Short range ordering in the melt and its manifestation in glassy Fe-Co-B Investigation by positron annihilation lifetime

Six sets of ribbons of Fe64Co21B15 metallic glass obtained by rapid quenching of the melt having different temperature and time history, selected with respect to the temperature-time evolution of melt viscosity, have been investigated by positron annihilation spectroscopy. The interest has been focused on the correlation between the expected melt precursor structure and the presence of ordered units (clusters, associations) in the amorphous matrix. Ribbons have been annealed for suitable times at temperatures below and above the crystallization temperature. Such selective thermal treatment of the samples has enabled to accentuate or anneal-out the effects of the presence of small ordered structural units as reflected by the positron annihilation lifetimes and positron affinities and by changes of electron structure obtained from electrical resistivity measurements. Fine structure of the amorphous matrix has been determined by analyzing the positron annihilation lifetime spectra with the use of a numerical Laplace transform method which provides continuous distributions of the position annihilation lifetimes. Differences in local ordering have been found for samples from master alloys with different melt histories.