Cooling rate governs aging and mechanical dynamics in metallic glasses

In this study, rods of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (at%) metallic glasses (MGs) were fabricated recently and six years ago, with diameters of 2, 3 and 5 mm. It is found that the relaxation enthalpy, activation volume, and structural disordering increase as cooling rate increases, while hardness demonstrates an inverse trend. These can be primarily attributed to the increase in the free volume content with increasing cooling rate, highlighting the crucial role of cooling rate in the deformation behavior of MGs. The effect of aging on the relaxation behavior and mechanical properties was further investigated through X-ray diffraction, dynamic mechanical analysis, differential scanning calorimetry, and nanoindentation. Notably, six-year ambient aging induces a slight structural relaxation, and this relaxation increases with increasing cooling rate. In stark contrast, samples annealed at high temperatures exhibit significant sub-T_g endothermic behavior, indicating that annealing temperature exerts a more pronounced influence on structural relaxation than aging duration. Intriguingly, the 2-mm sample manifests subtle aging-induced relaxation, which is further corroborated by ultrasonic vibration (USV) rejuvenation experiments. USV-treated as-cast samples exhibit enhanced rejuvenation effects compared to aged counterparts, accompanied by an elevated energy threshold for rejuvenation in aged samples – direct evidence of aging-induced structural stabilization. Both as-cast and aged systems demonstrate USV energy-dependent rejuvenation gradients, further validating the aging-resistance hypothesis. These findings collectively highlight that targeted cooling rate modulation offers a viable strategy for engineering MG mechanical properties.