Processing and properties of Al-Li-SiCp composites

Al-Li-SiCp composites were fabricated by a modified version of the conventional stir casting technique. Composites containing 8, 12 and 18 vol% SiC particles (40 μm) were fabricated. Hardness, tensile and compressive strengths of the unreinforced alloy and composites were determined. Ageing kinetics and effect of ageing on properties were also investigated. Additions of SiC particles increase the hardness, 0.2% proof stress, ultimate tensile strength and elastic modulus of Al-Li-8%SiC and Al-Li-12%SiC composites. In case of the composite reinforced with 18% SiC particles, although the elastic modulus increases the 0.2% proof stress and compressive strength were only marginally higher than the unreinforced alloy and lower than those of Al-Li-8%SiC and Al-Li-12%SiC composites. Clustering of SiC particles appears to be responsible for reduced the strength of Al-Li-18%SiC composite. The fracture surface of unreinforced 8090 Al-Li alloy (8090Al) shows a dimpled structure, indicating ductile mode of failure. Fracture in composites occurs by a mixed mode, giving rise to a bimodal distribution of dimples in the fracture surface. Cleavage of SiC particles was also observed in the fracture surface of composites. Composites show higher peak hardness and lower peak ageing time compared with unreinforced 8090Al alloy. Macro- and microhardness increase significantly after peak ageing. Ageing also results in considerable improvement in strength of the unreinforced 8090Al alloy and its composites. This is attributed to formation of δ′ (Al3Li) and S′ (Al2CuMg) precipitates during ageing. Per cent elongation, however, decreases due to age hardening. Al-Li-12%SiC, which shows marginally lower UTS and compressive strength than the Al-Li-8%SiC composite in extruded condition, exhibits higher strength than Al-Li-8%SiC in peak-aged condition.