Grain refinement mechanism of soft-magnetic alloys with nanocrystals embedded in amorphous matrix

To obtain uniform and stable nanostructure with fine α-Fe grains is very important for the wide applications of Fe-based nanocrystalline soft-magnetic alloys. In this study, the nanostructure evolution of the Fe_84.75Si₂B₉P₃C_0.5Cu_0.75 (at.%) alloy after annealing under different conditions was characterized in detail. It is found that the alloy exhibits excellent structural stability, which can maintain small α-Fe grains for a prolonged annealing time at low temperature. The increase of annealing temperature and/or annealing time will lead to the precipitation of compound phases in the intergranular amorphous interphase, which affects the α-Fe grains size greatly and determines the structural stability. The elemental mappings of the nanostructured alloys reveal that metalloid elements are enriched in the intergranular amorphous interphase, wrapping around α-Fe grains. The grain refinement and nanostructure stability of these alloys are derived from the shielding and soft-impingement effects of the core-shell like structure. The nanostructure stability is lost with the precipitation of compound phases in the intergranular amorphous interphase, owing to the break-down of the shielding layer, which results in the rapid coarsening of α-Fe grains by coalescence.