Enhanced stiffness and ductility in ZK61 magnesium matrix composites via bimodal structure design with MWCNT and nanodiamonds hybrid reinforcement

• Hybrid reinforcement with MWCNTs and nanodiamonds tailors dynamic recrystallization and produces a heterogeneous bimodal microstructure. • The hybrid composite exhibits weakened basal texture, enhanced 〈 c + a〉 dislocation activity, and elevated HDI hardening. • A superior strength–ductility synergy is achieved, with YS of 285 MPa, UTS of 350 MPa, and elongation of 10.2%. ZK61 magnesium matrix composites reinforced with 2 vol.% multi-walled carbon nanotubes (MWCNTs), 2 vol.% nanodiamonds (NDs) and their hybrid (1 vol.% MWCNTs + 1 vol.% NDs) were fabricated via powder metallurgy and hot extrusion. The effects of reinforcement type on microstructure evolution, dynamic recrystallization (DRX) behavior, dislocation activity and tensile performance were investigated. The results reveal that hybrid reinforcement promotes the formation of a refined heterogeneous microstructure with an intermediate fraction of coarse grains, a weakened basal texture, and enhanced 〈 c + a〉 dislocation activity. These combined effects enhance strain compatibility and hetero-deformation-induced (HDI) hardening, thereby achieving a superior balance of strength and ductility. The hybrid composite achieves a yield strength of 285 MPa, an ultimate tensile strength of 350 MPa and an elongation of 10.2%, outperforming the single-reinforced counterparts. The synergistic action of MWCNTs and NDs in facilitating DRX nucleation and stabilizing grain boundaries is identified as the key factor underlying the tailored microstructure and mechanical improvement. This study proves reinforcement hybridization is an effective strategy to overcome the stiffness-strength-ductility trade-off in Mg composites, offering promising potential for lightweight structural applications.