Microstructure and microsegregation in a directionally solidified quaternary Al-rich Al-Cu-Mg-Zn alloy

A quaternary A1-3.86Cu-0.89Mg-0.99Zn alloy was directionally solidified with a cooling rate of 0.78 K/s. Solidification path was determined by both experiment method and micromodel simulation. It was found that the phase solidified sequence is Liq. --> (Liq. + alpha) -, (Liq. + alpha + theta) --> (Liq. + alpha + theta + S) --> (Liq. + alpha + theta + sigma). The microstructure and microsegregation in this directionally solidified quaternary alloy were determined by SEM, image analysis and EPMA. The solidification microstructure of the quaternary alloy was dendritic, similar to those of Al-rich Al-Cu-Mg alloys solidified at the same condition. Both the fractions of solid formed and the solute concentration gradients in the dendrites were studied. While the volume fractions of solid formed were measured by image analysis, the solute concentration gradients within the dendrites were determined by an area scan technique. The EPMA standards used to measure the compositions were Al-lCu-0.5Mg alloy for Al, pure copper for Cu, MgO for Mg and pure zinc for Zn. The fractions of solid formed were also obtained from the concentration profiles, which were in agreement with those fractions obtained by image analysis. The model-calculated fractions of solid and the concentration gradients were in reasonable accord with the measured data The model used was a modified Scheil model including back diffusion in the solid, dendrite arm coarsening, and the effects of undercooling, which is similar to that used for Al-Cu-Mg alloys in an earlier study.