Reliability of current thermodynamic data in predicting phase stability of superalloys

The time-temperature-transformation (TTT) diagrams of Nimonic 263 and Hastelloy X are used to provide phase stability information for these two alloys. The results are then used to test the thermodynamic predictions using a commercial Ni-database. The experimental solvus of various phases (the gamma prime phase and the topologically close packed (TCP) phases) is used to check the thermodynamic calculation results. The comparison shows that the existing thermodynamic data generally do a very good job in predicting the stability of the gamma prime phase, but a less satisfactory job for the TCP phases. A method is suggested to provide critical experimental data needed to improve the thermodynamic parameters of the TCP phases. In addition, examples are presented to illustrate the importance of long-term exposure data in judging the real equilibrium phases of multicomponent superalloys.