Superplasticity of fine-grained Mg-10Li alloy prepared by severe plastic deformation and understanding its deformation mechanisms

The superplastic behavior and associated deformation mechanisms of a fine-grained Mg-10.1 Li-0.8Al-0.6Zn alloy (LAZ1011) with a grain size of 3.2 µm, primarily composed of the BCC β phase and a small amount of the HCP α phase, were examined in a temperature range of 473 K to 623 K. The microstructural refinement of this alloy was achieved by employing high-ratio differential speed rolling. The best superplasticity was achieved at 523 K and at strain rates of 10−4 -5 × 10−4 s−1, where tensile elongations of 550–600% were obtained. During the heating and holding stage of the tensile samples prior to tensile loading, a significant increase in grain size was observed at temperatures above 573 K. Therefore, it was important to consider this effect when analyzing and understanding the superplastic deformation behavior and mechanisms. In the investigated strain rate range, the superplastic flow at low strain rates was governed by lattice diffusion-controlled grain boundary sliding, while at high strain rates, lattice diffusion-controlled dislocation climb creep was the rate-controlling deformation mechanism. It was concluded that solute drag creep is unlikely to occur. During the late stages of deformation at 523 K, it was observed that grain boundary sliding led to the agglomeration of the α phase, resulting in significant strain hardening. Deformation mechanism maps were constructed for β-Mg-Li alloys in the form of 2D and 3D formats as a function of strain rate, stress, temperature, and grain size, using the constitutive equations for various deformation mechanisms derived based on the data of the current tests.