Strengthening and ductilization of the in-situ synthesized graphene/Ni composite through Cu microalloying

In-situ grown graphene nanosheets reinforced nickel-based composites (graphene/Ni composites) always suffer from the trade-off between strength and ductility due to the poor mechanical compatibility and interfacial bonding of reinforcement/matrix components. Microalloying has been established to be an effective strategy in strengthening the graphene/Ni composites; however, the influences of alloying elements on the microstructures and mechanical properties of the composite still remain unclear. This study introduces the Cu element into the Ni matrix for achieving a co-reinforcing effect with the well-dispersed in-situ graphene nanosheets. The experimental investigations and first-principles calculation reveal that a proper concentration of Cu dopant not only improves the bonding strength of the graphene/Ni interface by a good wettability, but also causes an abnormal grain growth in the Ni matrix and thus generates a bimodal structure. Consequently, the in-situ synthesized graphene/Ni composite with 1.5   wt.% Cu exhibits the optimal strength-ductility synergy with a high yield strength, ultimate tensile strength, and fracture elongation of 276   MPa, 575   MPa, and 49.9%, respectively. Compared to the undoped composite, the enhancements in these mechanical parameters reach 32.1%, 21.6%, and 28.9%, respectively. Despite the common strengthening mechanisms, such as the effective load transfer, dislocation strengthening, twin boundary strengthening, solid-solution strengthening, and Orwan strengthening, are contribute to the tensile strength of Cu microalloying graphene/Ni composites, it is determined that the hetero-deformation-induced hardening is playing a critical role. At the same time, the bimodal structure also delays the necking and further suppresses the crack propagation in the Ni matrix, enabling exceptional plasticity.