Temperature and Time Dependence of Elemental Segregation at Stacking Faults in Ni- and Co-Base Superalloys

Elemental segregationSegregation at stacking faultsStacking faults in the γ′ phase is a crucial part of the stacking-fault-based deformation of superalloysSuperalloys, and the local composition of a stacking faultStacking faults critically affects the resistance of γ′ to further shearing. Here, the process of elemental segregationSegregation at extrinsic stacking faultsStacking faults in γ′ is examined in Ni-base superalloySuperalloys CMSX-4CMSX-4 and Co-base superalloySuperalloys ERBOCo-4. By measuring fault compositions not only after deformation, but also after additional load-free annealing steps of different durations and temperatures between 700 and 900 °C, time- and temperature-dependent aspects of the segregationSegregation process are revealed. It is shown that elemental segregationSegregation continues to evolve toward an equilibrium composition after a fault has formed, indicating that fully formed stacking faultsStacking faults still provide a driving force for segregationSegregation processes and that their equilibrium composition can differ from that which they obtain during their formation. As the elemental segregationSegregation process is diffusionDiffusion-based, it is observed to be considerably slower at a lower temperature. In both alloys, the segregationSegregation trends observed after annealing depend on the annealing temperature: A lower temperature promotes a more γ-like fault composition associated with γ′ softening; conversely, at higher temperatures, the fault composition tends toward that of the η phase, which is considered beneficial in the context of local phase transformationLocal phase transformation strengtheningPhase transformation strengthening. As η-like segregationSegregation also involves the enrichment of slowly diffusing tungsten, kinetics likely play a role in the observed temperature dependenceTemperature dependence.