Localized recrystallization during creep in nickel-based superalloys GTD444 and René N5

Creep damage mechanisms in two nickel-based superalloys, directionally solidified (DS) GTD444 and single crystal (SX) René N5, crept at high temperature (982 °C) and low stress (179–206 MPa), have been studied. Electron backscatter diffraction analyses showed localized dynamic recrystallization in specimens crept to high strain levels and to final fracture. Recrystallization was observed around clusters of carbides and creep cavities, and was less common in areas away from the fracture surface. The average recrystallized grain diameter was 0.71 μμm, which, after 50 h at 982 °C, had grown to an average of ∼∼2.31 μμm. Growth of the recrystallized grains occurred by the dissolution of the γ′γ′ precipitates at the interface followed by discontinuous precipitation to relieve supersaturation. To consider the influence of localized recrystallization on macroscopic creep rates, a model for recrystallization-accelerated tertiary creep was developed. The model predicts that the axial strain rate increases by approximately one order of magnitude from the onset of recrystallization to rupture, comparing favorably to the experimentally measured accelerations in strain rate in the tertiary creep regime from 180 h to rupture. The observation of localized dynamic recrystallization provides new insight into the damage processes that occur in the tertiary creep regime.