Effect of extrusion temperature on microstructure and mechanical properties of a low-alloying and ultra-high strength Mg–Zn–Ca–Mn matrix composite containing trace TiC nanoparticles

Mechanical properties of microalloying Mg–2.2Zn–1.8Ca–0.5Mn (wt%) matrix composites reinforced by 0.5 wt% TiC nanoparticles before and after extrusion were investigated based on the detailed microstructural analysis. A uniform distribution of TiC nanoparticles was realized in the nanocomposite by the method of ultrasonic-assisted semisolid stirring. The morphology of eutectic Ca2Mg6Zn3 phases changed from plate-like in the free TiC nanoparticles region to lamellar in the dense TiC nanoparticles region for the as-cast nanocomposite. Both the grain structure and precipitates were obviously refined as the extrusion temperature decreased from 350 to 270 °C. The nanocomposite exhibited excellent tensile yield strength (352–428 MPa) which was governed by the extrusion temperature. The grain refinement strengthening with the contribution ratio of ∼80% to this strength increment was much higher relative to thermal expansion effect, Orowan strengthening and dislocation strengthening. Ultrafine recrystallized grain structure with a substantial of fine precipitates appeared in the nanocomposite extruded at 270 °C. The refined grain structure was not only due to dynamic recrystallization, but also the synergistic pinning effect of nano-TiCp, precipitated MgZn2 and α-Mn particles. The tensile toughness value of nanocomposite after extrusion improved with increasing the extrusion temperature. Massive micro-cracks formed along the remnant coarse Ca2Mg6Zn3 led to the structural failure during tension.