RAPIDLY SOLIDIFIED AL-CU ALLOYS .1. EXPERIMENTAL-DETERMINATION OF THE MICROSTRUCTURE SELECTION MAP

Laser rapid solidification experiments have been performed on Al-Cu alloys of hypereutectic composition 36, 40 and 44 wt% Cu. By taking thin foils from the surface of the laser traces it has been possible to study the resulting growth morphologies using TEM; the microstructural orientation allowing the morphologies to be correlated with their local growth velocities. Microstructural features in the velocity range 0.01-2.0 ms-1 have been studied, and eutectic (both with and without oscillatory instabilities), dendritic, cellular, banded and planar front growth have all been observed. By combining these results with earlier observations made on alloys with lower Cu concentrations, it has been possible to produce a microstructure selection map for Al-Cu alloys. The map correlates microstructure to growth velocity and composition in the ranges 0.01-2.0 ms-1 and 0-44 wt% Cu, i.e. the major part of the binary Al-Al2Cu eutectic (0-54 wt% Cu). As well as providing an interesting overview of solidification structures, several features of this map can be used to gain information on the Al-Cu phase diagram.