Thermodynamics of the Al2O3–Al4C3 system: II. Free energies of mixing, solid solubilities and activities

The phase diagram of the Al2O3–Al4C3 system is now based on unit constituents 1/5 Al2O3 and 1/7 Al4C3, and the formation of intermediate compounds 1/9 Al4O4C and 1/4 Al2OC below 2100 K is addressed in terms of free energies of mixing, with corresponding partial molar quantities. Some relations are derived involving extents of solid solubilities and activities, especially at the eutectoid temperature. It is shown that the positive excess amount, relative to the stoichiometric mole fraction, of 1/7 Al4C3 in the oxycarbide 1/9 Al4O4C, highest at that temperature, can be no greater than 3.98 × 10−10, whereas, based on a reasonable assumption, the negative amount should not exceed 6.9319488 × 10−5. Activities a 1 / 7 A l 4 C 3 and a 1 / 5 A l 2 O 3 calculated at the eutectoid temperature show that both 1/7 Al4C3 and 1/5 Al2O3 present remarkably large positive deviations from Raoultian behaviour, as attested by their activity coefficients at infinite dilution.