The calculated compositional phase diagram is shown in Fig. and compared with experimental results tabulated by Thompson and Waldbaum . The main feature of the phase diagram is a miscibility gap. Neglecting vibrational free energy-as typically assumed for solidsolutions studied with cluster expansion techniquesresults in a miscibility gap extending to temperatures much higher than experimentally observed in agreement with previous DFT studies . The inclusion of vibrational effects lowers the miscibility gap significantly, to values in much better agreement with experiment. For the CE based only on MLFF vibrational free energies, the highest immiscibility temperature is calculated to be 950 K at x ≃ 0.4 close to the experimental value of 770 K.