A twinning mechanism: Direct observation of the transition from stacking faults to Microtwins

The transition mechanism from stacking faults to microtwins in nickel-based superalloys remains debated despite its critical role in governing high-temperature creep behavior. Here, we directly observe the atomic-scale evolution of superlattice extrinsic stacking faults (SESFs) into microtwins during creep deformation at 815 °C/490 MPa using aberration-corrected scanning transmission electron microscopy (STEM). Transitional regions exhibit coexisting microtwin-like symmetry and residual stacking fault misalignment, confirming that sequential shearing of a/6 < 112> Shockley partial dislocations and subsequent atomic rearrangement drive SESF-to-microtwin conversion. Notably, microtwins exclusively originate within the γ′ phase, with no evidence of matrix-initiated twins. These findings experimentally validate the Kolbe atomic rearrangement mechanism while challenging hypotheses proposing matrix-driven twin formation in analogous systems. The results provide a foundational framework for understanding deformation pathways and designing creep-resistant alloys, particularly for applications demanding microstructural stability under extreme thermomechanical conditions.