Thermodynamics and diffusion in ternary Fe–Al–Cr alloys, Part I: Thermodynamic modeling

The development of an accurate thermodynamic model description for the system Fe–Al–Cr is hampered by the sparseness of thermodynamic data in this ternary system. This compelled us to measure activities in the system Fe–Al–Cr using the Knudsen Effusion Mass Spectrometry technique. We investigated two samples with target compositions 45.1Fe–39.8Al–15.1Cr (at.%) and 49.9Fe–19.9Al–30.2Cr (at.%) in the temperature range between 1150 K and 1550 K. We combined these measurements with a revised thermodynamic model description for this system based on the so-called Calphad approach. This approach allows us to predict thermodynamic properties in the ternary system from the Gibbs energy formulations of the binary subsystems. This combination was used to discriminate between the quality of the sparse experimental activity data available in the literature and our own experimental data. We found that additional iron activity measurements are necessary in the binary system Fe–Al. Additional activity measurements of iron, aluminum and chromium are also necessary in a compositional region close to this system. We show preliminary calculations of the on-diagonal thermodynamic factors, which appear to be largest in the compositional region close to the system Fe–Al. For a composition in the ternary system they decrease in the order aluminum, iron, chromium.