Microstructural evolution and mechanical properties of cast Al-3Li-1.5Cu-0.2Zr alloy during heat treatment

The microstructures and mechanical properties of cast Al-3Li-1.5Cu-0.2Zr alloy during heat treatment were investigated. The results showed that large amounts of secondary phases distributed along grain boundaries in as-cast state, and most of them dissolved into the α-Al matrix after solution treatment. The alloy had a very strong age-hardening response with the peak-hardness that was more than double for the as-quenched. TEM showed much fine δ′ (Al3Li) particles were homogeneously distributed after quenching, and long δ′ phases were observed during ageing. Fine T1 (Al2CuLi) precipitate was visible after ageing for 8 h at 175 °C, and both its number density and size increased with the increase of ageing time. Apart from δ′ and T1, θ′ (Al2Cu) phases of large aspect ratio were precipitated in the over-aged condition. As the ageing proceeded, the δ′ particles coarsened and the Precipitate-Free Zones (PFZs) developed, resulting in poor ductility and premature failure. The studied alloy was largely strengthened by the δ′ phase during ageing. After ageing at 175 °C for 32 h, a good mechanical property with ultimate tensile strength (UTS) = 382 MPa, yield strength (YS) = 307 MPa and elongation (EL) = 2.9% was obtained.