Free volume coalescence and void formation in shear bands in metallic glass

We have investigated the possibility of void nucleation from the coalescence of excess free volume generated in shear bands during deformation of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass. Excess free volume in a shear band results in excess free energy relative to a relaxed glass with less free volume. To calculate the free energy of the material in a shear band with excess free volume, we model it as having the same structure as a glass solidified at an elevated glass transition temperature, which we call the fictive temperature. This excess free energy can be correlated with a free volume chemical potential that provides a driving force for void nucleation. The results of this modeling indicate that any free volume generated in the shear band during deformation is unstable, with the consequence that voids are predicted to form spontaneously from the coalescence of free volume. These voids are then expected to coarsen. Under tension, void growth and linkage would be facilitated by a tensile stress state, perhaps leading to premature fracture, whereas a compressive stress state would hinder void growth. This may explain the common observation that failure under uniaxial tension occurs as the result of the propagation of a single shear band, whereas multiple shear bands can form under uniaxial compression without causing failure.