THE STRUCTURE OF RAPIDLY SOLIDIFIED RIBBONS OF AL-12 AT-PERCENT-ZN ALLOY

The structure of melt spun rapidly solidified ribbons of the Al‐12 at.% Zn alloy has been investigated by scanning electron microscopy and by X‐ray diffraction. Four solidification processes differing in the extent of associated segregation of zinc have been found to be operative during the solidification of the ribbon. These successive processes beginning from above the sites of good contact of the melt with the rotating plane of the copper substrate are as follows: massive planar growth of grains, arrayed dendritic growth, unaligned growth of dendrites, and star‐like dendritic growth out from fragments of dendrites. Smaller grains resulting from the last two mentioned processes have been also found within the regions located above the sites of poor contact of the melt with the substrate (lift‐off regions). All indicated successive growth modes starting with the epitaxial regrowth of grains have been also found to proceed during the solidification of the melt flowed over the already solidified alloy ribbon. Grain boundary precipitation of equilibrium β (≈ Zn) phase has been detected in all stages of solidification. High density of lattice defects built‐up during solidification is leading to the immediate intragranular heterogeneous precipitation of β phase at elevated temperatures within the two‐phase region without the formation of intermediate metastable precipitates. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA