Interfacial microstructure and strengthening mechanisms of SPSed Al/GNP nanocomposite subjected to multi-pass friction stir processing

Al/0.1 wt% graphene nanoplatelet (GNP) composites were fabricated successfully by the combination of spark plasma sintering (SPS) and solid-state multi-pass friction stir processing (FSP). The dispersion of GNPs in the Al matrix and the consolidation of the SPSed compacts were significantly enhanced by increasing the number of FSP passes and the most homogeneous dispersion was obtained in the 3-pass FSPed composite. Additionally, the multiple FSP passes fracture the Al2O3 layer deposited on the Al particles in the SPSed composite and thus allow a direct physical interaction of GNPs with the Al matrix to form an effective Al/GNP interface for an efficient load transfer. The grain refinement of the Al/GNP composite was also improved significantly from ∼22 μm in the SPSed condition to ∼2.73 μm after the 3-pass FSP because of dynamic recrystallization. The detailed TEM analysis confirms the physically bonded Al/GNP interface with the tight mechanical interlocking between the lattice fringes of the GNP and Al matrix. The tensile strength of the Al/GNP composite fabricated with the combined approach was increased with the number of FSP passes. The ultimate tensile strength (UTS) of the SPSed Al/0.1GNP composite was increased from ∼111 MPa to ∼135 MPa (∼ 22% increment) and ∼ 145 MPa (∼ 31% increment) after 1–2 pass FSP and 3-pass FSP, respectively. The Hall-Petch strengthening and efficient load transfer through the tightly bonded Al/GNP interface were identified as the dominant strengthening mechanisms in the fabricated composites.