Evolution of shear bands, free volume and hardness during cold rolling of a Zr-based bulk metallic glass

Two-dimensional cold rolling was performed on a bulk metallic glass (BMG) with composition Zr58.5Cu15.6Ni12.8Al10.3Nb2.8. The average shear band density continuously increased with plastic strain. By comparing with published data, a common relation is proposed whereby the average shear band density scales with the square root of the induced true plastic strain, independent of changes in BMG composition or loading mode. Also, the measured exothermal heat release preceding the glass transition, and thus also the free volume, was found to increase linearly with shear band density. Based on an analysis of the measured shear band densities and enthalpy changes, it is concluded that the free volume of both the matrix and the shear bands must evolve continuously during deformation. Finally, the measured hardness during cold rolling was found to decrease initially within the low deformation regime and then increase at higher deformations with a minimum at a strain of ∼0.073. By recognizing the commonality in the shear band formation among different loading modes, the contributions of shear band generation and residual stresses to the hardness changes were separated. The initial decrease in hardness was attributed to free volume generation and a softening of the material, while the subsequent increase in hardness was related to the evolution of compressive residual stresses during cold rolling. It is suggested that these competing mechanisms affecting hardness may help to explain the different observations in the literature concerning the influence of pre-deformation.