Calculation of phase diagrams for the metastable Al-Fe phases forming in direct-chill (DC)-cast aluminum alloy ingots

In direct-chill (DC)-cast 1xxx-and 5xxx-series Al sheet-ingots, the presence of mainly Fe and some 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 Al6Fe and AlmFe compounds in addition to the stable Al3Fe, and hence the fir-tree defect. Since the Al-Fe and Al-Fe-Si phase diagrams are not useful in predicting the metastable phase formation, a binary phase diagram study was conducted to calculate the Al-Al6Fe and Al-AlmFe metastable phase equilibria using a thermodynamic software and an Al-alloy database. The Al-Al3Fe phase diagram was calculated using the existing Gibbs energy data which gives the eutectic point at 1.85wt% Fe and the eutectic temperature as 654 °C. The missing Gibbs energy data for the metastable phases were estimated using substitutional and graphical methods and the phase diagrams were calculated. In the Al-Al6Fe phase diagram, the eutectic temperature is depressed from 654 °C (equilibrium) to 648 °C and the eutectic point is shifted from 1.85wt% Fe to 3.4wt% Fe. In the Al-AlmFe phase diagram, the eutectic temperature is 643 °C and the eutectic point is at 4.6wt% Fe. The verification of the calculated eutectic temperatures was carried out by DSC measurements which were conducted on samples removed from Al-Fe alloy rods directionally grown in a Bridgman-type solidification furnace. A good agreement is observed between the calculated and measured values.