Study On Microscopic Deformation Mechanism Of Nanopolycrystalline Aluminum And Its Composites

Particles reinforced metal matrix composites (PMMCs) have excellent performance and been widely applied in many fields. However there are still many problems in the practical production and processing process. For example, the strength and ductility of some PMMCs cannot meet the requirements in engineering. It is necessary to further understand the performance and reinforcement mechanism of PMMCs. Nowadays more and more attentions have been paid to the study on nanopolycrystalline metal and their composite materials by molecular dynamics method. At nanoscale, we can more clearly understand the mechanical properties and deformation mechanism of nanopolycrystalline metal and their composite materials by analyzing the evolution of microstructure in tensile tests. In this thesis, the tensile behavior of metal polycrystalline Al and β-SiC/Al composite materials are respectively simulated using molecular dynamics method. The focus of the study is the mechanical properties and the microscopic deformation mechanism of metal polycrystalline Al and β-SiC/Al composite materials.Several polycrystalline Al bulk models with different grain sizes were built by Voronoi method, based on which the tensile tests were simulated. The fraction of atoms at grain boundary will increase after relaxation. With the decrease of the grain size, atoms at grain boundary will increase. The atomic energy distribution of the polycrystalline Al bulk model presents a basin-shape image. And the atomic energy of grain boundary is higher than that of internal grain. The deformation mechanism and the influence of different grain size of polycrystalline A1 bulk on the tensile deformation were investigated with the help of the change of the energy in tension. The results showed that the yield strength of polycrystalline A1 bulk will decrease with the decrease of the grain size, which follows the inverse Hall-Petch relationship. For larger grain-size models of polycrystalline Al bulk some atoms as the source of dislocation appear firstly in tension and form the cross dislocation. The dislocation continues to extend to the grain boundary which is the barrier of the extention and hence strengthens the material. At the same time, there are grain boundary sliding and grain rotation happen. For smaller grain-size models of polycrystalline Al bulk a large number of defect atoms are produced in grain boundary, and then cracks will appear. In addition, the yield strength of polycrystalline Al bulk will decrease with the increase of temperature and the decrease of strain rate.Then the microstructure and tensile behavior of β-SiC/Al composite material models with the same volume fraction and different SiC particle size were simulated. The results indicate that the smaller SiC particle size, the more grain-boundary atoms after relaxation. It focused on the deformation and reinforcement mechanism by observing the change of the energy during tensile test. The results suggested that the yield strength of composite materials decrease with the decrease of the SiC particle size. The yield strength and elastic modulus of j3-SiC/Al composite materials are improved in comparision with polycrystalline Al. In additon, the dislocations are produced in the region of the aluminum matrix near grain boundary which will extend with the increase of the deformation. When the strain reaches a certain value, the cracks will appear in the aluminum matrix. However, SiC cannot be damaged in tension. Moreover, the yield strength of P-SiC/Al composite materials decreases with the increase of temperature and the decrease of strain rate.