Hot deformation behavior and microstructural evolution for dual-phase Mg–9Li–3Al alloys

The hot deformation behavior of dual-phase Mg–9Li–3Al alloys was investigated by the isothermal hot compression tests using the Gleeble-3500 thermal-mechanical simulation testing system over a temperature range from 473 to 623 K and a strain rate range of 0.001–1 s−1. The flow curves exhibited obvious serrations of periodic fluctuation at high strain rates, which can be considered as the Portevin-Le Chatelier effect. The relationship among flow stress, strain rate, and deformation temperature was analyzed. The deformation activation energy (Q) and some basic material factors (A, n, and α) were calculated based on the Zener–Hollomon equation. An approach of processing map composed of power dissipation and instability domains was established by the dynamic material model to reveal the hot workability. The flow instability domain only occurred at low temperatures and high strain rates. When the Mg–9Li–3Al alloy was deformed at 473 K and the strain rate of 1 s-1, numerous deformation twins were formed in α-Mg phases and, meanwhile, the β-Li phase was deformed and broken. When the temperature was increased to 573 K, the synergetic deformability between α-Mg and β-Li phases was improved due to the activation of more slip systems. However, the proportion of dynamic recrystallization was still low at the strain rate of 0.001 s-1. The needle-shaped α-Mg phase precipitated out in the β-Li matrix when the alloy was deformed at 623 K and the strain rate of 0.001 s-1. Its formation was attributed to the deformation-induced transformation. Moreover, the α-Mg phase can retard the dislocation movement and grain growth during deformation, leading to the precipitation/dispersion hardening.