AZ91 alloy nanocomposites reinforced with Mg-coated graphene: Phases distribution, interfacial microstructure, and property analysis

A new organic chemical reduction method was successfully used to synthesize magnesium-coated graphene (GNPs), and xGNPs/AZ91 nanocomposites with different contents were fabricated by vacuum hot-pressing sintering. The microstructure of the composite was mainly composed of the matrix (α-Mg) and the precipitated phase (β-Mg₁₇Al₁₂) with different morphologies such as rods, spindles, and granules. The coarse irregular β phases precipitated along the grain boundaries, while fine rod-like β phases were distributed inside the crystal grains. With the increase of GNPs content, the grain and structure are significantly refined under the action of two mechanisms of increasing the nucleation rate and hindering the growth of grains. The average grain size of the 2.5-wt% GNPs/AZ91 composite dropped from 40.78 µm to 25.39 µm, a reduction of 37.7%. In addition, the orientation relationship (OR) between β-Mg₁₇Al₁₂ and α-Mg was shown as [—3 —1 —1]_β-Mg17Al12)∥[1 —1 0 —1]_α-Mg. Further, finer β phases were further precipitated in the grain boundaries and matrix. Moreover, the β precipitated phase and the GNP, as well as the GNP and magnesium-matrix formed a nano-scale contact interface and a diffusion bonding interface, thereby greatly enhancing the interface bonding strength between GNP and the matrix. Compared with AZ91 alloy, the grain refinement and load transfer caused by GNPs increased the microhardness of the composite by 17.6% and the friction coefficient was decreased by 37.4%. The significant improvement in the wear resistance of the composites was due to the effect of the lubricating layer formed by GNPs on the wear surface, which changed from severe delamination wear to slight delamination and abrasive wear behavior.