High pressure torsion of a Vitreloy bulk metallic glass near the glass transition temperature

Disk shaped specimens of commercial Zr44Ti11Cu10Ni10Be25 bulk metallic glass (Vitreloy 1b) were subjected to extreme hydrostatic pressure and simultaneous plastic shear deformation by high pressure torsion method at elevated temperatures below (610 K and 620 K) and around the glass transition (630 K). Coupled calorimetric and X-ray diffraction experiments revealed that the amorphous structure of the alloy is very stable up to 620 K, however, destabilization takes place close to the glass transition (Tg) at 630 K. Series of hardness measurements were performed by depth-sensing nanoindentation for the as-cast and the thermo-mechanically treated samples. The shape and the mean value of the hardness distribution of the disks processed below Tg are similar to that of the as-cast glass, nevertheless the hardness distribution becomes broader accompanied with a significant overall increase when the Vitreloy 1b glass is subjected to extreme shear deformation close to Tg.