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, α-AlFeSi, β-AlFeSi and Mg2Si were identified by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). MTDATA predicts that α-AlFeSi and Mg2Si should form during equilibrium solidification. DSC results confirm that β-AlFeSi is a metastable phase. The eutectic reaction resulting in α-AlFeSi formation during directional solidification does 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 β-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 α-AlFeSi phase by the eutectic reaction dominates the phase selection.