Intermetallic phase selection during solidification of Al-Fe-Si(-Mg) alloys

Commercial wrought aluminium alloys often contain a mixture of intermetallic phases. The exact composition of the alloy and the casting conditions will directly influence the selection and volume fraction of each such phase in the starting ingot or billet. This paper aims to show how the relative ease of formation of each different phase changes with composition and how recent advances in understanding of the thermodynamic and kinetic factors influencing phase selection allow us to predict such changes. Firstly, the balance of intermetallic phases changes from binary Al-Fe phases to ternary Al-Fe-Si phases as the alloy type changes from 1xxx series (commercially pure Al) through to 6xxx type alloys, consistent with calculations of interdendritic liquid composition development and phase diagram predictions. Secondly, the addition of Mg at levels of >1.0 wt% to otherwise dilute Al-Fe-Si alloys is seen to promote Al, Fe over other binary Al-Fe phases, a trend that is also observed on the addition of traditional grain refiners (Al-Ti-B). Microstructural observations show that one effect of the Mg is to alter the primary aluminium solidification morphology from largely columnar to heavily dendritic, providing a similar local environment in which the intermetallic phases nucleate and grow as is the case when the material is grain refined. This indicates that the primary phase morphology may influence the selection of the secondary (intermetallic) phase.