Synergistic effects of size constraint and structural heterogeneity on the serrated flow dynamics of amorphous medium-entropy alloy

In this work, the size-dependent serration flow behavior and mechanical response in pristine and annealed TaTiZr amorphous medium-entropy alloys (AMEAs) were examined by uniaxial compression testing using micropillars with diameter ranging from 750 to 2000 nm. The TaTiZr AMEA was synthesized via magnetron sputtering, and subsequently annealed below its glassy temperature for promoting increasingly elemental segregation and structural heterogeneity. The deformation behavior of micropillars is size and structural heterogeneity dependent, changing from the catastrophic highly localized shear banding at large annealed pillars to quasi-homogeneous multiple shear deformation at small as-deposited pillars. Meanwhile, statistical analysis on the serrated flow behavior demonstrates that both the average stress drop and elastic energy density gradually decrease with raising the pillar diameter or annealing temperature. However, their distributions exhibit an obvious shift from power-law scaling to Gaussian-like behavior, corresponding to a transition from the self-organized criticality behavior towards the chaotic response, and consequently the degradation of plastic deformability. The underlying dynamic mechanism is rationalized by the theoretical analysis of shear transformation zone (STZ) density in each sample. In addition, the yield strength of micropillars confirms to the size effect of “smaller is strong”, and could be further enhanced via improving the structural heterogeneity. The associated strengthening and deformation mechanisms are elucidated based on the STZ theory and mean-field theory, respectively. Our findings highlight the critical role of geometric constraint and structural relaxation in governing plastic deformation and optimizing mechanical properties of AMEAs.