Growth related metastable phase selection in a 6xxx series wrought Al alloy

Bridgman furnace solidification has been used to study directional solidification of a model 6xxx series alloy. At solidification front growth velocities in the range of 5-120 mm/min, alpha -AlFeSi, beta -AlFeSi and Mg2Si were identified by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). MTDATA predicts that alpha -AlFeSi and Mg2Si should form during equilibrium solidification. DSC results confirm that beta -AlFeSi is a metastable phase. The eutectic reaction resulting in alpha -AlFeSi formation juring directional solidification doss not take place at a cooling rate of 10 K/min. SEM and TEM observations show that intermetallic particle size strongly depends on the Bridgman growth velocity. Quantitative XRD results show that the maximum proportion of metastable beta -AlFeSi intermetallic phase is produced at relatively low growth velocities in the range of 30-60 mm/min. Both DSC and XRD analysis confirm that at higher growth velocities, such as 120 mm/min, the formation of alpha -AlFeSi phase by the eutectic reaction dominates the phase selection. (C) 2001 Elsevier Science B.V. All rights reserved.