Viscosity effects on the flow and fracture of metallic glasses and other viscous materials

The purpose of this study was to further understand the effects of changes in viscosity on the flow and fracture of metallic glasses. The viscosity of metallic glasses under various testing conditions were measured via three types of experiments. Estimates of global sample viscosity were made from tensile test data at temperatures from room temperature to above the glass transition temperature for several metallic glasses at strain rates ranging from 10^-1 to 10^-3 s^-1. Dynamic mechanical analysis (DMA) was used to investigate effects of temperature, frequency, pre-annealing, pre-straining, and sample orientation on global sample complex viscosity. Instrumented tensile tests were conducted to measure the local viscosity within shear bands. The global viscosity measurements from DMA and tensile test estimates showed good agreement. The local viscosity within a shear band was found to be several orders of magnitude lower than the global viscosities.

Mechanical tests including microhardness, nanoindentation, bending over a mandrel, and tensile tests were conducted on Mg₈₅Cu₁₀Ca₅ metallic glass ribbons, as well as observations of fracture surface features. High pressure shear experiments were conducted on a number of metallic glass ribbons to investigate the effects of superimposed pressure on flow of metallic glasses. It was found that superimposed pressure does not significantly affect the flow of the metallic glass ribbons tested.

Model experiments on viscous materials were performed to mimic flow and fracture in metallic glasses. It was found that the viscosity and toughness of model experiments as plotted against the fracture surface feature sizes showed similar behavior to the same parameters in metallic glasses. Model experiments conducted on viscous materials and those conducted on metallic glasses were mathematically modeled. It was found that the viscous materials were well described by the model, but that the model did not well describe higher toughness (i.e. >20 MPa m^1/2) metallic glasses. This was attributed to the lack of a shear band thickness parameter in the mathematical model and to multiple shear banding and crack tip blunting in tough metallic glasses.