Plastic Deformation and Annealing of Zr- and Cu- Based Bulk Metallic Glasses

Plastic deformation in crystalline metals occurs by the motion and multiplication of lattice defects called dislocations, resulting in strain hardening. Metallic glasses, in contrast, lack long-range order; therefore, dislocation-mediated plasticity is not possible in these materials. Metallic glasses undergo plastic deformation through the nucleation and propagation of defects called shear bands, which cause strain softening. This strain softening can be recovered after annealing, i.e., annealing makes the plastically strained metallic glasses harder, in sharp contrast to the annealing-induced softening typically observed in crystalline metals. During annealing, hardness initially recovers more rapidly in heavily deformed specimens than in lightly deformed ones, at a rate that varies inversely as the shear band spacing. With increasing annealing time and temperature, hardness further increases, at the same rate whether pre-strained or not. If the deformed and annealed metallic glass is plastically deformed again, reversible softening is observed. To our knowledge, this is the first time that such reversible softening in metallic glasses has been reported. These hardness changes were correlated with shear band patterns around/underneath a Vickers indent. Shear bands produced during indentation of as-cast glass were semi-circular and radial, consistent with the maximum stress distribution beneath the indenter. In contrast, shear bands in the pre-strained glass were irregular and convoluted, and appeared to be a mixture of shear bands produced during the preceding compression and those in as-cast glass. This indicates that shear band regions are softer than the surrounding undeformed matrix and act as preferred sites for subsequent deformation during indentation, consistent with the macroscopic strain softening vi observed after plastic deformation. After annealing, shear bands tend to recover the originally semi-circular and radial shapes associated with the hardness increase