Time-dependent subcritical crack growth and its mechanism in Ni-based single crystal superalloys at 500 °C

The subcritical crack growth behavior under sustained loading and its microscopic mechanism in <100> oriented single crystal (SC) Ni-based superalloys subjected to a relatively low temperature condition (500 °C) are critically investigated. An evident time-dependent crack growth (TDCG) phenomenon is observed even at such a temperature condition, but its rate (da/dt) is significantly small and weakly dependent on of the crack length, i.e. the stress intensity (K) value. The cross-sectional observation of a crack tip region by electron microscopy (SEM/TEM), together with the conventional fractography, reveals the following points: the macroscopic crack path shows pronounced branching influenced by the {100} planes; most dislocations in the γ’ phase near the crack tip are rectilinearly aligned and locked on the {100} planes; the crack tip dislocations provide the microscopically preferable oxidation paths. These points strongly suggest that the dislocation-mediated micro-oxidation process plays a major role in the observed anisotropic TDCG mechanism that eventually leads to the anomalous da/dt - K property.