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 beta-relaxation, E-beta. This result suggested that the fundamental process for STZ activation is the mechanically activated beta-relaxation. Since the E-beta value and the glass transition temperature T-g of BMGs have a linear relation, that is, because E-beta approximate to 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-beta 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-beta (or W-STZ) and result in improvements in the mechanical properties of the BMG. (C) 2014 AIP Publishing LLC.