Characterization and properties of soft magnetic (Fe_0.5Co_0.5)₇₅B₂₁Nb₄ metallic glasses subjected to cryogenic treatment and relaxation annealing

The effect of cryogenic treatment (CT) and relaxation annealing on the average nearest neighboring distance of atom (d_m), thermodynamic stability, soft magnetic properties, microhardness (H_v), and corrosion resistance of as-spun (Fe_0.5Co_0.5)₇₅B₂₁Nb₄ metallic glasses (MGs) is studied. On the premise of maintaining a fully amorphous phase, appropriate CT and relaxation annealing are conducive to achieving the synergistic effect of increasing saturation magnetization (M_s) and reducing coercivity (H_c). Shallow CT at 213 K optimally enhances the soft magnetic properties of MGs. Given its low activation energy of nucleation and increased activation energy of growth, appropriate CT is beneficial for achieving uniform annealed nanocrystals in amorphous phases. The correlation between free volumes (FVs) and potential energy suggests that the variation in H_c depends on the expansion and contraction behavior of amorphous phases after different CT processes. The fitting formulas of H_c–d_m and M_s–H_v correlations demonstrate that soft magnetic parameters have a solid linear relationship with the contents of FVs and degree of dense random packing. Moreover, pitting resistance is improved by appropriate CT and relaxation annealing. This improvement is characterized by the promotion of the stability of the Nb-rich passive film formed during electrochemical corrosion in 3.5wt% NaCl solution.