Atomistic study of the effect of grain size and reinforcement particle on mechanical behavior of magnesium / silica nanocomposite

Abstract

In the present study, the atomistic simulations were carried out to investigate the mechanical properties and deformation mechanisms of nanocrystalline magnesium with different grain sizes. Also, the effect of reinforcement particle position and structure on the mechanical behavior of nanocrystalline magnesium / (amorphous/crystalline) silica nanocomposite under uniaxial compression loading was studied by using the molecular dynamics method. Firstly, eight structure with the mean grain sizes from 5 to 24 nm of nanocrystalline magnesium were constructed using the Voronoi tessellation method, and used to study the deformation mechanism. The simulation results showed that the sample with an average grain size of 14 nm had the optimum mean flow stress. Furthermore, by increasing the average grain size to values close to coarse-grained structures (> 25 nm), an increasing in the dislocation density and elastic modulus was observed. Moreover, there are two distinct deformation mechanisms activated by decreasing grain size. The first one, the dislocation-dominated deformation mechanism, belonged to large average grain size (≥ 14 nm) samples, and the second one, the grain boundary sliding deformation mechanism, was observed in small grain-sized simulation samples. In the following, nanocrystalline Mg nanocomposite simulation samples with different amorphous silica nanoparticle positions were studied, and it was observed that the presence of nanoparticle in the triple junction among three grains had the main effect on the strength of nanocomposites compared to the samples with nanoparticle at the grain boundary or inside the grain. Accordingly, the maximum strength and fracture strain was observed in the simulation samples with nanoparticles in the triple junctions. The investigated structures showed that the partial dislocations and aggregation of FCC structure stacking faults as lamella structure defects were observed in all simulated nanocomposite samples. Also, the local atomic shear strain contour obtained by simulation of nanocrystalline magnesium reinforced by amorphous and crystalline silica nanoparticles showed different responses of amorphous and crystalline nanoparticles during compression loading. Furthermore, amorphous and crystalline silica with the same size have different effects on the mechanical properties of the magnesium: in which, amorphous silica not only increases the ultimate compressive strength but also increases the strain before fracture compared to crystalline silica. The results of this research can expand the use of amorphous ceramic nanoparticles as reinforcement instead of crystalline ceramic nanoparticles in the development of nanocomposite materials.