In situ quantification of the nucleation and growth of Fe-rich intermetallics during Al alloy solidification

Real-time in situ synchrotron X-ray tomographic microscopy was used to gain new insights into and quantify the nucleation mechanisms and growth kinetics of β-Al5FeSi intermetallics during solidification of an aluminium Al-7.5Si-3.5Cu-0.6Fe (wt.%) alloy. Three new insights were obtained. First, the plate-like β-intermetallics appeared to nucleate mainly on or near the primary aluminium dendrites and to a lesser extent off the oxide skin on the surface of the specimen. Second, for this alloy composition, β-intermetallic formation was largely complete before the formation of Al-Si eutectic. Third, the β-intermetallics formed via fast lateral growth, wrapping around and in between the primary dendrite arms. Further, the nucleation and growth dynamics of β-intermetallics were quantified as a function of undercooling in a functional form that could be easily used in microstructural simulations. The frequency of intermetallic interaction mechanisms, such as plate nucleation vs. impingement and branching, were also quantified.