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Although deformation twinning has been demonstrated to improve the strain hardening and ductility of metals and alloys with low stacking fault energies (SFEs), the optimum grain size for maximum strength-ductility combination still needs to explore. In this work, we selected Cu- 30 wt% Zn alloy with extremely low SFE (7 mJm^-2) acting as a model material. Specifically, Cu- 30 wt% Zn samples with different grain sizes ranging from 628 nm to 30.6 µm were prepared by equal-channel-angular pressing (ECAP) and subsequent annealing. Tensile test revealed that the maximum strength-ductility combination (ultimate tensile strength of 565 MPa and ductility of 20%) corresponds to a mean grain size of 3.8 µm. Electron backscatter diffraction (EBSD) indicated that pre-existing annealing twins in fine-grained Cu- 30 wt% Zn alloy annihilated at the initial deformation stage (<10% tensile strain) via detwinning of thin twin …