Achieving high strength in graphene nanoplatelets reinforced Mg-Zn-Mn matrix composites via liquid-state molding technology and low temperature deformation process

The graphene nanoplatelets (GNPs) reinforced Mg-4Zn-0.5Mn (wt%) matrix composites (GNPs/ZM40) with different concentrations (0.1, 0.3, 0.5 wt%) of GNPs were processed by liquid-state molding method combined with low temperature extrusion process. The microstructure of the prepared materials was analyzed by means of scanning electron microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS), transmission electron microscope (TEM), X-ray diffractometer (XRD) and optical microscope (OM). And the tensile test at room temperature was carried out to reveal the mechanical properties of the materials. The results showed that the 0.3 wt% GNPs/ZM40 composite exhibited the outstanding mechanical properties of yield strength (YS: ∼466 MPa) and ultimate tensile strength (UTS: ∼532 MPa). In as-cast materials, the addition of GNPs led to the refinement of equiaxed grains and eutectic phases in the composites, closely related with the promotion of heterogeneous nucleation by GNPs during the solidification. After low temperature extrusion, the prominent dynamic recrystallization (DRX) took place in as-extruded alloy and GNPs/ZM40 composites, producing plenty of fine grains. In contrast to the alloy, the average grain sizes of the composites with different GNPs content were reduced, whereas the degrees of recrystallization in the composites increased. These differences in microstructural evolutions originated primarily from the inhibiting effect of GNPs on the grain growth and the boost to DRX by the high-density dislocations in magnesium matrix caused by GNPs. Accordingly, the high YS of the composites could be largely ascribed to fine-grain strengthening, dislocation strengthening effect and load transfer mechanism.