Eutectoid decomposition of Zn-Al splat-quenched and bulk specimens

Abstract

The structures and overall transformation rates of the cellular eutectoid decomposition products observed in thin-foil and bulk specimens of Zn-40.6 at.% Al alloys are compared. The thin-foil alloys were prepared by splat-quenching onto room temperature and warm substrates and the microstructures of electron-transparent areas were examined by transmission electron microscopy. It is found that lamellar structures formed in the thin-foils at significantly lower temperatures and with considerably smaller interlamellar spacings than in the bulk specimens investigated hitherto. Thermal analysis of bulk alloys conducted during the exothermic cellular reaction is used to determine the specimen size dependence of the amount of recalescence. The temperature dependence of the Zn-Al interlamellar spacings and the time-temperature-transformation diagram for eutectoid Zn-Al alloys, isothermally transformed without recalescence, are then established. The transformation temperature dependence of the interlamellar spacing obtained here is used to give a more accurate estimate of the Zn-Al interfacial energy. Transformation curves for the thin-foil specimens are calculated using the corrected transformation diagram for bulk material and differences between the relative dispositions of the curves for bulk and thin-foil specimens account for the observation, at low temperatures, of granular structures in the former and predominantly lamellar structures in the latter.