Thermodynamic modeling and experimental investigation of the Ni-rich corner of the Ni-Al-Hf system

Ni-based superalloys with the addition of Hf show promising oxidation resistance and exhibit further improvements in thermal barrier coating (TBC) lifetime. To understand the beneficial effects of Hf on oxidation resistance, it is essential to have phase equilibrium information for some key ternaries, such as Ni-Al-Hf. The phase relationships in the ternaries and multi-component systems serve as a roadmap for alloy design and processing control. In this study, the phase constituents of selected Ni-rich Ni-Al-Hf ternary alloys and their microstructures were investigated in both as-cast and long-term annealed states using SEM, XRD and EPMA. A thermodynamic description of the Ni-Al-Hf system was developed based on those of the constituent binaries and ternary experimental data obtained in the present study as well as those reported in literature. The calculated liquidus projection and isothermal sections at 1273, 1403 and 1433 K are in accord with the experimental results.