Microsample testing of alloys proposed for high temperature gas turbine applications

Modern aerospace, gas turbine engines consist of three primary sections: the compressor, the combustion system and the turbine. Thermal and mechanical property requirements of materials employed in the blades and disks of the turbine stage where hot gas temperatures can exceed 1350°C are particularly stringent. The use of thermal barrier coating systems, an aluminide bond coat, a thermally grown oxide and a ceramic top coat, applied on turbine blades have permitted increased operating temperatures, greater efficiencies and advanced performance. Similarly, the application of powder metallurgy methods has facilitated the production of turbine disks with corresponding capabilities. The efforts of this dissertation measure the mechanical properties of alloys for constituents directly linked to mitigating deformation and improving system durability. This includes the orientation-specific strength and intermediate temperature creep of individual grains extracted from a polycrystalline Ni-based superalloy for turbine disk applications and the elevated temperature tensile strength and creep resistance of two ruthenium nickel aluminides proposed for bond coat applications.;A heat treated, polycrystalline Ni-based superalloy matching the chemistry of Rene 104 contained individual crystallographic grains on the order of 10 mm which could be oriented by electron back scattered diffraction. Room temperature microsample testing of specimens extracted from these grains along crystallographic orientations of [-1 28 -7], [2 2 -1] and [-1 1 0] showed that the Young's moduli of this material compared favorably with the moduli predicted by the elastic constants of pure Ni and another Ni-based superalloy. Room temperature micro-tensile experiments conducted beyond yield for specimens oriented along the [-1 28 -7], [2 2 -1] and [-1 1 0] directions revealed 0.2% flow stresses of 800 MPa, 1050 MPa and 750 MPa, respectively. Intermediate temperature creep experiments conducted on oriented microspecimens to specifically investigate the microtwinning deformation mechanism previously observed in testing of polycrystalline specimens exhibited creep strain rates on the order of 1x10 - 10s-1 which were difficult to measure and compare. Single crystal properties measured in this study contribute valuable mechanical property data to crystal plasticity models currently being developed for polycrystalline Ni-based superalloys.;Micro-tensile experiments conducted to measure the elevated temperature properties of RuNiAl and RuNiAlCoCr reveal dramatic improvements in elevated temperature tensile strength, as compared to platinum modified nickel aluminide bond coats. Stress relaxation experiments confirm that the creep strength of ruthenium nickel aluminides is at least equal to that of platinum modified nickel aluminide bond coats and have been successfully modeled using the Sherby-Dorn description of power-law creep. Measured temperature-dependent creep response of the ruthenium nickel aluminide alloys is qualitatively normalized by an activation energy of approximately Q=300kJ/mol which is consistent with the reported value of Q=304kJ/mol for the diffusion of Ru in Ni. This suggests that creep in these alloys is controlled by the lattice diffusion of Ru atoms in a Ni matrix. Based on models predicting thermal barrier coating life, ruthenium nickel aluminides exhibit mechanical properties which could improve the performance and durability of gas turbine engines.