Thermomechanical control of pathway-dependent relaxation in metallic glasses

Structural relaxation influences the mechanical stability of metallic glasses, but the mechanisms by which thermal annealing and cyclic loading interact to control it are still far from resolved. Here, we systematically investigate the coupled effects of cyclic loading and sub-T_g annealing on the stress relaxation of Cu₄₆Zr₄₆Al₈ metallic glass, and establish a connection among relaxation time evolution, activation energy spectra, and dynamic heterogeneity. Both cyclic loading and sub-T_g annealing progressively slow stress relaxation, as reflected by longer characteristic relaxation times, smaller stretched exponents, and a shift in the activation energy spectrum toward higher energies. Relaxation time spectra further reveal a redistribution of the dominant modes toward slower and more constrained processes, indicating suppressed atomic mobility and enhanced dynamic heterogeneity. These effects are interpreted within the Coupling Model, in which strengthened dynamic constraints among neighboring flow units increase the cooperative character of relaxation and amplify the macroscopic slowdown. Molecular dynamics simulations support this interpretation by showing increased five-fold local symmetry, reduced orientational disorder, and diminished participation in low frequency soft modes. Comparative experiments further show that, under the present conditions, cyclic loading promotes accelerated aging rather than rejuvenation, owing to the incomplete recovery of flow units whose intrinsic relaxation times exceed the available recovery interval. These results establish a physically consistent and experimentally validated framework of pathway-dependent relaxation in metallic glasses and provide a physically grounded basis for tailoring their time-dependent mechanical stability through coupled thermo-mechanical histories.