Mechanical properties of 7075 aluminium matrix composites reinforced by nanometric silicon carbide particulates

Abstract

Aluminium composites reinforced by particles have received considerable attention because of their superior mechanical properties over monolithic aluminum matrix. Over the last ten years, nanocomposites with nano-sized reinforcements have become a revolutionary progress for composites because they have different strengthening mechanisms as compared to that in composites with micro-sized reinforcements. Consequently novel properties can be expected from the nanometric particulate reinforced composites. The aim of this project was to fabricate SiC (50nm)/7075 aluminium composites via a modified powder metallurgy and extrusion route. Ageing treatment was used to increase the strength of the composites and mechanical tests, including tensile test and abrasive wear test, were performed. The effects of nanometric silicon carbide particulates to the ageing behaviours and mechanical properties of the composites have been studied by optical metallography, scanning electron microscopy and transmission electron microscopy. It was found that the dispersion of nanometric silicon carbide was not homogeneous, but tended to disperse along grain boundaries. Clustering of these nano-reinforcements was also found within the grains. This was particular true when the amount of nano-reinforcement increased to 5%. Compared with the monolithic 7075 alloy, the 1 vol.% SiC (50nm)/7075 aluminium had a higher strength because of effective dislocation pinnings by the reinforcements, while 5% SiC (50nm)/7075 had a much lower strength and ductility because of severe aggregation of nanometric particulates. Nanometric silicon carbide was not as effective as the micro ones in improving abrasive wear resistance of aluminium, this was because of micro-cracking in the aggregation and relatively large abrasive grit. In summary, the addition of a small amount of SiC nanoreinforcements has a high potential to further strengthen 7xxx aluminium alloy. However, the clustering of reinforcements in the matrix will detrimentally affect the strength and ductility of the alloy. The wear resistance of nanometric particulate reinforced composites was inferior to those with micrometric reinforcements. It is suggested that by improving the dispersion of nanometric reinforcements, as well as putting in reinforcememts with different sizes, the mechanical properties and wear resistance can both be increased.