Structural Origins for Enhanced Thermal Stability and Glass-Forming Ability of Co–B Metallic Glasses with Y and Nb Addition

The effects of Y and Nb addition on thermal stability, glass-forming ability (GFA), and magnetic softness of Co75B25 metallic glass (MG) were comprehensively investigated. The experimental results indicated that the thermal stability, GFA, and magnetic softness of the studied MGs increase in the order Co75B25 < Co73Nb2B25 < Co71.5Y3.5B25 < Co69.5Y3.5Nb2B25. The structural origins of the improved properties were revealed by ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. Results showed that the B-centered prism units are the primary structure-forming units of the four MGs, connect through vertex-, edge-, and face-shared (VS, ES, and FS) atoms, and Co-centered units tend to connect with Co/B-centered units via the intercross-shared (IS) atoms. The addition of Y and Nb not only plays the role of connecting atoms but also enhances both bond strengths and the fractions of icosahedral-like units in increasing order Co75B25 < Co73Nb2B25 < Co71.5Y3.5B25 < Co69.5Y3.5Nb2B25, which is conducive to the enhancement of the structural stability, atomic packing density, and viscosity, thereby improving thermal stability and GFA. In addition, the improvement of structural stability and homogeneity leads to enhanced magnetic softness.