Effect of substitution of Cu by the same group element (Ag, Au) on the soft magnetic properties of Fe83.3Si4B8P4Cu0.7 alloy was investigated. Ag-/Au-substituted alloys have a similar hetero-amorphous structure with Fe-Si-B-P-Cu alloy, and consists of large α-Fe grains with a high distribution ratio and residual amorphous phase. The large coercivity, low permeability and unsatisfied saturation magnetization of Fe84.32Si3.87B7.59P4.16Ag0.05 and Fe83.47Si3.89B7.88P4.04Au0.71 alloys are attributed to their large α-Fe grain size in comparison to Fe83.3Si4B8P4Cu0.7 alloy according to Herzer's random anisotropy model. Cu atoms supersaturated in hetero-amorphous Fe-Si-B-P-Cu matrix results in the formation of α-Fe grains with smaller grain size around large numbers of pre-existing nuclei during annealing than those of Ag-/Au-substituted Fe-Si-B-P-Cu alloys. The competition driven nanocrystallization mechanism governs the balance between the nucleation and growth of α-Fe grains, and the saturation degrees, surface diffusivities and competition abilities of seed elements Ag, Au and Cu finally determine the dependency between their microstructure and soft magnetic performances. The Cu seed element plays an optimal role in controlling the microstructure and soft magnetic properties of NANOMET® alloys.
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