Microstructural evolution and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy during heat treatment

The microstructure, electrical conductivity and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy during heat treatment were investigated. The results indicated that a population of secondary phases enriched along grain boundaries in as-cast studied alloy, most of which dissolved into the alpha-Al matrix after solution treatment. Profound enhancement in mechanical properties of the studied alloy was obtained after T6 heat treatment. TEM observations revealed competitive precipitation of T-1 (Al2CuLi) and S' (Al2CuMg) phases during ageing, while no theta ' (Al2Cu) phase was observed. The anomalous decrease in conductivity observed during ageing was attributed to the precipitation of T-1 phases. Fine delta ' (Al3Li) particles were homogeneously distributed after quenching, gradual coarsening of which was observed with prolonged ageing time. As ageing proceeded, the delta ' coarsened and delta '-Precipitate-Free Zones (PFZs) developed, resulting in diminishing ductility observed from as quenched to peak-aged state. Precipitation and continuous coarsening of T-1 phases kept on at the expense of gradual delta ' dissolution. The improvements of ductility observed in over-aged samples arose mainly from the presence of T-1, S' phases, Al-3(Li,Zr) composite precipitates and progressive delta ' dissolution. Moreover, continuous coarsening of T-1 and S' phases contributed primarily to the strength loss in over-aged studied alloy.