (1.21 × 10 ⁻⁵ m s ⁻¹).