| Metallic glasses are metal alloys whose atomic arrangements have no long range order. Metallic glasses deform in fundamentally different ways from their crystalline counterparts. This study uses analytical and molecular dynamics methods to study the deformation mechanisms and shear band processes in metallic glasses.; Analytical models were analyzed to determine the possible effect of shear band heating on the mechanical behavior. The heat generation predicted from these models is insufficient to reduce the viscosity in the shear band enough to explain the flow localization that dominates the mechanical behavior of metallic glasses at high stresses and low temperatures. Instead, the increase in free volume is responsible for the localization and reduction in viscosity.; A simple four component Lennard-Jones molecular dynamics model was used to characterize the behavior of the free volume during deformation. The deformation behavior observed was typical of that seen experimentally involving both elastic and plastic strains, load history dependence and strain rate sensitivity. Changes in free volume and excess strain were monitored during deformation to demonstrate how the stress state affects the distribution of free volume and how regions with excess free volume will preferentially deform. Under monotonic shear loading, the system developed a shear band in the region with the highest free volume. The same system developed a shear band under much lower stress under cyclic fatigue loading. This fatigue behavior can be related to the notion of directional shear transformation zones. Once a particular cluster of atoms undergoes a shear transformation, it does not return to its original state on unloading. Furthermore, in subsequent cycles although this cluster will not transform again, the surrounding region is more likely to have further shear transformations. This localization of the deformation process accelerates the shear banding process and is used to rationalize the poor fatigue properties commonly reported for metallic glasses. |
