| Abstract Scope |
Microtwin formation is a critical deformation mechanism in superalloys that has typically been associated with poor creep resistance in γ′-strengthened Ni-based superalloys. However, microtwins have been commonly observed in Haynes® 244® and related alloys containing the γ′′′-Immm-Ni2(Cr,Mo,W) precipitate, and their formation does not appear to be detrimental to the alloys' creep resistance. This presentation will highlight mechanisms for microtwin formation in this and similar alloy systems using atomistic density functional theory calculations and electron microscopy techniques, which have been pioneered by this year's awardee, Prof. Michael Mills. In this presentation, we will show that due to the combination of six orientation variants, unique precipitate shearing pathways, and the presence of misfit interfacial dislocations, intrinsic stacking faults initially form and act as seeds for highly localized microtwins within the precipitates. We believe that long-range microtwin formation then only proceeds when a favorable set of precipitate orientations are locally present to enable long-range dislocation decorrelation. Finally, we will demonstrate how an enhanced understanding of these processes can enable the design of new alloys with novel deformation mechanisms. |