Abstract
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, CO2 and 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 1000h, 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=.5092nm and c=.8336nm, α=90°, β=90° and γ=120°.
Highlights
While new and promising oxy-fuel combustion turbine power generation systems are being developed, materials performance of candidate superalloys is a vital area of research
The base alloy remained unaffected by the high temperature oxidation and in its stabilized + ’ microstructure
The twinning zone was comprised of a Ni-matrix and the slab-shaped grain that appeared in a brighter contrast, i.e. the so-called “U phase”
Summary
While new and promising oxy-fuel combustion turbine power generation systems are being developed, materials performance of candidate superalloys is a vital area of research. This work is the continuation of the earlier effort, aiming to understand the crystal structure of the U phase through electron diffraction and high-resolution transmission electron microscopy (HR-TEM). The QSTEM package has an option for simulating thermal diffuse scattering by running multiple simulations with slight variations in atomic position simulating atomic thermal vibrations, summing the varied exit waves into a final exit wave incorporating thermal variation in the atomic positions. This option was used with 50 variations, set for a temperature of 300 K
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