Graduation date: 2008
Deformation of metallic glasses requires the existence of free volume to allow atomic movement under mechanical loading. Accordingly, the present research seeks to understand how free volume variations in alloys of identical compositions affect the fatigue and fracture behavior. By annealing below the glass transition temperature, the free volume of a Zr-based bulk metallic glass was varied via structural relaxation. Differential scanning calorimetry was used to quantify enthalpy differences between the relaxed and as-cast materials which are then related to free volume differences. Although structural relaxation showed a pronounced effect in reducing the fracture toughness, a reduction in free volume increases the fatigue strength of the bulk amorphous alloy. Mechanistically, the fatigue properties associated with a free volume variation differ significantly with respect to crack initiation. Surprisingly, the fatigue crack-growth behavior was found to be relatively insensitive to bulk free volume differences. Depth-profiled Doppler broadening spectroscopy (DBS) was utilized to perform local depth profiling of fatigue fracture surfaces to characterize local free volume differences. It’s demonstrated that the intense deformation near a fatigue crack tip result in a local increase in free volume, which in turn determines the local flow properties. The effect of residual stresses on the fatigue and fracture behavior was also investigated. The superimposition of compressive stresses induced by thermal tempering during processing was found to retard fatigue crack propagation and to improve the fracture toughness.