Non-equilibrium reactions in 6xxx series Al alloys

Abstract

Laboratory scale techniques have been developed to investigate particular aspects of nucleation and growth during solidification. These techniques have been used to isolate and study under controlled conditions the nucleation and growth aspects of the solidification that may occur in industrial processes such as direct chill (DC) casting, where cooling rates and growth velocities can be relatively high. The resultant intermetallic phase selection has been studied in Al alloys, where a wide range of equilibrium and metastable intermetallics can form. This paper describes the use of the entrained droplet technique together with intermetallic phase identification by transmission electron microscopy (TEM) to study the solidification sequence in a model 6xxx series Al alloy. In the entrained droplet technique, 1–1000 nm liquid droplets are entrained in a solid matrix which acts as a heterogeneous nucleation catalyst, and their solidification is monitored using differential scanning calorimetry (DSC). In the majority of commercial Al alloys, solidification takes place by the formation of primary Al, followed by secondary eutectic and peritectic reactions to form small quantities of intermetallic particles in the interdendritic regions. In the model 6xxx series Al alloy, the solidification sequence observed using the entrained droplet technique after partially melting to 640°C and re-solidifying at a cooling rate of 5 K min −1, showed that the cubic α c-AlFeSi phase was formed via three reactions: (i) the predicted equilibrium peritectic reaction L+Al 13Fe 4→α-Al+α c-AlFeSi; (ii) a non-equilibrium eutectic reaction L→α-Al+α c-AlFeSi; and (iii) a ternary eutectic reaction L→α-Al+α c-AlFeSi+Mg 2Si. Only the peritectic and non-equilibrium eutectic reactions were readily observed during solidification at a cooling rate of 5 K min −1 from the fully molten state. The literature suggests that all the three reactions may operate during commercial casting operations.