The study investigates the rejuvenation of planar defects in CrCoNi medium-entropy alloys (MEAs) during cyclic mechanical loading. Short-range order (SRO) in these alloys is known to influence their mechanical properties, but the link between SRO and the exceptional performance has been unclear. The researchers used in situ nanomechanical testing and energy-filtered four-dimensional scanning transmission electron microscopy (4D-STEM) to directly observe the evolution of stacking faults (SFs) and twin boundaries (TBs) in single-crystal CrCoNi at room temperature. They found that SFs and TBs initially reversible in the first cycles become irreversible after 1000 cycles, indicating a reduction in SF energy and rejuvenation. Molecular dynamics (MD) simulations further revealed that the destruction of SRO by dislocation glide triggers this rejuvenation, leading to a more ductile state with increased entropy and free energy. This process creates softer zones within the matrix, enhancing the overall damage tolerance of the alloy. The study provides critical insights into the role of SRO in the superior mechanical properties of MEAs and highlights the importance of optimizing SRO for enhancing material performance.The study investigates the rejuvenation of planar defects in CrCoNi medium-entropy alloys (MEAs) during cyclic mechanical loading. Short-range order (SRO) in these alloys is known to influence their mechanical properties, but the link between SRO and the exceptional performance has been unclear. The researchers used in situ nanomechanical testing and energy-filtered four-dimensional scanning transmission electron microscopy (4D-STEM) to directly observe the evolution of stacking faults (SFs) and twin boundaries (TBs) in single-crystal CrCoNi at room temperature. They found that SFs and TBs initially reversible in the first cycles become irreversible after 1000 cycles, indicating a reduction in SF energy and rejuvenation. Molecular dynamics (MD) simulations further revealed that the destruction of SRO by dislocation glide triggers this rejuvenation, leading to a more ductile state with increased entropy and free energy. This process creates softer zones within the matrix, enhancing the overall damage tolerance of the alloy. The study provides critical insights into the role of SRO in the superior mechanical properties of MEAs and highlights the importance of optimizing SRO for enhancing material performance.