Sulfur-suppressed surface crystallization of Fe-based amorphous alloys

Understanding the mechanism governing surface crystallization of Fe-based amorphous soft-magnetic alloys is of paramount importance to their industrial production. In this study, the surface microstructural characteristics and elemental depth-distribution of the (Fe0.78Si0.09B0.13)100-xSx (x = 0-0.1) ribbons were carefully characterized. Accordingly, the surface crystallization mechanism and sulfur-suppressed surface crystallization as well as its effect on magnetic properties were thoroughly discussed. We found that the crystallization of Fe78Si9B13 (at. %) ribbon is exclusively confined to the free surface with a limited quantity of alpha-Fe(Si) grains, which stems from the local composition change caused by the oxidation of the constituent Si, B elements and the heterogeneous nucleation due to the precipitated oxides. Surprisingly, the addition of S element can effectively suppress the surface crystallization and readily obtain amorphous ribbons with enhanced magnetic softness. The underlying mechanism should be attributed to the formation of Fe-S clusters and the dense packing of Fe atoms in the near free surface layer. The increased atomic packing density will impede the diffusion of Si, B elements to the ribbon's free surface, and thereby avoiding oxidation and ensuring uniform composition. As a result, amorphous ribbons with the elimination of surface crystallization can be obtained after adding S content exceeds 0.05 at.%, which facilitates the reduction of coercivity (Hc) while preserving saturation induction (Bs). These findings are important and provide a new strategy for suppressing surface crystallization, which is of great significance to the wide application of sulfur-containing raw materials and the large-scale industrial production of high performance soft-magnetic materials.