Quasi-peritectic solidification reactions in 6xxx series wrought Al alloys

Secondary intermetallic phase formation during directional solidification of two 6xxx series wrought Al alloys at low growth velocities of 5-30 mm/min has been investigated using differential scanning calorimetry, transmission electron microscopy and scanning transmission electron microscopy. Thermodynamic calculations predict that a quasi-peritectic reaction, L + Al13Fe4 → α-Al + α-AlFeSi, should occur during equilibrium solidification of the alloys. However, no composite Al13Fe4/α-AlFeSi particles, but composite Al13Fe4/β-AlFeSi particles and triple phase junctions have been observed for the first time, indicating a divorced metastable β-AlFeSi quasi-peritectic reaction, L + Al13Fe4 → α-Al + β-AlFeSi. More detailed analysis suggests that the metastable β-AlFeSi quasi-peritectic reaction is more favourable both at nucleation and during growth. No unique orientation relationship was found between primary Al13Fe4 and peritectic β-AlFeSi. The nucleation and growth of peritectic phases and the morphology evolution of the two intermetallic phases, Al13Fe4 and β-AlFeSi, are discussed. © 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved