| With the constant application of metal matrix composites in modern society, people put forward higher requirements for the materials: a certain strength, good toughness and wear resistance in some structural components, thus the researches of trimodal aluminum matrix composites have been developed rapidly in recent years. Taking A1 alloy as an example, the typical trimodal composites consist of three micro-constituents: ultrafine grained(<800nm) Al, coarse-grained(>800nm) Al and reinforced particles. It can realize the controllable preparation of materials and meet the need of mechanical properties by reasonable constitution of the components. In this paper, the microstructure and mechanical properties of B4C reinforced aluminum matrix composites have been researched through the method of experimental study combined with finite element simulation.In the experimental study, AI5083-B4C trimodal composites were firstly prepared by powder metallurgy method, and then the microstructure of materials was characterized and the mechanical properties were tested. The results showed that the trimodal composites were composed of ultrafine-grained A15083, coarse-grained A15083 and B4C, and the average particle size was 381nm,970nm, and 793nm separately. B4C particles distributed uniformly in the A15083 matrix, and the matrix-reinforcement interface was clean and well-bonded. The tensile test showed that the yield strength of trimodal composites was 427MPa and the tensile strength was 477MPa. It is noted that the yield strength of the composites is 50% higher than the commercial A15083 alloy. The strengthening mechanisms includes direct strengthening and indirect strengthening, and the indirect strengthening includes fine grain strengthening, dislocation strengthening, dispersion strengthening. According to the results of analyzing the strengthening mechanisms, fine-grain strengthening makes a great contribution to the strength of the composites, and it is predicted that the yield strength of the composites is 432MPa by mixed strengthening mode, which has a good agreement with the experiment results. The hardness of the composites decreased slightly after annealing, which showed that the stability of the composites was fine.In the aspect of finite element simulation, the axisymmetric unit cell model which can descript A15083-B4C composites was firstly established, and then were simulated by ANSYS software. By comparing the simulation results with the experiment results using spherical particles as reinforced phase, we can see that the calculated yield strength and the tensile stress composites is 449MPa and 485MPa separately in considering no thermal residual stress, showing a relative error of 5.2% and 1.7%. The results show that the yield strength is 438MPa, and the tensile strength is 476MPa in considering thermal residual stress, showing a relative error of 2.6% and 0.2%. Therefore, it will be more accurate to analyze the stress-strain curves of composites in considering the effect of the thermal residual stress and adopting thermal-structure coupling method. The effect of meso-scale characteristics such as particle shape, volume fraction, and aspect ratio on the mechanical properties of the composites were simulated respectively, and it shows that the sharp corner of the cylindrical particle will induce severe stress concentration, while the interface of spherical particle leads to a more uniform stress distribution. In addition, the strength will increase with the increase of the volume fraction in a certain range. Moreover, when the particles with different aspect ratio are aligned along the load direction, the increase of the aspect ratio is beneficial to the improvement of the strength in the case that same volume fraction of B4C particles is used. Besides, the thermal residual stress of A15083-B4C composites from 450℃ to 20℃ was simulated, and it can be seen that the matrix bears residual tensile stress, while the particle bears residual compressive stress. The distribution and magnitude of the thermal residual stress are distinct from two different shaped particles, and the stress of spherical particle is relatively small and uniform, thus it can effectively reduce the thermal residual stress in the composites using sphere-like particles... |
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