The Relationship of Microstructure to Fracture and Corrosion Behavior of a Directionally Solidified Superalloy

Fractography;High temperature corrosion;Superalloy

Trexler, Matthew David ; Materials Science and Engineering

University:Georgia Institute of Technology

Department:Materials Science and Engineering

关键词: Fractography;    High temperature corrosion;    Superalloy;   

Others  :  https://smartech.gatech.edu/bitstream/1853/14617/1/trexler_matthew_d_200705_phd.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

SUMMARYGTD-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 characterizethe microstructural evolution of the material and its effects on material properties. Theas-cast microstructure of GTD-111 is highly inhomogeneous as a result of coring. Thecurrent post-casting heat treatments do not effectively eliminate the inhomogeneity. Thisinhomogeneity affects properties including tensile strength, fracture toughness, fracturepath, and corrosion behavior, primarily in terms of the number of grains per specimen. Thegoal of this work was to link microstructural features to these properties.Quantitative fractography was used to determine that the path of cracks during failureof tensile specimens is influenced by the presence of carbides, which are located in theinterdendritic regions of the material as dictated by segregation. The solvus temperatureof the precipitate phase, Ni3(Al, Ti), was determined to be 1200C using traditional metallography,differential thermal analysis, and dilatometry. A heat-treatment was designedto homogenize the microstructure for tensile testing that isolates the carbide by dissolvingall of the eutectic Ni3(Al, Ti) precipitate phase, which is also found in the interdendriticareas.High temperature oxidation/sulfidation tests were conducted to investigate the corrosionprocesses involved when GTD-111 DS is utilized in steam and gas combustion turbineengines. The kinetics of corrosion in both oxidizing and sulfidizing atmospheres were determinedusing thermogravimetric analysis. Additionally, metallography of these samplesafter 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 sulfurbearing environment. In summary, this work correlates the inhomogeneous microstructureof GTD-111 DS to tensile fracture, and the corrosion process in turbine engines.