Improving the mechanical properties of Zr-based bulk metallic glass by controlling the activation energy for β -relaxation through plastic deformation

The mechanism of plastic deformation in bulk metallic glasses (BMGs) is widely believed to be based on a shear transformation zone (STZ). This model assumes that a shear-induced atomic rearrangement occurs at local clusters that are a few to hundreds of atoms in size. It was recently postulated that the potential energy barrier for STZ activation, W_STZ, calculated using the cooperative shear model, is equivalent to the activation energy for β-relaxation, E_β. This result suggested that the fundamental process for STZ activation is the mechanically activated β-relaxation. Since the E_β value and the glass transition temperature T_g of BMGs have a linear relation, that is, because E_β ≈ 26RT_g, the composition of the BMG determines the ease with which the STZ can be activated. Enthalpy relaxation experiments revealed that the BMG Zr₅₀Cu₄₀Al₁₀ when deformed by high-pressure torsion (HPT) has a lower E_β of 101 kJ/mol. The HPT-processed samples accordingly exhibited tensile plastic elongation (0.34%) and marked decreases in their yield strength (330 MPa). These results suggest that mechanically induced structural defects (i.e., the free volume and the anti-free volume) effectively act to reduce W_STZ and increase the number of STZs activated during tensile testing to accommodate the plastic strain without requiring a change in the composition of the BMG. Thus, this study shows quantitatively that mechanically induced structural defects can overcome the compositional limitations of E_β (or W_STZ) and result in improvements in the mechanical properties of the BMG.