Influence of cooling characteristics on glass formation of FeB and NiSiB metallic alloys

Abstract

One common approach to the glass-forming process of rapidly quenched metallic alloys is to prevent the formation and growth of crystalline nuclei in the undercooled melt. Homogeneous nucleation is the dominant mechanism for moderately high cooling rates. In the frequency of a kinetic model, glass-forming ability is defined in terms of a critical cooling rate to produce amorphous foils with quenched-in crystals with volume fractions less than a given value. In a preliminary paper a heat flow model for the formation of amorphous foils by cooling a melt film of given thickness on a heat conducting substrate with constant heat transfer coefficient was presented allowing for a more realistic description of the cooling process as well as spatial differences in the as-quenched state in the amorphous foils. Practically the thermal contact may be limited to a rather short time in certain rapid solidification techniques as in twin-roller experiments or may, according to recent results on single-roller experiments, undergo a sudden change in a smaller heat transfer coefficient during the ribbon-wheel adherence period. Therefore, in this work, the influence of time-dependent foil-substrate heat transfer conditions on the glass formation process and the as-quenched state of the amorphous foils is considered.