Effect of Particle Size on Properties of SiCp/Al Composites and Its Numerical Simulation
|School||Harbin Institute of Technology|
|Keywords||SiCp/Al composite particle size effects strain gradient plasticity theory finite element method|
In this paper, SiCp/Al composites for delegates, both the experimental and numerical approaches were applied to investigate the particle size effects of the particle reinforced metal matrix composites from the point of view of macro- and meso- mechanics. In the present experiment, the SiCp/Al composites containing 20% volume fraction of SiC particles were fabricated by a powder metallurgy technique. The particles and the reinforced matrix were selected to be polycrystallineα-SiC particles with the average diameter of 5μm, 20μm and 56μm and commercial pure aluminum powders with an average diameter of 10μm, respectively. The microstructures were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile properties at room temperature of the composites containing particles with different sizes were studied. The effects of particle size on the thermal expansion behaviors and thermal conductivity of the SiCp/Al composites were analyzed. Based on the experimental results, the method which combines the Taylor nonlocal strain gradient plasticity theory and the finite element method were utilized to simulate the effects of particle size on the tensile deformation behaviors of the composites. The two-dimensional plane strain and random distribution multi-particle unite cell model is designed to simulate deformation behavior of the composites, thermal residual stress field, superposition of residual stress field at the mechanical behavior of the composites. A new method for determining yield strength of the composites was proposed.The experimental results indicated that the dislocation density and thermal residual stress of the composites are decreased with the increasing of particle size. The investigation of the tensile properties of the composites showed that the particle size affects the tensile deformation behaviors of the composites. The yield strength, tensile strength and work hardening rate of the composites were decreased with increasing of particle size. When the particle size was less than 5μm, the particle size effect was very significant. With the increasing of particle size, the micro-yield strength of the composites decreased firstly, and then increased. The fracture mechanism of the composites is also affected by the impact of particle size. On the base of analysis of fractographies,it showed that along with the increase of the particle size, the fracture mechanism of composites was converted into particles fracture from matrix ductile fracture gradually. By using the Eshelby equivalent inclusion approach which include the effect of particle cracking during deformation and dislocation strengthening, the influence of particle size on the deformation behavior of the composites were well simulated and calculated results were satisfactorily agreed with the experiment.The investigation of the thermal expansion of the composites showed that the particle size affects the thermal expansion behaviors of the composites. The composite containing small particle showed lower thermal expansion coefficients (CTEs) and lower increasing rate with temperature. The relative higher residual thermal stresses, generated during the producing within the composite containing small particle, associated with the local plastic deformation of the matrix should be responsible for the observed differences in CTEs of the investigated composites with various particle sizes. The thermal conductivity results showed that the thermal conductivity of the composites also decreased with decreasing particle size.In finite element model, the Taylor-based nonlocal theory (TNT) of plasticity was used to study the elasto-plastic constitutive equation of the matrix, avoiding the constitutive contain high-order and facilitating the calculation. Two-dimensional multi-particles model based on the Taylor-based nonlocal theory (TNT) of plasticity was successfully applied to investigate the particle size effects on the deformation behavior of particle reinforced metal matrix composites. The model had captured the particle size effect very well; thereby overcoming the lack of the traditional elastic-plastic theory which can not explain the particle size effects. It had been clarified that the flow stress and work hardening rate of the composites increased with decreasing particle size under a constant volume fraction of the reinforcement. With the increase of particle size, the strengthening mechanism of the composites transited from load transfer strengthening mechanism to matrix dislocation strengthening. The failure mode of the composites can also be changed by modify the particle size. The composites reinforced with large particle tended to fail through particle fracture, whereas the composites reinforced with small particle tended to fail through void nucleation, growth, and coalescence in the matrix regions near particles. The result was also consistent with the experiment. Through research the relationship between the load transfer from matrix to particles and the composites’strain, a new method for determining yield strength of the composites was proposed. Simulation results showed that with the increase of particle size, the thermal residual stress of the composites decreased. Thermal residual stress had little effect on the macro-stress-strain curve of the composites. Square particles were more conducive than spherical particles, leading to increase the flow stress of the composites.