| Summary: | SUMMARY
GTD-111 DS is a directionally solidified superalloy currently used in turbine engines.
To accurately predict the life of engine components it is essential to examine and characterize
the microstructural evolution of the material and its effects on material properties. The
as-cast microstructure of GTD-111 is highly inhomogeneous as a result of coring. The
current post-casting heat treatments do not effectively eliminate the inhomogeneity. This
inhomogeneity affects properties including tensile strength, fracture toughness, fracture
path, and corrosion behavior, primarily in terms of the number of grains per specimen. The
goal of this work was to link microstructural features to these properties.
Quantitative fractography was used to determine that the path of cracks during failure
of tensile specimens is influenced by the presence of carbides, which are located in the
interdendritic regions of the material as dictated by segregation. The solvus temperature
of the precipitate phase, Ni3(Al, Ti), was determined to be 1200C using traditional metallography,
differential thermal analysis, and dilatometry. A heat-treatment was designed
to homogenize the microstructure for tensile testing that isolates the carbide by dissolving
all of the eutectic Ni3(Al, Ti) precipitate phase, which is also found in the interdendritic
areas.
High temperature oxidation/sulfidation tests were conducted to investigate the corrosion
processes involved when GTD-111 DS is utilized in steam and gas combustion turbine
engines. The kinetics of corrosion in both oxidizing and sulfidizing atmospheres were determined
using thermogravimetric analysis. Additionally, metallography of these samples
after TGA revealed a correlation between the presence of grain boundaries and sulfur attack,
which led to catastrophic failure of the material under stress-free conditions in a sulfur
bearing environment. In summary, this work correlates the inhomogeneous microstructure
of GTD-111 DS to tensile fracture, and the corrosion process in turbine engines.
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