Microstructural transitions in Al–Cu ribbons manufactured by planar flow casting

Morphological transitions in Al–4.2wt%Cu and Al–14.3wt%Cu ribbons manufactured by planar flow casting were analyzed by a combination of optical, scanning and transmission electron microscopy as a function of the solidification velocity. There is a {200} preferred orientation of the α-Al phase at the wheel surface of the Al–4.2wt%Cu ribbon, but no distinguishable preferred orientation in the Al–14.3wt%Cu ribbon. The absolute stability velocities of the Al–4.2wt%Cu and Al–14.3wt%Cu ribbons were determined to be 2.28 and 8.40 ms−1, respectively. For the Al–4.2wt%Cu ribbon, a segregation-free columnar layer was developed near the bottom of the ribbon, where the solidification velocity was higher than the critical value of 2.28 ms−1. At lower solidification velocities transitions occurred to cellular columnar, and then equiaxed dendritic layers. In Al–14.3wt%Cu ribbons, however, no segregation-free layer was found and only cellular columnar and equiaxed dendritic layers were observed, due to the solidification velocity being lower than the critical velocity for absolute interface stability for an Al–14.3wt%Cu ribbon of 8.40 ms−1. A solidification sequence has been proposed for the formation of Al–4.2wt%Cu ribbons.