Unambiguous Stacking Fault Analysis for Unraveling Shearing Mechanisms and Shear-Based Transformations in the L12-Ordered γ′ Phase

With its precipitationPrecipitation strengthening effect, the L12-ordered γ′ phase contributes substantially to the mechanical propertiesMechanical properties of superalloysSuperalloys; therefore, understanding the microscopic mechanisms by which it can be sheared is of key importance. A commonly used method to study these mechanisms involves high-resolution imaging in the transmission electron microscope in \(\langle {110}\rangle \) projection which enables straightforward discrimination between intrinsic and extrinsic stacking faultsStacking faults as well as microtwinsMicrotwin. However, the complex or superlattice nature of these stacking faultStacking faults structures, which provides key information on their formation mechanism, is not necessarily revealed in this projection. In the present work, an experimental approach is presented to resolve this ambiguity and reliably determine the complex or superlattice nature of a stacking faultStacking faults in the L12 structure by additionally imaging the fault in a nearby \(\langle {211}\rangle \) projection, which is achieved by tilting the specimen by 30°. The method is demonstrated using two different examples in single-crystalline Co-base superalloysSuperalloys. In the first example, the approach enabled the direct experimental verification of two key aspects of the well-known Kolbe mechanism for superlattice extrinsic stacking faultStacking faults formation, namely, the complex nature of the leading intrinsic stacking faultStacking faults segment and the occurrence of diffusionDiffusion-mediated re-ordering. In the second example, microscopic details of the shear-based transformation from the cubic L12-γ′ to the hexagonal D019-χ phase are revealed, accounting for additional complexities—again including a re-ordering process—arising from the superlattice ordering of both phases.