Ultra-high strength yet superplasticity in a hetero-grain-sized nanocrystalline Au nanowire

•Nanocrystalline nanowire always exhibits strength-ductility trade-off. Here, we provide in situ atomic-scale evidence that hetero-grain-sized nanocrystalline nanowires are simultaneously ultra-strong and ductile, with no strength-ductility trade-off.•The hetero-grain-sized nanocrystalline nanowire exhibits a super uniform elongation of ∼ 236% and high strength of ∼ 2.34 gigapascals at room temperature.•The in situ atomic-scale observations revealed that the dislocation activities, grain boundary plasticity, and surface atoms diffusion all contribute to the super elongation ability of hetero-grain-sized nanocrystalline nanowire.