Metallic glasses exhibit enormous yield strengths that exceed those of the strongest superalloys, but their use as structural materials is limited by their low ductility, the direct consequence of shear banding. The long-term research goal of this proposal is to characterize shear band propagation in bulk metallic glasses so that the extraordinary strengths of this unique class of materials can be exploited for use in structural applications. In pursuit of this goal, the research objectives are to test the hypotheses that 1) shear bands in bulk metallic glasses propagate simultaneously rather than progressively across a sample, 2) shear band velocities are typically on the order of millimeters per second during serrated flow, and 3) shear band velocity is a function of the imposed strain rate. The final failure event will be characterized, and the conditions under which a shear band propagates catastrophically will be determined. The research approach requires macro and microscale testing at strain rates spanning several orders of magnitude in concert with temporally and spatially resolved measurements of strain. Characterization techniques to be employed include high-speed video imaging and data acquisition from strain gages during uniaxial compression testing, microcompression testing using nanoindentation, scanning electron microscopy, and acoustic emission measurements of shear banding events.
NON-TECHNICAL SUMMARY: ?Metallic glass? - the name suggests something quite remarkable. Metallic glasses and conventional metal alloys are typically combinations of several elements, such as aluminum, copper, nickel, and titanium. But whereas the atoms in conventional metals are arranged in orderly, crystalline arrays, the atomic arrangements in metallic glasses have no long-range order. The result is that the mechanical behavior of metallic glasses is fundamentally different from that of their crystalline counterparts. Most notably, metallic glasses are extraordinarily strong, but unfortunately, they exhibit macroscopically brittle behavior. Since bulk metallic glasses comprise a relatively new class of materials, the current understanding of their mechanical behavior is far less sophisticated than that of crystalline metals, particularly with respect to the atomic level and microscopic mechanisms of plastic deformation. The research goal of this proposal is to investigate the microscopic deformation mechanisms in metallic glasses through a combination of mechanical testing, materials characterization, and modeling so that the potential of metallic glasses as structural materials can be fully realized. A laboratory-based workshop on metallic glasses for first generation undergraduates majoring in engineering at Santa Clara University (SCU) will enhance the cultural capital of these students at an early point in their academic careers to maximize their opportunities for enrichment and success in college. This workshop will be adapted for use in other SCU outreach efforts for underserved high school students in the community. Results and insights gained from assessments of outreach to first generation college students and members of underrepresented groups will contribute to the development of best practices for promoting the participation of these students in Science, Technology, Engineering, and Mathematics (STEM) fields. At a local level, these efforts will increase the number of students from first generation and underrepresented groups majoring in STEM fields.
