Different Modes of Shear during Quasistatic Compression of a Bulk Metallic Glass

The macroscopically brittle behavior of bulk metallic glasses is caused by a microstructural phenomenon known as shear banding, and in mechanical testing of metallic glasses, particularly compression testing, this shear banding behavior manifests as a series of sudden stress drops. Materials scientists refer to this behavior as “serrated flow” whereas the physics community describes this general phenomenon as slips or slip “avalanches.” Analysis of the statistics of these stress drops reveal that two distinct types of slip events occur during serrated plastic flow of bulk metallic glasses. Slips of small stress drop magnitude occur in stochastic fits and starts as evidenced by their stress drop rate profiles. When averaged, the stress drop rate profile of all small slips is parabolic and symmetric. These small slips correspond to the progressive or incremental propagation of a shear band, and optical microscopy reveals shear bands that do not propagate across the entire specimen. A sharply peaked stress drop rate profile characterizes the large serrations. These large slips correspond to simultaneous shear with uniform sliding over the entire shear plane as confirmed by direct high–speed imaging and image correlation. For the first time in metallic glasses, we have acquired synchronized load and acoustic emission data for the large slips. These data suggest that a percolation or shuffling of slipping weak spots precedes the sharp load drop, with a single acoustic burst occurring when the rapid load drop begins.