Compositional complexity buffers free-volume sensitivity and serrated flow in metallic glasses

Processing history imprints metallic glasses (MGs), yet whether compositional complexity desensitizes structure and mechanics to quench rate remains unresolved. We use large-scale molecular dynamics along a controlled Cu-Zr complexity ladder, Cu₅₀Zr₅₀, Cu_47.5Zr_47.5Al₅, and Cu₄₅Zr₄₅Al₅Ti₅, vitrified over 10¹¹-10¹⁵ K·s^-1 and probed by spherical nanoindentation. Additionally, composition-resolved Cu _<i>x</i> Zr_100-<i>x</i> sweep (<i>x</i> = 40-65 at.%) and a microalloying series Cu_{50-<i>z</i>/2}Zr_{50-<i>z</i>/2}Al _<i>z</i> , (<i>z</i> = 1-5 at.%) disentangle configurational entropy-driven effects from enthalpic and structural covariates. Atomic free volume is obtained from radical-Voronoi tessellation; non-affine rearrangements are quantified by Falk-Langer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>D</mml:mi></mml:mrow> <mml:mrow><mml:mi>min</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> </mml:msubsup> </mml:math> field and clustered in three dimensions. Three quantitative descriptors capture the dispersion of free volume and its quench rate sensitivity as a function of compositional complexity. Increasing compositional complexity narrows free-volume distributions across quench rates and systematically reduces the fast-slow disparity. A two-axis reconciliation emerges: within binary Cu-Zr, configurational entropy peaks near equiatomic and minimizes rate sensitivity, whereas across alloy families (binary→ternary→quaternary), increased species diversity and size/enthalpy mismatch further suppress sensitivity. Structure-property co-variation is consistent: at fixed rate, hardness, modulus and elastic recovery increase, while serration density, STZ number density, and plastic-zone volume decrease. Radial-distribution metrics and indentation-induced icosahedral losses corroborate enhanced short/medium-range stability. Compositional complexity thus provides a quantitative lever for processing-tolerant, high-performance Cu-Zr-based MGs.