Kinetics, thermodynamics and mechanism of metastable aluminum-aluminum(m) iron phase and fir-tree zone formation in DC-cast aluminum-iron-silicon alloy ingots

In DC-cast 1xxx-series aluminum alloy ingots, the presence of Fe and Si, and cooling rates increasing from < 1°C/s in the ingot center to ∼20°C/s near the surface cause the formation of metastable intermetallic Al6 Fe and AlmFe compounds in addition to the stable Al 3Fe. The highly contrasting etching/anodizing response of the Al 6Fe and AlmFe particles forming in the adjoining central and peripheral regions, respectively, creates the fir-tree zone (FTZ) defect in DC-cast ingots. This, in turn, causes streaking in Al sheets.; The need was great for a good fundamental understanding of the mechanism of metastable Al-A1mFe eutectic phase formation. Therefore, the mechanism was studied via the kinetic (experimental: Bridgman growth and DSC analyses), thermodynamic (macroscopic: phase diagram computation), and crystallographic (atomistic) approaches.; The controlled growth of Al-Al3Fe, and metastable Al-Al xFe, Al-Al6Fe, and Al-AlmFe eutectic phases was obtained in AA1050 and its binary analogue Al-0.33wt% Fe alloys via Bridgman growth at various growth velocities (V = 0.15 -60 mm/s). For each phase, by acquiring a series of growth-velocity-dependent invariant (for binary alloys) and univariant ( for AA1050) melting temperatures (T) via DSC, the binary and ternary melting and growth kinetics were determined as T vs ln(V) plots extrapolated to V = 0.001 mm/s.; Using the binary melting kinetics and tentative growth kinetics plots, the Gibbs free energy parameters of binary phases in the AA12S database were optimized, and the previously non-existent binary and ternary stable and metastable equilibrium phase diagrams (computed at V = 0.001 mm/s) and stable and metastable kinetic and dynamic phase diagrams (computed at the DC-casting velocity; V = 1 mm/s) were obtained via ThermoCalc(TM).