Phases in as-spun and aged binary aluminium-lithium ribbons

Abstract

As-spun aluminium-lithium ribbons, containing up to 10wt% Li, and an Al-1.79 wt % Li ribbon aged at different temperatures (room temperature, 66, 175 and 200 °C) were investigated by transmission electron microscopy (TEM) and microhardness testing. It was found that the metastable phase Al3Li (δ′) did not form either in the as-cast ribbons containing less than 1.79 wt% Li or in the featureless zone of Li-rich ribbons (Li content >3 wt%) but it did form, as the stable phase AlLi (δ), in the segregated zone of the later ribbons. The study of the early stage of precipitation showed that the mechanism of formation of δ′ depended on the ageing temperature. At a low temperature (66°C), the δ′ phase occurred by homogeneous spinodal decomposition. Whereas, it formed by heterogeneous nucleation at higher temperatures (175 and 200°C). These results are consistent with the presence of a spinodal curve within the miscibility gap (α + δ′). All investigations performed both on the as-cast and aged ribbons did not reveal any evidence for the formation of G.P. zones. It was also observed in this study that contamination of aluminium-lithium ribbons by impurities such as O2 and CO2 could alter the precipitation process and lead to the formation of lithium carbonate.