Fig. 1 Schematic illustration of the in situ catalytic synthesis and stir casting-hot rolling process for the fabrication of Cu-NPs@GNPs/Al-10Si composites

Fig. 2 a SEM image of spherical precursor powder. b, c SEM and high-resolution SEM images of Cu-NPs@GNPs. d TEM image of Cu-NPs distributed on GNPs. High-resolution TEM image of e graphene layers, f a typical Cu-NP

Fig. 3 a XRD pattern of in situ synthesized Cu-NPs@GNPs. b Raman spectrum of in situ synthesized Cu-NPs@GNPs and pure GNPs before and after ball milling process. c TGA and DTA profile of in situ synthesized Cu-NPs@GNPs

Fig. 4 SEM images of a, b 180 r-120 min ball milled 2.5 wt% Cu-NPs@GNPs/Al powders, c, d 180 r-120 min and 360 r-90 min ball milled Cu-NPs@GNPs/Al powders, e, f 180 r-120 min and 360 r-90 min ball milled mixture of (Cu-NPs + GNPs)/Al powders

Fig. 5 Si morphologies of Al-10Si and composites reinforced by Cu-NPs@GNPs: a modified Al-10Si, b, c modified 0.3 and 0.5 wt% Cu-NPs@GNPs/Al-10Si, d unmodified Al-10Si, e, f unmodified 0.3 and 0.5 wt% Cu-NPs@GNPs/Al-10Si

Fig. 6 Si morphologies of a modified Al-10Si, b modified 0.5 wt% (ex situ Cu-NPs/GNPs)/Al-10Si composite, c modified 0.195 wt% (Cu-NPs)/Al-10Si composite, d modified 0.325 wt% (Cu-NPs)/Al-10Si composite

Fig. 7 Raman results of the 1.0 wt% (Cu-NPs@GNPs)/Al-10Si bulk sample. a Optical image from the sample showing the scanned area of 20 μm × 20 μm outlined by white square. b Raman spectrum of some points from the map. c D-band intensity. d G-band intensity

Fig. 8 HRTEM images of microstructure in 0.5 wt% Cu-NPs@GNPs/Al-10Si composite: a GNPs and Al2Cu, b, c GNPs, d Al2Cu in the aluminum grain boundary, e, f Al2Cu in the aluminum crystal

Fig. 9 Elemental mapping images of 0.5 wt% Cu-NPs@GNPs/Al-10Si: a Al2Cu in the aluminum matrix, b-d Al, Cu, Si elements distribution in (a), e Al2Cu at the Al-Si interface, f-h Al, Cu, Si elements distribution in (e)

Fig. 10 Stress-strain curves of a Al-10Si and composites reinforced by various content of Cu-NPs@GNPs, b Al-10Si and composites reinforced by various content of Cu-NPs, c composites reinforced by Cu-NPs, mixture of Cu-NPs and GNPs, ex situ Cu-NPs/GNPs and in situ synthesized Cu-NPs@GNPs. d Comparison of strengthening efficiency and total elongation related to reinforcement contents of metal composites reinforced by different types of reinforcement fabricated by stir casting method