Abstract: To advance materials with superior performance, the construction of gradient structures has emerged as a promising strategy. In this study, a gradient nanocrystalline-amorphous structure was induced in Zr₄₆Cu₄₆Al₈ bulk metallic glass (BMG) through ultrasonic vibration (UV) treatment. Applying a 20 kHz ultrasonic cyclic loading in the elastic regime, controllable gradient structures with varying crystallized volume fractions can be achieved in less than 2 s by adjusting the input UV energy. In contrast to traditional methods of inducing structural gradients in BMGs, this novel approach offers distinct advantages: it is exceptionally rapid, requires minimal stress, and allows for easy tuning of the extent of structural gradients through precise adjustment of processing parameters. Nanoindentation tests reveal higher hardness near the struck surface, attributed to a greater degree of nanocrystal formation, which gradually diminishes with depth. As a result of the gradient dispersion of nanocrystals, an increased plasticity was found after UV treatment, characterized by the formation of multiple shear bands. Microstructural investigations suggest that UV-induced nanocrystallization originates from local atomic rearrangements in phase-separated Cu-rich regions with high diffusional mobility. Our study underscores the tunability of structural gradients and corresponding performance improvements in BMGs through ultrasonic energy modulation, offering valuable insights for designing advanced metallic materials with tailored mechanical properties.

Key words: Bulk metallic glass, Nanocrystallization, Ultrasonic vibration, Plasticity, Structural heterogeneities