Magnesium and its alloys have been considered for engineering applications in various fields due to their low density and high strength to weight ratio. However, low ductility and strength as well as poor cold workability, have limited its usage. In the present study, the manufacturing and characterization of Mg/nanosilica nanocomposite and the analysis of its mechanical behavior as well as the explanation of the phenomena seen in the laboratory results and the governing mechanisms, were performed using MD simulations. For this purpose, the effect of adding nanosilica powder with both amorphous and crystalline structures as well as the effect of weight percentages of the reinforcement phase in the Mg/nanosilica nanocomposite, was investigated. In this regard, first bulk samples of amorphous and crystalline silica were made by spark plasma sintering method, then the hardness and modulus of indentation of amorphous and crystalline silica were measured using nanoindentation test. Furthermore, Mg matrix nanocomposite with 0.25, 0.5 and 1%wt. of amorphous and crystalline nanosilica was prepared using the accumulative extrusion (AE) process at 300°C. Similarly, monolithic samples were prepared from pure Mg. The mechanical properties of the nanocomposite strips were eva‎luated using the cold compression test after extrusion and after the annealing process at 400°C. The mechanical behavior of amorphous and crystalline silica was investigated during the compression test and nanoindentation test by simulation. Also, the effect of grain size on the mechanical properties and deformation mechanism of Mg nanocrystals was investigated. Then, the Mg nanocrystal matrix nanocomposite was simulated after adding amorphous and crystalline silica nanoparticles with different positions relative to the grain boundary during the uniaxial compression test. Experimental results showed that crystalline silica samples had a higher hardness and indentation modulus than amorphous silica. MD simulation of nanoindentation test showed that amorphous silica has a lower indentation modulus and a higher elastic strain than crystalline silica. The compression test results showed that both structures have three stages of deformation. The results of the compression test on samples of nanocomposite strips showed that the yield strength of the sample contains 0.5%wt. of amorphous silica was increased up to 26% after extrusion and 235% after annealing. The ultimate compressive strength increased up to 20% without reducing the ductility of the nanocomposite compared to the monolithic sample and the other nanocomposite samples. In addition, X-ray diffraction, EBSD and EDS analysis showed that there was an insignificant difference in the orientation of the basal planes of pure magnesium and the nanocomposite strips along the extrusion direction. It resulted in reducing the grain size due to the uniform distribution of amorphous silica nanoparticles after the AE process compared to the samples of pure magnesium and magnesium reinforced by crystalline silica nanoparticles. The simulation results showed that the sample with an average grain size of 14 nm had the optimum mean flow stress. In addition, there are two distinct deformation mechanisms with the decrease in the grain size. On the other hand, for nanocomposite samples with different positions of amorphous and crystalline silica nanoparticles in the matrix, it was observed that the presence of nanoparticles at the triple junction between three grains has a superior effect on the strength of the nanocomposite. The ultimate strength and fracture strain when using amorphous silica nanoparticles are higher than nanocomposites reinforced with crystalline silica nanoparticles. This investigation opens a fascinating research window to study the effect of other amorphous ceramic nanoparticles on magnesium and other metals and to improve their mechanical and physical properties and, as a result, their applications.

ابوذر طاهري زاده، علي مالكي

امير لهراسبي، مسعود پنجه پور، بهزاد نيرومند