Abstract
The effects of alloying elements, M (M = Nb, Ta, and V), on the stability of D022 γ”-Ni3M precipitates at elevated temperatures were investigated in Ni-22Cr-based ternary and quaternary alloys using heat-treated diffusion-multiple and bulk samples with discrete chemical compositions, with a final goal to improve the precipitate stability and the temperature capability of the Alloy-718-type Ni-based superalloys. Our microstructural characterization indicated that a complete replacement of Nb with Ta stabilized the γ” precipitates at temperatures up to 800 °C. A partial replacement of Ta with V was found to stabilize the precipitates even at 900 °C. Differential scanning calorimetry and high-temperature X-ray diffraction experiments demonstrated that the D0a-Ni3M structure was stable at elevated temperatures in the Ni-Cr-Ta ternary system. Lattice parameter measurements at room temperature suggested that a partial replacement of Ta with V decreased the lattice misfit between the fcc γ matrix and the γ” precipitate phases along the a- and c-axes of the tetragonal γ” crystal structure. The improved γ” precipitate stability was discussed in terms of the chemical driving force, misfit strain, and diffusion kinetics viewpoints.
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