High-temperature-oxidation-induced ordered structure in Inconel 939 superalloy exposed to oxy-combustion environments

In the integrated oxy-fuel combustion and the turbine power generation system, turbine alloys are exposed to high temperature and an atmosphere comprised of steam, CO₂and O₂. While surface and internal oxidation of the alloy takes place, the microstructure in the subsurface region also changes due to oxidation that results in the loss of the strengthening precipitates. In an earlier study of the oxidation of Inconel 939 Ni-based superalloy exposed to oxy-fuel combustion environment for up to 1000 h, a high-temperature-oxidation-induced phase transformation in the sub-surface region was noticed and a two-phase region formed at the expense of strengthening γ' phase. While one of the two phases was identified as the Ni-matrix γ solid solution, face-center-cubic) phase, the other product phase remained unidentified. In this study, the crystal structure of the unknown phase and its orientation relationship with the parent Ni-matrix phase was investigated through electron diffraction and high-resolution transmission electron microscopy. It was determined that the crystal structure of the unknown phase could be modeled as a ternary derivative of the ordered η-Ni₃Ti phase (D0₂₄) structure with lattice parameters of a=.5092 nm and c=.8336 nm, α=90°, β=90° and γ=120°.