Grain Boundary Engineering of a Low Stacking Fault Energy Ni-based Superalloy
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NICKEL-BASED superalloys are principal materials used in high-temperature components of modern gas turbine engines and have been shown to exhibit excellent creep, fatigue, and corrosion resistance at elevated temperatures.[1–4] The unique attributes of Ni-based superalloys can be attributed to their underlying microstructure that is comprised of coherent intermetallic particles of Ni3Al (c¢) with an ordered L12 crystal structure contained within an austenitic (c) face-centered cubic (FCC) nickel matrix.[5] Grain boundary engineering (GBE) or controlling the distribution and relative fractions of twin and other high coincident site lattice (CSL) boundaries with R < 29 has been an active topic of research in recent years. Pertaining to Ni-based superalloys, various studies have reported improvements in creep resistance, intragranular stress corrosion cracking, and fatigue crack growth resistance associated with having large fractions of ‘‘special grain boundaries’’ that break up the interconnectivity of the pre-existing random grain boundary network[6–8] in polycrystalline Ni-based superalloys. CSL boundaries with R values containing lower static energies (R < 29) have been shown to have longer
JOSHUA MCCARLEY and SAMMY TIN are with the Illinois Institute of Technology, 10 West 32nd Street, Chicago, IL 60616. Contact e-mail: [email protected] RANDOLPH HELMINK and ROBERT GOETZ are with the Rolls-Royce Corporation, 450 S Meridian St, Indianapolis, IN 46225. Manuscript submitted August 3, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
fatigue life due to their ability to offer enhanced resistance to the multiplication and eventual accumulation of dislocations brought forth during fatigue. This accumulation of defects results in an increase of the dislocation density which saturates as fatigue continues, resulting in a hardening response to crack initiation and extended fatigue life.[9–11] At elevated temperatures, the physical and mechanical properties of polycrystalline materials are strongly influenced by the underlying texture and character of the grain boundaries. For isotropic polycrystalline materials, each respective grain possesses a specific orientation, which together forms a grain boundary network that consists of a random distribution of grain boundary orientations. The degree of coherency of each grain and its neighbors or CSL is heavily influenced by the orientations found at the boundaries. Boundaries with a low CSL index of R > 29 contain large concentrations of vacancies and crystalline defects that ultimately contribute to lowering the mechanical integrity of the material by weakening its interface while simultaneously promoting diffusive mechanisms triggered at elevated temperatures. These mechanisms accelerate diffusion and sliding along the grain boundaries, which decreases the materials resistance to creep deformation.[12–16] Alternatively, fewer crystalline defects and vacancies within the grain boundaries are found when neighboring grains are favorably oriented and form high CSL boundar
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