The structure and mechanical properties of composites consisting of a metal matrix based on aluminum and its alloys of different compositions (AA-3003 and AA-5154) and graphene synthes sized in situ under a layer of molten salts were investigated depending on the chemical composition and grain size of the matrix. Aluminum matrix composites of three compositions were studied in as-cast coarse-grained, deformed fine-grained (grain size < 1 mm), and deformed sub microcrystalline (grain size < 1 μm) states in order to compare the structural characteristics of composites with different grain sizes. The composites were subjected to deformation with a split Hopkinson (Kolsky) bar and to dynamic-channel angular pressing. The hardness and dynamic mechanical properties of the composites were measured at strain rates $\dot{\epsilon }$ from 1.8 − 4.7 × 10³ to 1.6 − 2.4 × 10⁵ s⁻¹. It was found that grain refinement induced a sharp increase in the hardness of composites with various compositions (by a factor of 1.6–2.6). A correlation of the elastic-plastic properties of the aluminum matrix composites with the grain sizes and chemical compositions of the matrices was established. A transition from coarse-grained to sub microcrystalline structure was shown to improve the elastic-plastic properties on average by a factor of 1.5. It was proved that the reinforcing effect of graphene increased with the decreasing grain size of the matrix. Mechanisms of reinforcement of the aluminum matrix composites using graphene were proposed.

The development of novel methods for industrial production of metal-matrix composites with improved properties is extremely important. An aluminum matrix reinforced by “in situ” α-Al₂O₃ nanoparticles was fabricated via direct chemical reaction between molten aluminum and rutile TiO₂ nanopowder under the layer of molten salts at 700–800 °C in air atmosphere. Morphology, size, and distribution of the in situ particles, as well as the microstructure and mechanical properties of the composites were investigated by XRD, SEM, Raman spectra, and hardness and tensile tests. Synthesized aluminum–alumina composites with Al₂O₃ concentration up to 19 wt.% had a characteristic metallic luster, their surfaces were smooth without any cracks and porosity. The obtained results indicate that the “in situ” particles were mainly cube-shaped on the nanometer scale and uniform matrix distribution. The concentration of Al₂O₃ nanoparticles depended on the exposure time and initial precursor concentration, rather than on the synthesis temperature. The influence of the structure of the studied materials on their ultimate strength, yield strength, and plasticity under static loads was established. It is shown that under static uniaxial tension, the cast aluminum composites containing aluminum oxide nanoparticles demonstrated significantly increased tensile strength, yield strength, and ductility. The microhardness and tensile strength of the composite material were by 20–30% higher than those of the metallic aluminum. The related elongation increased three times after the addition of nano-α Al₂O₃ into the aluminum matrix. Composite materials of the Al-Al₂O₃ system could be easily rolled into thin and ductile foils and wires. They could be re-melted for the repeated application.

A novel method of creating new lightweight aluminum-metallic composite materials in halide melts at temperatures of 973–1073 К in air atmosphere is proposed. The method for synthesizing aluminum-based metallic composite materials containing up to 2 wt. % graphene sheets uniformly distributed in a metal matrix is entirely new, having no analogies in current science and practice. The synthesis of graphene nanosheets in a metal matrix is a one-step simultaneous process taking place directly in molten aluminum in alkali halide melt without the necessity of a separate stage of synthesis and introduction of graphene. This has the potential to facilitate the inexpensive synthesis of aluminum-graphene composites with a high concentration of graphene. Aluminum-graphene composites formed according to this method are characterized by a high uniformity of graphene films with linear dimensions from 100 nm to 50 µm and a thickness from 1 nm to 3 nm in the metal bulk. No aluminum carbide forms under synthesis; aluminum-graphene and aluminum-graphite composites are resistant to corrosion in a NaCl solution. The hardness, strength and ductility of aluminum-graphene composites are at least 2–3 times as high as the initial aluminum material, proportional to the concentration of graphene.