Scratch-induced work hardening of an atomically flat bulk metallic glass by stress-driven structural ordering

Although the enhanced structural relaxation is usually believed to be an important contributor to work hardening of metallic glasses subjected to triaxial stress state, an direct observation of relaxation process in response to work hardening has not been achieved in metallic glasses. Here we show that by nanoscratching on an atomically flat bulk metallic glass surface, the small atomic force microscopy tip with a radius of ≈ 10 nm brings about a large hydrostatic stress within stressed volume, which enables a densifying plastic flow via enhanced structural relaxation and leads to the work hardening behavior, as evidenced by an obvious decrease in friction force signals within scratched regions. Further examination on the atomic structure beneath the scratched surface using high resolution transmission electron microscopy reveals a relaxed structural configuration, which is indicated by disperse clusters of medium-range order scale in the case of line scratching and nucleated nanocrystals in the case of cyclic scratching. This study provides a compelling evidence for stress-driven structural relaxation, greatly deepening the understanding of work hardening mechanism in metallic glasses.