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

Abstract 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+Al 13 Fe 4 → α-Al+α-AlFeSi, should occur during equilibrium solidification of the alloys. However, no composite Al 13 Fe 4 /α-AlFeSi particles, but composite Al 13 Fe 4 /β-AlFeSi particles and triple phase junctions have been observed for the first time, indicating a divorced metastable β-AlFeSi quasi-peritectic reaction, L+Al 13 Fe 4 → 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 Al 13 Fe 4 and peritectic β-AlFeSi. The nucleation and growth of peritectic phases and the morphology evolution of the two intermetallic phases, Al 13 Fe 4 and β-AlFeSi, are discussed.