Combined strengthening effect of nanocrystalline matrix and graphene nanoplatelet reinforcement on the mechanical properties of spark plasma sintered aluminum based nanocomposites

Al-0.5 wt% GNP nanocomposites with both microcrystalline and nanocrystalline matrices, were synthesized via spark plasma sintering. The strengthening effects of nanocrystalline Al matrix and GNP reinforcement have been systematically studied. Nanocrystalline Al compact (without GNPs), in comparison to microcrystalline one, exhibited 57% and 53% improvement in yield strength and ultimate tensile strength. Addition of 0.5 wt% physio-chemically functionalized GNPs in the nanocrystalline Al matrix led to further enhancement in the yield strength and ultimate tensile strength by 85% and 44%, respectively. This is attributed to the homogenous distribution of the functionalized nanometric GNPs with high elastic modulus and strength, resulting in 99% densification. The elastic moduli of the nanocomposites measured by nanoindentation matched well with the values estimated by applying micromechanics models, owing to the very good densification of the nano composites. Contributions from different strengthening mechanisms viz. load transfer, grain refinement, CTE mismatch and Orowan strengthening were calculated. Load transfer strengthening mechanism showed highest (73%) contribution in enhancing the strength of the nanocomposite. The mode of failure in microcrystalline Al compact was ductile and changed to mixed mode with the incorporation of GNPs. On the contrary, the Al-GNP nanocomposite with nanocrystalline matrix exhibited completely brittle mode of fracture.