Wrought magnesium alloys are attractive for automotive industry applications due to their low density and high specific strength. However, commercial magnesium alloys, such as AZ31B sheet usually have poor formability at room temperature due to limited activity of slip systems. Additionally, due to the twinning deformation mechanism activated in specific loading directions, magnesium alloys exhibit an asymmetric stress-strain response in uniaxial tension and compression tests. The formability of magnesium alloys can be improved by deforming at elevated temperatures; however, warm forming requires more complex tooling setup which increases the cost of the forming operation. Alternatively, the formability can be improved by the addition of rare-earth elements such as Ce, Nd, Y and Gd, for example, which have been shown to weaken the basal texture. The aim of this work is to compare and identify the effect of initial crystallographic texture on the mechanical response of commercial AZ31B-O and rare-earth ZEK100 magnesium alloy sheets in light of known deformation mechanisms operating at different orientations and strain rates. Tensile and compression testing was performed on AZ31B and ZEK100 sheets along different directions to characterize the material response under a wide range of temperature (23-250°C) and strain rate (0.001s-1-1000s-1). A digital image correlation system (DIC) was used to capture the evolution of plastic strain during the deformation. Three point bend tests were also performed on the AZ31B samples and surface strain on the tensile surface of the bend was measured using the DIC method. The instantaneous R-values and their evolution with respect to the plastic strain were measured from the tensile and compressive DIC data. The ZEK100 sheet exhibits strong in-plane anisotropy both in tension and compression as the orientation changes from the RD to TD. The low to high strain rate experiments reveal a significant orientation dependence of the strain rate sensitivity of ZEK100. In contrast, the rate sensitivity of AZ31B, while pronounced, does not depend upon loading direction. In the RD, the rate sensitivity of ZEK100 is manifest in significant changes in yield strength, but only mild changes in hardening rate. In contrast, along the TD, the yield strength is not affected by strain rate, while the hardening rate increases significantly with strain rate. This behavior is attributed to different deformation mechanisms being activated at different strain rates depending on the load path, sheet orientation and texture. As temperature is increased, the degree of anisotropy and asymmetry is reduced for both alloys.
An evolving asymmetric/anisotropic continuum-based approach, adopting Cazacu-Plunkett-Barlat (CPB) yield surfaces, is proposed to model the complex behavior of magnesium alloys at room and elevated temperatures. The model shows that the material response of AZ31B at room temperature is highly anisotropic and asymmetric; while a minor asymmetry between uniaxial tension and compression results is predicted at elevated temperature, consistent with experimental data. The proposed material model is used to simulate 3-point bending experiments on AZ31B and predict both the load-displacement response as well as the distribution of strains on the outer bend radius. The results of the simulation using the new “evolving continuum model” compare well with experimental data over predictions using other material models such as classical von Mises and non-evolving CPB based models.