Optimizing microstructure and mechanical properties of bimodal-structured magnesium matrix composites by regulating the remelting time of powder thixoforming

A kind of bimodal graphene oxide (GO) reinforced ZK60 Mg matrix composite, characterized by the constitutes of fine grain (FG) zones reinforced by GO and coarse grain (CG) zones, was prepared via powder thixoforming, and its microstructure was optimized through adjusting the partial remelting time to achieve a desirable strength-ductility coordination. Both the volume fractions and grain sizes of CGs could be regulated at different levels by changing the heating duration. The composite thixoformed at 60 min, having 30.1 vol% CGs with average grain size of 15.2 μm, achieves the highest yield strength of 192 MPa and ultimate tensile strength of 316 MPa, as well as a high elongation of 21.8% and toughness of 61.9 MJ m−3, possessing the most remarkable coordination of strength and ductility among the as-reported homogeneous-structured Mg-6Zn composites reinforced by carbonaceous nanomaterials. The macroscopic strain of the composites becomes more uniform as the remelting time increases due to the enhanced co-deformation ability between FG and CG constitutes, which contributes to higher ductility. The back stress strengthening makes a large contribution to the strengthening of the composites. Bimodal structure induced toughening, enhanced dislocation storage capability of the FG zones, and crack blunting contribute to the toughness of the composites. This work provides a promising approach for designing and fabricating bimodal-structured metal matrix composites with good comprehensive mechanical performances.