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Poor formability at room temperature is a key problem that limits the application of Mg alloys. Formability is closely dependent on both large work-hardening capacity and high tensile elongation. Herein, we develop a Mg-3Al-1Zn-0.4Mn (wt%) alloy with heterogeneous grains, obtaining an exceptionally high difference between ultimate tensile strength (UTS) and yield strength (YS) (UTS−YS: 164 MPa) and a good tensile elongation (22%). The uneven deformation of large and small grains is mainly affected by the size difference between grains rather than texture. The high strain hardening is attributed to the pile-up of geometrically necessary dislocations at grain boundaries between large and small grains due to uneven microstrain. The fraction of non-basal dislocations increases greatly with the increase of tensile strain, contributing to the improved ductility. Thus, we put forward a new strategy for improving the …