Severe plastic deformation induced by high-pressure torsion (HPT) causes significant changes in the microstructure and thermo-mechanical properties of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass. Although HPT does not directly lead to clear crystallization of the amorphous alloy, transmission electron microscopy reveals changes in the medium-range order. This is accompanied with an increase in the alloy microstrains and a slight decrease in the crystallization enthalpy. These effects are due to the combined action of the large shear strains and the temperature rise that occurs during HPT. Indeed, the time evolution and spatial distribution of temperature during HPT, calculated using a heat-conduction equation, evidence that values of temperature close to the glass transition, T_g, are achieved throughout the HPT disk. However, contrary to conventional sub-T_g thermal annealing, which induces mechanical hardening and embrittlement of the glass, HPT increases the net amount of excess free volume in the amorphous structure, leading to a mechanical softening effect.