Ni-Based Superalloys

From Northwestern University Center for Atom-Probe Tomography
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Superalloys are a class of precipitate-hardened alloys that achieve high strength and corrosion resistance above the temperature limits (> 800°C) accessible to martensitic steels. Nickel-based superalloys have been primarily developed for high temperature structural applications within land-based gas turbine and aviation jet engines. In current manufacturing processes, it is common to cast engine components, such as nozzle guide vanes and turbine blades as single crystals, thus eliminating the potential for grain boundary embrittlement and the need for grain boundary strengtheners such as C, B, and Zr. Subsequently, the compositional development of single-crystal Ni-based superalloys is primarily driven by the aim to raise the operating temperature, while maintaining or ideally improving the mechanical properties for high temperature applications, particularly creep properties. Toward this end, current commercial superalloys, e.g., René N6 and CSMX-10M, are comprised of as many as 12 or 13 microalloying elements, including high melting-point refractory elements. Refractory element additions, such as Nb, Re, Mo, or W, have proven to be potent strengtheners and improve the high temperature properties. The principle strengthener is L12-ordered precipitates of Ni3Al (γ') in a face-centered cubic concentrated solid solution (γ). Refractory additions are known to reduce the kinetics in which the γ'-precipitates coarsen, which is critical to improving the operational use of commerical alloys.

Our research efforts on model Ni-based superalloys are two-fold, to understand: (i) the physical mechanisms that drive the early-stage γ/γ' phase separation experimentally (APT, TEM, and HREM) and simulationally; and, (ii) the influence of refractory elemental additions (W, Re, Ta, Nb and Ru) on the morphological development, elemental partitioning, and the coarsening behavior of the γ'-precipitates (APT, SEM, TEM, XRD, and AFM). The investigated Ni-Al-Cr alloys were cast and prepared by Dr. Ronald Noebe at NASA Glenn research center.