AbstractHeterogeneous TiC/AZ61 nanocomposites, consisting of TiC-rare coarse grain (CG) bands and TiC-rich fine grain (FG) zones, were fabricated to simultaneously improve the strength and ductility of nanoparticles reinforced Mg matrix composites. The fraction of CG bands could be optimized by adjusting the mechanical ball milling time to change the proportion of powders with different morphologies. It was found that composites began to form a heterogeneous bimodal grain (HBG) structure after 12 h ball milling. With further increasing the ball milling time from 12 h to 30 h, the proportion of spherical powder decreased, the volume fraction of CG bands decreased from 48.4% to 11.7%. Excellent comprehensive mechanical properties (ultimate tensile strength: 417 MPa, yield strength: 323 MPa, and elongation: 10.2%) were achieved for the composite with ∼25 vol% CG bands after 20 h of ball milling. Moreover, the HBG-20 h composite had significant additional strengthening at ultimate tensile strength owing to the existence of geometrically necessary dislocations (GNDs) inside the coarse grain bands, which were introduced by mechanical incompatibility between the CG band and FG zone. Such dislocations provided optimum back-stress work hardening at the HBG-20 h composite due to its suitable CG band fraction (∼ 25 vol%), contributing to the high strain-hardening.