Mechanical behaviour of nanometric-scale and micrometric-scale SiC particulate reinforced A1 7075 matrix composites

摘要

This research work involves the study of the effects of the nanometric-scale silicon carbide particulates (n-SiCp) on the mechanical properties of the Al 7075 alloy. In the last few years, nanometric-scale reinforcements have shown the potential for revolutionary changes in the metal matrix composites to replace the existing monolithic alloys and their composites reinforced with micrometric-scale reinforcements. In this study an attempt was made to introduce hard nanometric-scale particulates in a high strength (Al 7075) matrix for possible achievement of novel properties in the composites. The project work was divided into: (a) synthesis of both the monolithic Al and its composites using the powder metallurgy (P/M) technique, (b) tensile testing and dry abrasive testing of all the materials, (c) explanation of the experimental results obtained and the microstructural characterization of all the materials using diffractometry, metallographic techniques and microscopy. Firstly, both the unreinforced Al and its composites were synthesized using the P/M and hot extrusion route. Reinforcement particulates of micrometric-size (μ-SiCp) and nanometric-size (n-SiCp) along with their different volume fractions were added separately to study their effects on the mechanical behaviour of the Al rix. T6 (solution treatment and artificial ageing) heat treatment was used to increase the hardness and strength of all the materials. Ageing behaviour was studied to observe any effect of the reinforcement particulates on the ageing kinetics and hardness of the composites. X-ray diffraction was performed to determine the crystal structures of all the materials and any reaction phase formed in the composites. Tensile tests were performed to measure the stiffness, 0.2% yield strength, ultimate tensile strength and ductility of all the materials at both room and levated temperatures. Dry abrasive wear tests were used to measure wear rates of all the materials. The n-SiCp were not uniformly dispersed in the Al matrix and clustered mainly at the grain boundaries. Clustering increased with increases in the volume fraction of the n-SiCp. Stiffness of the composites increased and the ductility decreased with an increase in the volume fraction of the reinforcement particulates at both room and elevated temperatures. Enhancement in the stiffness of the composites was higher at elevated temperature than at room temperature. Strength of the composites remained the same in comparison with that of the monolithic Al and no change was observed with increases in the volume fraction of the n-SiCp. The n-SiCp proved to be a better reinforcement than the traditional μ-SiCp in terms of imparting higher ductility to the composite. The abrasive wear rate decreased with the addition of the n-SiCp in the Al matrix. It was found to be dependent on the particulate size and its volume fraction. The wear resistance of the μ-SiCp/Al composite was higher than those of the n-SiCp/Al composites and it was attributed to the large size of the μ-SiCp. Fractography and microscopy using optical, scanning electron and transmission electron microscopy were performed for failure analysis and microstructural analysis of all the materials. The incorporation of the reinforcement particulates altered the nature of fracture from being ductile in the unreinforced Al to brittle in the composites. At elevated temperature, the fracture mechanism transformed from brittle to ductile rupture in the composites. In summary, inclusion of the n-SiCp in a very little amount demonstrated that they have the ability to further improve the performance of the high strength Al 7xxx series alloys. Non-uniform dispersion and clustering of the particulates are considered to be the main hindrances in utilising their full potential. It is believed that after fixing these issues the strength and other mechanical properties of the Al matrix can be further increased.