On Plastic Deformation Mechanism Of Aeronautical Metal Matrix Nanocomposites

As a kind of emerging nanomaterials, light metal matrix nanocomposites (MMNCs) with high strength-to-weight ratio are becoming more and more attractive in recent years. However, it has been known that the nano-sized reinforcements are easy to cluster together during the process of material preparation, which deeply reduces the mechanical properties of light metal matrix nanocomposites. Therefore, the cluster effect of the reinforcements in the MMNCs is necessary to be taken into account, which is especially a key issue at studying their properties.In this thesis, a new micromechanical constitutive model is developed to describe the overall elasto-plastic response of carbon nanotube (CNT) reinforced metal matrix nanocomposites. The pernicious influences of CNT clusters and misorientation angle on the mechanical properties of the nanocomposites are considered in the proposed model. The cluster effect is introduced into the new model by using the statistically-averaged equivalent length (l) and equivalent diameter (D) of CNT clusters to replace the original length and diameter, and the equivalent size is figured out with a logarithmic normal distribution; the misorientation angle (θ) is considered by modifying physically-based strength model of short fiber-reinforced composites. At the same time, a constitutive model for particle-reinforced metal matrix nanocomposites is provided too. To consider the important cluster effect of nano-sized ceramic particles on the mechanical properties of MMNCs, the primary particle size is amended by the equivalent particle size in the newly established constitutive model. The equivalent particle size is deduced through defining a probability density distribution function of the size of particle clusters and a clustering extent function.Based on the Matlab platform and genetic algorithm with global optimization, a multi-variable nonlinear optimization program is developed. By means of inverse analysis of parameters, the program can be used to determine the parameters in constitutive model of two typical MMNCs, SiCnp/A356 and CNTs/Al. Furthermore, these proposed models are validated by experimental results, and the results demonstrate that the new models can describe the thermo-viscoplastic deformation very well under the quasi-static or dynamic loading. In addition, some prediction results of new models indicate that there exists a maximum strength in investigating the volume fraction of reinforcements effect on the variation of stress. This is because there has a severely cluster effect at higher volume fraction of reinforcements, which induces the decrease of the MMNCs’ strength.