The Ni-Ni3Al phase diagram: thermodynamic modelling and the requirements of coherent equilibrium

The Ni-rich portion of the Ni-Al phase diagram is examined in light of data on coherent equilibrium between the γ (Ni-Al solid solution) and γ' (Ni 3 Al) phases, as well as on the thermodynamic requirements of coherent equilibrium. The model of Ardell and Maheshwari was used to calculate the difference between the incoherent and coherent solubility limits over the temperature range of 400-800 °C. New data on the elastic constants of Ni 3 Al as a function of temperature and composition, as well as the Gibbs free energy functions of the most recent thermodynamic model of the Ni-Al alloy system, were used in the calculations. With these input parameters the predicted differences between the incoherent and coherent solubility limits are very small, and inconsistent with recent experimental data on the variation of the coherent solubility with the γ' volume fraction. The inconsistency is a consequence of the large curvature of the Gibbs free energy function for the γ phase obtained from the thermodynamic model. These findings are discussed in the context of the accuracy of thermodynamic modelling of phase diagrams.