Low stacking fault energy governs deformation mechanisms in the γ matrix of Co-based superalloys

Understanding plastic deformation mechanisms in the γ matrix is essential for the development of next-generation Co-based superalloys. Since magnetic transitions in Co-based matrices can occur within service temperature ranges, stacking fault energies (SFEs) must be evaluated with explicit consideration of magnetic state effects. In this work, SFEs of pure Co and Co-rich alloys are calculated to quantify their compositional and temperature dependences in both ferromagnetic (FM) and paramagnetic (PM) states. The predicted values show good agreement with available experimental and theoretical data. For pure Co, the SFE increases with temperature in the FM state and decreases in the PM state; when the magnetic contribution is considered, the PM (Mag.) SFE exhibits the same trend as its FM counterpart. Co-rich alloys, by contrast, display a distinct temperature response, highlighting the dominant role of magnetic entropy in the PM state. Low SFEs predicted for Co-rich alloys at elevated temperatures favor deformation twinning, consistent with experimental observations. These results demonstrate that magnetic-state-dependent SFE calculations provide a robust framework for predicting plastic deformation modes and guiding the alloying design of Co-based alloys.