CALCULATION OF PHASE-DIAGRAMS FOR THE METASTABLE AL-FE PHASES FORMINGIN DIRECT-CHILL (DC) CAST-ALUMINUM ALLOY INGOTS

In direct-chill (DC)-cast 1xxx- and 5xxx-series Al sheet-ingots, the p resence of mainly Fe and some Si, and cooling rates increasing from le ss than or equal to 1 degrees C/s in the ingot center to similar to 20 degrees C/s near the surface cause the formation of metastable interm etallic 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 diagram s are not useful in predicting the metastable phase formation, a binar y phase diagram study was conducted to calculate the Al-Al6Fe and Al-A lmFe metastable phase equilibria using a thermodynamic software and an Al-alloy database. The Al-Al3Fe phase diagram was calculated using th e existing Gibbs energy data which gives the eutectic point at 1.85wt% Fe and the eutectic temperature as 654 degrees C. The missing Gibbs e nergy data for the metastable phases were estimated using substitution al and graphical methods and the phase diagrams were calculated. In th e Al-Al6Fe phase diagram, the eutectic temperature is depressed from 6 54 degrees C (equilibrium) to 648 degrees C and the eutectic point is shifted from 1.85wt% Fe to 3.4wt% Fe. In the Al-AlmFe phase diagram, t he eutectic temperature is 643 degrees C and the eutectic point is at 4.6wt% Fe. The verification of the calculated eutectic temperatures wa s carried out by DSC measurements which were conducted on samples remo ved from Al-Fe alloy rods directionally grown in a Bridgman-type solid ification furnace. A good agreement is observed between the calculated and measured values.