Influence of size-dependent free volume variation and distribution on the thermoplastic flow behavior of Zr-based bulk metallic glass

The size-dependent microstructural evolution behavior has a significant impact on the thermoplastic flow behavior of metallic glass in the supercooled liquid region. In this work, double-cup extrusion tests, corresponding finite element method simulations and nanoindentation measurements of Zr55Cu30Al10Ni5 metallic glass samples with different sizes, are employed to study the relationship between the size-dependent free volume variations and thermoplastic flowability. Experimental results and theoretical analyses demonstrate that the free volume content increases with increasing sample size. The change tendencies of free volume content and micro-hardness are in a negative correlation. The variation regularity of extruded cup height ratio verifies that the thermoplastic flowability of Zr-based metallic glass is enhanced with rising sample size. Furthermore, the size-dependent free volume variations and distributions induced by internal microstructural evolution behaviors result in the evident variations of micromechanical properties. The results of this study will contribute to a more in-depth understanding of the size-dependent thermoplastic deformation behavior of Zr-based metallic glass.