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

Abstract

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.

Acknowledgements

Support for this work has been provided by the DARPA Accelerated Insertion of Materials (AIM) Program under contract F33615-00-C-5215 and by the Air Force Office of Scientific Research, for model development, through the MEANS-2 theme grant # FA9550-05-1-0135.