Structural evolution and the Hall-Petch relationship in an Al-Mg-Li-Zr alloy with ultra-fine grain size

Experiments were conducted on an Al-5.5% Mg-2.2% Li-0.12% Zr alloy to investigate the feasibility of introducing an ultra-fine grain size using equal-channel angular (ECA) pressing and of retaining an ultra-fine grain size at elevated temperatures. It is shown that ECA pressing is capable of reducing the grain size from an initial value of ∼ 400 μm to a value of ∼ 1.2 μm. However, the microstructure after ECA pressing is heterogeneous, with many areas of equiaxed grains having high angle grain boundaries and some regions of subgrains with boundaries having low angles of misorientation. Unlike earlier experiments on Al-Mg binary alloys, it is demonstrated that the grain size of the Al-Mg-Li-Zr alloy is reasonably stable up to temperatures as high as ∼ 700 K because of the presence in the matrix of a fine dispersion of β′-Al₃Zr precipitates. Microhardness data confirm the Hall-Petch relationship for grain sizes above ∼ 2 μm produced by annealing at temperatures above ∼ 673 K, but the Hall-Petch relationship breaks down at smaller grain sizes because of variations in the volume fraction of the δ′-Al₃Li precipitates.