A study of the microstructure, mechanical properties and failure behaviour of Al-Li-Co melt-spun ribbons: effect of Al₉Co₂ phase particle precipitation

Abstract

The present work is performed on three alloys, namely Al-2.8Li-0.1Co, Al-2.8Li-0.4Co and Al-2.8Li-0.8Co (all wt%) produced by melt-spinning. Their microstructures in the as-melt-spun condition as well as following ageing at 473 K are studied by light and transmission electron microscopy where the phases present have been identified using X-ray diffractometry. The mechanical properties of these alloys are characterized by longitudinal tensile tests as well as by hardness measurements. The results reveal that increasing the cobalt content and hence the volume fraction of Al₉Co₂ dispersoids is very effective in increasing both yield and tensile strengths, in addition to a noticeable improvement in the ductility. Simple empirical equations are proposed to determine the yield strength in the as-melt-spun and aged conditions. The Young's modulus for the first alloy has been calculated from a bending test and is found to lie between 80 and 83 GPa depending on the ageing condition. Another advantage of increasing the Al₉Co₂ concentration appears in promoting high-energy transgranular fracture instead of the low-energy intergranular fracture usually observed in Al-Li alloys subjected to ageing.