It was pointed out that the deformation mechanisms which operated during the intermediate-temperature creep of Ni-based polycrystalline superalloys were poorly understood. The creep deformation sub-structure was characterized here in René 88DT following rapid cooling from the super-solvus temperature to yield a fine γ’-phase precipitate microstructure. After creeping at modest strain levels (up to 0.5%) at 650C under an applied tensile stress of 838MPa, micro-twinning was found to be the predominant deformation mode. This surprising result was confirmed by using diffraction contrast and high-resolution transmission electron microscopy. It was noted that micro-twinning occurred via the sequential movement of identical 1/6[11¯2] Shockley partials on successive (111) planes. This mechanism necessitated re-ordering within the γ’-phase precipitates, in the wake of the twinning partial dislocations, so that the L12 structure could be restored. A quantitative model for the creep rate was derived on the basis that the re-ordering process was rate-limiting. The model was shown to be in reasonable agreement with experimental data. The results were also analyzed with regard to previous studies performed under similar deformation conditions.

Microtwinning during Intermediate Temperature Creep of Polycrystalline Ni-Based Superalloys - Mechanisms and Modelling. G.B.Viswanathan, S.Karthikeyan, P.M.Sarosi, R.R.Unocic, M.J.Mills: Philosophical Magazine, 2006, 86[29-31], 4823-40