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This work was undertaken to investigate the influences of Mg content on the microstructure and mechanical properties of cast Al-2Li-2Cu-0.2Zr alloy. The addition of increasing amounts of Mg led to continuous grain refinement of as-cast base alloy accompanied by the increased volume fraction of secondary phases distributed along grain boundaries or interdendritic regions. The tensile test results showed that Mg addition could greatly enhance the mechanical properties of the base alloy after subjected to the solution treatments and artificial ageing. The precipitation behavior and microstructural evolution were also investigated. The addition of Mg would facilitate the nucleation of δ′ (Al3Li) phases after quenching. The presence of Mg was observed to lower the growth rate of δ′ and δ′-precipitation-free zones (δ′-PFZs) of the base alloy. Precipitation of θ′ (Al2Cu) phase would be almost suppressed, and S′ (Al2CuMg) phase was consequently introduced due to Mg additions. The precipitation of S′ phases was encouraged at the expense of T1 (Al2CuLi) phases as the Mg content increased. The increased amount of S′ phases tended to coalesce to form coarse laths distributed in uneven manner for alloys with relatively high Mg content (≥ 1 wt%). No improvements in mechanical properties were observed in alloys with heterogeneous distribution of coarse S′ laths. The optimal Mg addition in cast Al-2Li-2Cu alloy was 0.5 wt%. [ABSTRACT FROM AUTHOR]

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 α-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 (Al 2 CuLi) and S′ (Al 2 CuMg) phases during ageing, while no θ′ (Al 2 Cu) phase was observed. The anomalous decrease in conductivity observed during ageing was attributed to the precipitation of T 1 phases. Fine δ′ (Al 3 Li) particles were homogeneously distributed after quenching, gradual coarsening of which was observed with prolonged ageing time. As ageing proceeded, the δ′ coarsened and δ′-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 δ′ 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 δ′ dissolution. Moreover, continuous coarsening of T 1 and S′ phases contributed primarily to the strength loss in over-aged studied alloy. [ABSTRACT FROM AUTHOR]

The microstructures and mechanical properties of cast Al-3Li- x Cu-0.2Zr alloy were investigated. The results showed that Cu addition could slightly refine the grain size of as-cast alloy. Cu element was significantly enriched at the grain boundaries, and Cu-rich intermetallic compounds were formed in the Cu-containing alloys during casting. The peak hardness increased and the time to peak ageing decreased with the increase of Cu content. Cu addition also had a pronounced influence on the precipitation behavior of Al-3Li- x Cu-0.2Zr alloys, i.e. the number density, size and distribution of T 1 -Al 2 CuLi and θ ′-Al 2 Cu phases. The tensile strength was effectively improved after 2 wt% Cu addition compared with Al-3Li-0.2Zr alloy. A good combination of strength to ductility could be obtained in Al-3Li-2Cu-0.2Zr alloy after solution and ageing 32 h at 175 °C. The precipitation-free zones (PFZs) were developed with the coarsening of δ′-Al 3 Li particles in the peak-aged (PA) and over-aged (OA) state, promoting the poor ductility and premature failure. [ABSTRACT FROM AUTHOR]