Shear band processes in bulk metallic glasses.

摘要

Metallic glasses and conventional metal alloys are typically combinations of several elements, such as aluminum, copper, nickel, and titanium. Unlike all conventional metals, however, in which the atoms are arranged in orderly arrays (the hallmark of a crystal), the atomic arrangements in metallic glasses have no long-range order. The result is that the deformation mechanisms of amorphous metallic alloys are fundamentally different from their crystalline counterparts. This study employs uniaxial mechanical testing with high-speed data acquisition, multi-axial mechanical testing, nanoindentation, and materials characterization methods to study shear band processes and plasticity in bulk metallic glasses.; Uniaxial compression experiments were performed to make detailed measurements of shear band propagation. These measurements are used to demonstrate that the heating that occurs during shear band propagation is insufficient to dramatically reduce the viscosity in the shear band and lead to the flow localization that characterizes the deformation of bulk metallic glasses at high stresses and low temperatures. The orientation of shear bands formed in compression suggests a normal stress dependence for shear band propagation, consistent with the free volume theory of deformation that requires atomic dilations for flow. A series of nanoindentation experiments were performed to quantify the mechanical behavior of bulk metallic glasses at the length scale of distributed free volume. A size dependence for plasticity was observed, again consistent with a statistical distribution of free volume in the glass. Finally the free volume theory and the thermodynamics of nucleation are used to demonstrate that voids can form during deformation of metallic glasses due to free volume coalescence; this modeling explains the asymmetry between the amount of plastic strain observed during uniaxial tension and compression of bulk metallic glasses. In tension, the growth of voids is assisted by a tensile stress state, leading to premature fracture, as compared to a compressive stress state that retards void growth.