Microtwinning during intermediate temperature creep of polycrystalline Ni-based superalloys: mechanisms and modelling

Deformation mechanisms, operative during intermediate temperature creep of Ni-based polycrystalline superalloys, are poorly understood. The creep deformation substructure has been characterized in Renè 88DT following rapid cooling from the super-solvus temperature, yielding a fine γ′-precipitate microstructure. After creep to modest strain levels (up to 0.5% strain) at 650°C and an applied tensile stress of 838 MPa, microtwinning is found to be the predominant deformation mode. This surprising result has been confirmed using diffraction contrast and high-resolution transmission electron microscopy. Microtwinning occurs via the sequential movement of identical 1/6[11–2] Shockley partials on successive (111) planes. This mechanism necessitates reordering within the γ′ precipitates in the wake of the twinning partials, so that the L12 structure can be restored. A quantitative model for creep rate has been derived on the basis that the reordering process is rate-limiting. The model is in reasonable agreement with experimental data. The results are also discussed in relation to previous studies under similar deformation conditions.