EFFECT OF CARBON ON SURFACE CRYSTALLIZATION OF AMORPHOUS FE-B-SI ALLOY INDUCED BY SELECTIVE OXIDATION

The relationship between the annealing atmosphere and the magnetic properties of amorphous Fe78.5B13Si8.5 and Fe80.5B14Si3.5C2 alloys has been studied, showing that annealing in dry H2 significantly improves core loss in amorphous ribbons, giving much better results than annealing in an H2H2O atmosphere. A boron depletion zone containing 0-3 mol.% B and 9-11 mol.% Si or containing 5-7 mol.% B, 3-4 mol.% Si and 0-2 mol.% C in the surface layer of amorphous FeBSi and FeBSiC ribbons respectively was detected by Auger electron spectroscopy under the oxide film formed during annealing in H2H2O. When the amorphous ribbons are annealed in H2H2O, the H2O in the annealing atmosphere selectively oxidezes boron in the alloy to form a B2O3 film and a boron depletion zone; the alloy in this zone is then crystallized into α-Fe. The thickness of the surface crystalline layer induced by selective oxidation is more significant in FeBSi than in FeBSiC, possibly because the apparent dew point in the annealing atmosphere near the amorphous ribbon surface decreases with decarburization reaction in the FeBSiC system. © 1990.