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Many researches have shown that nanotwinned structure and graphene reinforcing phase can improve the mechanical properties of aluminum (Al) metal, but the synergistic effect of the two on nanoindentation mechanical response of Al matrix composites is rarely reported. In this research, based on molecular dynamics simulations, the nanoindentation mechanical properties and microscopic deformation mechanisms of graphene/nanotwinned Al matrix composites are studied. The effect of the inclination angle and spacing of the twin boundaries (TBs), the insertion position of the graphene sheet, the graphene coating and the layer number of the graphene sheet on the nanoindentation mechanical properties of the Al matrix composites is analyzed. The nanoindentation mechanical properties of nanotwinned Al strongly depend on the inclination angle of TBs, and the dislocation slip modes mainly includes three types: hard mode I (dislocation pile-up penetration mode), soft mode I (partial dislocation parallel twin boundary slip mode) and hard mode II (dislocation limited slip mode). Moreover, as the twin boundary spacing increases, the dislocation slip mode starts to weaken until the nanoindentation mechanical properties are insensitive to the twin inclination orientation. The insertion of the graphene sheet weakens the indentation mechanical properties of the nanotwinned Al matrix composite, which is attributed to the interaction of the graphene with the dislocations and the subsequent absorption of part of them. The weakening effect depends on the distance between the graphene/Al interface and the plastic zone. What's more, graphene coating can improve the indentation mechanical properties of composites. In addition to the load bearing effect of graphene in the elastic stage, this strengthening effect is also attributed to the combined action of graphene and TBs in the plastic stage, which makes the dense dislocation network widely distributed around the indentation of graphene coating and induces the interfacial strain hardening. [ABSTRACT FROM AUTHOR]