Annealing Optimization of High Induction (> 1.7 T) Fe_85−(x+y)Co_xCu_yB₁₅ Alloys With Enhanced Magnetic Properties

In this work, the influence of the substitution of Co and Cu for Fe on the thermal stability, crystallization process, crystal structure and magnetic properties of Fe_85−(x+y)Co_xCu_yB₁₅ (x = 2.5, 5, 7.5; y = 0.6, 1.2) as-quenched alloys in the form of the ribbons have been studied. Thermal analysis showed that the average activation energy of the α-Fe phase crystallization decreases from 216.1 to 168.6 kJ/mol for Fe_84.4−xCo_xCu_0.6B₁₅ and increases from 157 to 208.4 kJ/mol for Fe_84.8−xCo_xCu_1.2B₁₅. The relationship between the annealing temperature (260 °C to 440 °C for 20 minutes in vacuum) and magnetic properties (saturation induction, coercivity and core power losses) was determined for isothermally vacuum-annealed. The optimal annealing conditions corresponding to the minimum value of power core losses were found for temperatures between 300 °C and 330 °C. Optimally vacuum-annealed alloys are characterized by high saturation induction of 1.67 to 1.78 T, a relatively low coercivity of 14.7 to 26.4 A/m, core power losses at 1 T/50 Hz of 0.21 to 0.39 W/kg and magnetic permeability μ′ up to 250 to 967. The X-ray diffraction was used to monitor the crystal structure evolution during annealing. It confirmed the occurrence of three stages: glass relaxation, early α-Fe(Co) crystallization stage for the minimum value of core power losses and extensive two-phase crystal growth stage with high saturation induction and deteriorated coercivity and core power losses. Transmission electron microscopy examination verified that optimally annealed alloy was the composite nanomaterial built of α-Fe(Co) nanocrystals immersed in an amorphous matrix.