Influence of γ'-Ni3(Al, Ti) precipitate morphology and lattice misfit on the oxidation behavior of Ni-based superalloy IN740H

Abstract

IN740H, a γ'-lean Ni-based superalloy, has recently been code-approved for fabricating boiler structural components in Advanced-Ultrasupercritical (A-USC) power plants. However, prolonged service exposure leads to γ'-Ni₃(Al, Ti) precipitate coarsening, altering γ' morphology and lattice misfit (δ), which may compromise its oxidation resistance. This study addresses this gap by evaluating the oxidation response of IN740H with systematically varied γ' characteristics through thermal aging at 800 °C for 4 h to 10,000 h. Aging resulted in γ' precipitate growth from ∼18 nm (spherical) to ∼200 nm (cuboidal), with δ increasing from < 0.16 % to ∼0.52 %. Oxidation behavior was investigated via thermogravimetric analysis at 760°C for 100 h and long-term furnace tests at 760 °C for 1000 h. All samples formed a duplex oxide scale comprising an outer Cr₂O₃ layer doped with TiO₂ and an inner Al₂O₃ layer. Short-term testing revealed accelerated oxidation kinetics in samples with higher δ, indicating an inverse correlation between lattice misfit and oxidation resistance. In contrast, long-term exposure revealed a non-linear correlation, influenced by γ' coarsening. Samples with thermally stable, large cuboidal γ' precipitates exhibited superior oxidation resistance through sub-parabolic internal oxidation kinetics. In contrast, samples with finer γ' precipitates exhibited γ' instability, resulting in near-linear internal oxidation kinetics. This study underscores the critical role of γ' morphology and lattice misfit in determining the oxidation performance of IN740H under prolonged high-temperature exposure.