The evolution of relaxation modes during isothermal annealing and its influence on properties of Fe-based metallic glass

Isothermal annealing can enhance the soft magnetic properties of Fe-based metallic glasses, but usually causes brittleness. In this work, we studied the relaxation kinetics of Fe-based metallic glasses below glass transition temperature and the evolution of soft magnetic properties and plasticity. We found that the metallic glass first experiences secondary (beta) relaxation and then transforms into primary (alpha) relaxation upon isothermal annealing. The magnetic properties are enhanced predominantly during beta relaxation stage but don't change very much during alpha relaxation stage. The plasticity doesn't change very much during beta relaxation stage but gets deteriorated during alpha relaxation stage. The structural origin of the changes is also studied. These results prove that it is probable to achieve metallic glasses with both excellent soft magnetic properties and excellent mechanical properties by controlling the relaxation process, which may provide a universal protocol for enhancing the functional performance of soft magnetic metallic glasses.