LASER RESOLIDIFICATION OF THE AL-AL2CU EUTECTIC - THE COUPLED ZONE

Laser rapid solidification experiments have been performed on hypoeutectic Al-Cu alloys (26, 27 and 28 wt%) and TEM foils have been produced by means of longitudinal cuts through the centre, or from the surface, of the laser traces. From the orientation of the microstructures present in the foils it has been possible to relate microstructural features to local growth rate during solidification. Several growth morphologies have been observed: regular eutectics at up to approximately 0.2 ms-1, dendritic/cellular structures from approximately 0.05-0.6 ms-1 and bands from approximately 0.6 ms-1. The transition from eutectic to dendritic structures at such velocities requires a revision to the shape of the coupled zone at large undercoolings which is, however, consistent with current theories of rapid solidification. The utility of combined rapid solidification experiments and theory for the determination of the metastable phase diagram has been demonstrated.