FORMATION OF METASTABLE LL2-AL3Y THROUGH RAPID SOLIDIFICATION PROCESSING

A microstructure composed of coherent metastable L1(2) second-phase particles in a matrix of a supersaturated Al solid solution has been produced via rapid solidification processing of Al-Y alloys. Splat-quenched samples of the nominal compositions Al with 1, 3. 5, and 8 at.% Y and droplet samples made with the droplet emulsion technique were characterized by differential scanning calorimetry (DSC), differential thermal analysis (DTA), scanning electron microscopy, X-ray diffraction (XRD), and transmission electron microscopy. The 5 and 8 at.% Y splat samples exhibit a uniform fine dispersion of coherent metastable L1(2) second-phase particles with a lattice parameter of 0.4234 nm, while the 1 at.% Y splat sample exhibits a cellular structure. Thermal cycling of splat-quenched Al-3at.%Y showed an exothermic signal on first heating at around 240-degrees-C corresponding to the partial decomposition of the metastable structure, with some retained L1(2) phase structure detectable by XRD following heating of the sample to 550-degrees-C in the DSC. DTA of the droplets revealed undercoolings in excess of 100-degrees-C for the alloy composition range examined.