Superior strength–toughness synergy of heterogeneous TiB2p/AZ91 composites containing hard/soft zones via tailoring bimodal grain structure

Magnesium matrix composites show great prospects due to lightweight and high specific strength, but strength-toughness tradeoff has limited their engineering applications. Here, a novel strategy for strengthening and toughening TiB2p/AZ91 (TiB2p: TiB2 particles) Mg matrix composites via tailoring heterogeneous configuration containing hard/soft zones and bimodal grain structure is proposed. The results show that hot extrusion deformation can achieve heterogeneous structure of hard and soft zones resulting from alternating arrangement TiB2p, which induces the formation of coarse and fine grain zones. With the decrease of extrusion temperature, both the hard zone spacing and the number of coarse grains have a great decrease. In particular, TiB2p/AZ91 composites extruded at 400 °C and 350 °C exhibit obvious bimodal grain structure, while almost uniform fine grains are obtained in the composites extruded at 300 °C. This contributes to the excellent comprehensive mechanical properties of TiB2p/AZ91 composites at 400 °C and 350 °C, with UTS of 345 MPa, 388 MPa, and elongation of 12.1%, 10.1%, respectively. Compared with AZ91 alloy, the strength of composites extruded at 400 °C and 350 °C is significantly improved, while the ductility is not sacrificed, indicating that superior strength–toughness synergy is achieved. The improved toughness is ascribed to the increased strain hardening capacity, suppressing and deflecting effects of the hard zones on the crack propagation. Lower extrusion (300 °C), however, leads to the reduced toughness of the composites, which is due to the premature initiation of cracks caused by smaller hard zone spacing and large amount of precipitated phase.