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Twin boundary (TB) strengthening in nanotwinned metals experiences a breakdown below a critical spacing at which softening takes over. Here, we survey a range of nanotwinned materials that possess different stacking fault energies (SFEs) and understand the TB strengthening limit using atomistic simulations. Distinct from Cu and Al, the nanotwinned, ultralow SFE materials (Co, NiCoCr, and NiCoCrFeMn) intriguingly exhibit a continuous strengthening down to a twin thickness of 0.63 nm. Examining dislocation slip mode and deformation microstructure, we find the hard dislocation modes persist even when reducing the twin boundary spacing to a nanometer regime. Meanwhile, the soft dislocation mode, which causes detwinning in Cu and Al, results in phase transformation and lamellar structure formation in Co, NiCoCr, and NiCoCrFeMn. This study, providing an enhanced understanding of dislocation mechanism in nanotwinned materials, demonstrates the potential for controlling mechanical behavior and ultimate strength with broadly tunable composition and SFE, especially in multi-principal element alloys.
