INFLUENCE OF GRAPHENE CONTENT ON MICROSTRUCTURAL REFINEMENT AND MECHANICAL PROPERTIES OF AA6063/SIC NANOCOMPOSITES

This paper presents a mechanism-centred interpretation of stir-cast AA6063 hybrid nanocomposites containing 1.5 wt.% SiC and 0–1.0 wt.% graphene. Rather than reporting characterisation tests separately, the study links optical grain size, X-ray diffraction (XRD) crystallite size and dislocation density to microhardness, tensile strength and impact energy to identify a reinforcement threshold. Five material conditions were examined: base AA6063 (B), 1.5 wt.% SiC (C1), and SiC plus 0.2, 0.6 or 1.0 wt.% graphene (C2–C4). Relative to B, C3 (1.5 wt.% SiC+0.6 wt.% graphene) reduced the measured optical grain size from 52.4 to 24.8 μm (52.7%) and the XRD crystallite size from 62.962 to 55.846 nm (11.3%), while dislocation density increased from 2.52 to 3.21×10⁻⁴ nm⁻². The same composition increased Vickers microhardness from approximately 65 to 90 HV (38.5%) and ultimate tensile strength from about 135 to 164.9 MPa (≈22%). Increasing graphene to 1.0 wt.% reversed the refinement trend: grain size rose to 31.6 μm, crystallite size to 61.091 nm, dislocation density decreased to 2.68×10⁻⁴ nm⁻², and both hardness and tensile response declined. A dimensionless microstructure–strength coupling index, based solely on measured normalised quantities and without fitted weighting factors, increased from 1.00 for B to 1.42 for C3 before falling to 1.21 for C4. SEM/EDS observations independently support the transition from improved dispersion at C3 to carbon-rich agglomeration and localized porosity at C4. The results establish 0.6 wt.% graphene as a reinforcement threshold for the present stir-cast system and explain its property maximum through simultaneous Hall–Petch refinement, interfacial load transfer and dislocation obstruction, balanced against the toughness penalty introduced by rigid reinforcement.