Microstructures and mechanical properties of ultralight cast Al-3Li-XMg-0.1Zr alloys

Abstract

Ultralight cast Al-3Li-XMg-0.1Zr alloys (X = 1, 2, 3, 4) were studied by microscopy methods and tensile tests. Increasing the addition of Mg, grains in as-cast alloys were refined as a result of the intensified constitutional supercooling. After the low-to-high two-stage solution heat treatment, coarse δ′-Al3Li, Al12Mg17, δ-AlLi and S1-Al2MgLi second phases were dissolved into the α-Al matrix. Meanwhile, plenty of fine δ′-Al3Li phases and some core-shell Al3(Zr, Li) composite particles were precipitated. During the following ageing procedure, δ′ phases were coarsened following LSW theory while the size of composite particles kept almost unchanged due to the low diffusion rate of zirconium in aluminum. Magnesium was found to be able to promote not only the precipitation but also the coarsening of δ′-Al3Li phases in AlLi alloys. After ageing heat treatment at 175 °C for 32 h, Al-3Li-2 Mg-0.1Zr alloy presented optimum comprehensive mechanical performance with the ultimate tensile strength, yield strength and elongation equal to 368 MPa, 247 MPa and 4.7% respectively under the synthetic effect of grain refinement, promoted solution strengthening and the relatively narrow δ′-Al3Li precipitate free zones.