A Thermodynamic Approach to Guide Reactive Element Doping: Hf Additions to NiCrAl

A method based on thermodynamic modeling was developed to determine optimal amounts of Hf additions to Al₂O₃-forming, γ-γ′ NiCrAl alloys. The alloy ability to maintain Hf in solution was set by the Hf concentration required to form HfO₂ at the oxygen activity defined by the alloy/Al₂O₃ equilibrium. This Hf tolerance decreased with increasing temperature and increased with increasing γ′ fraction. The latter was due to the higher solubility of Hf in γ′, compared to γ. The validity of the procedure was evaluated by oxidizing a series of NiCrAl–Hf alloys in dry air at 1000–1200 °C. The experimental results followed the predicted trends, although the Hf tolerance tended to be overestimated. The applicability of the criterion, and potential routes for improved predictability, were discussed by considering the influence of the compositional changes occurring at the metal surface during the transient and steady-state stages of the oxidation process.