Strengthening/weakening effect of graphene orientation angle on mechanical properties of AZ91 magnesium matrix composites

Graphene, as the reinforcing phase of magnesium matrix composites, can effectively improve the material strength, elastic modulus, and other properties. However, the random distribution of graphene in the matrix (i.e., random orientation angle) leads to different reinforcement effects on the matrix. To gain a deeper understanding of the impact of monolayer graphene (1LG) with varying orientation angles on the properties of Mg-9Al-1Zn (AZ91 (wt.%)) magnesium alloy, molecular dynamics (MD) simulations are employed to analyze the mechanical properties of AZ91/1LG composites under uniaxial tension. The simulation results show that Young's modulus and tensile strength of AZ91/1LG composites decrease gradually with the increase of the orientation angle of the 1LG. The Young's modulus and tensile strength of AZ91/1LG composites can be improved by the 1LG orientation angle of 0°~10°, where the two are enhanced by 21.7% and 19.7% respectively, at an orientation angle of 0°. However, the Young's modulus and tensile strength of 1LG are decreased for orientation angles of 20°~90°. Atomic structure evolution analysis revealed that the deformation mechanism of AZ91/1LG nanocomposites mainly depended on the load transfer ability of 1LG with different orientation angles, the bonding ability with AZ91 magnesium alloy matrix and the change of dislocation density. By fitting the formula to the tensile strength of AZ91/1LG composites with different orientation angles of 1LG, it is found that the simulated data of the AZ91/1LG composites containing a 1LG has a maximum relative error of about 10% concerning the fitted empirical formula to calculate the data. The maximum relative error for AZ91/1LG composites containing multiplate 1LG with different orientation angles is 7%. In addition, the interaction between graphene and dislocations in AZ91 magnesium matrix was further explained by transmission electron microscopy (TEM) and phase-field-crystal (PFC) simulation. It can provide some technical guidance for the experimental process design of AZ91/1LG composites.