COMPOSITION AND PRECIPITATION INHOMOGENEITIES IN MELT-SPUN AL-CU AND AL-ZN RIBBONS

Non-uniform distributions of solute content in the supersaturated alpha-Al matrix and inhomogeneous precipitation of metastable/stable phases have been studied in rapidly solidified Al-Cu and Al-Zn ribbons. Double-peaked X-ray diffraction lines from the alpha-Al matrix were observed in three as-quenched Al-Cu alloys and one annealed Al-Zn alloy. For as-quenched Al-Cu ribbons, variations in composition, microstructure and precipitate density across the ribbons result from local differences in cooling rates. These differences originate from recalescence during the solidification. The double-peaked diffraction lines for these alloys correspond to two compositions of the alpha-Al phase with less copper solute in the top-side than in the wheel-side of the ribbons. Subsequently, annealing of the samples leads to inhomogeneous precipitation of the metastable THETA' phase across the whole thickness of the ribbons. Such inhomogeneities do not appear in as-quenched Al-Zn alloys which are quenched into a single alpha-Al phase. Splitting of the diffraction lines in the annealed ribbons arises from differences in the composition of two different alpha-Al matrix phases. The alpha-Al phase between the R phase lamellae has a low zinc content, while the alpha-Al phase where high densities of Guinier-Preston zones are retained has a higher zinc concentration.