Microtwinning and other shearing mechanisms at intermediate temperatures in Ni-based superalloys

In Ni-based superalloys, microtwinning is observed as an important deformation mechanism at intermediate temperature and low stress and strain rate conditions. Current knowledge concerning this unusual deformation mode is comprehensively reviewed, and fundamental aspects of the process are further developed using state of the art experimental and modeling techniques. The nature of microtwins and the microtwinning dislocations at the atomic level have been determined using High Angle Annular Dark Field Scanning Transmission Electron Microscopy imaging. The results unambiguously confirm that the operative twinning dislocations are identical Shockley partials a/6〈1 1 2〉, and that they propagate through the γ′ precipitates in closely-separated pairs on consecutive {1 1 1} planes. The rate-limiting process of the microtwinning deformation mechanism is the diffusion-controlled reordering in γ′-phase. It is shown that reordering requires very simple, vacancy-mediated exchange between Al and Ni atoms. The energetic aspect of the vacancy-mediated exchanges is studied for the first time using ab initio calculations. The concept of reordering as a rate-limiting process is generalized and shown to be relevant for other, previously reported deformation mechanisms in superalloys such as a〈1 1 2〉 dislocation ribbons, and superlattice intrinsic and superlattice extrinsic stacking fault formation. Other diffusion phenomena associated with microtwinning, such as segregation of heavy elements, is also discussed and supported by experimental evidence. The influence of the γ/γ′ microstructure on microtwinning deformation mode is also discussed in light of observations and phase-field dislocation modeling results.