Heat of crystallization and microstructure of amorphous Pd82Si18 and Fe80B20 alloys

Abstract

For X-ray amorphous Pd82Si18 and Fe80B20 alloys prepared by a melt-spinning method with various wheel velocities, the relationship between the heat of crystallization and microstructure was studied using differential scanning calorimetry and optical and transmission electron microscopy. The result of TEM shows that the X-ray amorphous Pd82Si18 alloy has a uniform microstructure without crystals and the X-ray amorphous Fe80B20 alloy contains uniformly-dispersed α-Fe particles with nanometer sizes which are undetectable by the ordinary X-ray diffraction analysis. The Avrami exponent n for the crystallization of the X-ray amorphous alloys supports the microstructural features. The variations in the heat of crystallization with the cooling rate estimated from the ribbon thickness correspond to the volume fraction of crystalline phases for Pd82Si18 alloys and the particle size of α-Fe for Fe80B20 alloys. The heat of crystallization is proposed as a parameter expressing the degree of non-crystallinity for the alloys. The critical cooling rates for the formation of X-ray amorphous phases mean the minimum cooling rate to avoid the formation of crystals with a detectable 'volume fraction' for Pd82Si18 alloys and that with a detectable 'size' for Fe80B20 alloys.