Calculation of Phase Diagrams and Verification of the Eutectic Temperatures for the Solidification of Metastable Al-Fe Phases in DC-Cast Aluminium Alloys

摘要

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 Al_6Fe and Al_mFe compounds besides the stable Al_3Fe, and hence the fir-tree defect. Since the Al-Fe and Al-Fe-Si phase diagrams are not useful for predicting the metastable phase formation, a binary phase diagram study was conducted to calculate the Al-Al_6Fe and Al-Al_mFe metastable phase equilibria using thermodynamic software and an Al alloy database. The Al-Al_3Fe phase diagram was calculated using the existing Gibbs energy data which gives the eutectic point at 1.85wt% Fe and 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-Al_6Fe 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-Al_mFe phase diagram, the eutectic temperature is 643 °C and the eutectic point is at 4.6wt% Fe. Verification of the calculated eutectic temperatures was carried out by DSC measurements which were conducted on samples removed from Al-Fe alloy rods grown at various cooling rates in a Bridgman-type directional solidification furnace. A good agreement is observed between the calculated and measured values.