Monotonic and cyclic anisotropics of an extruded Mg-Al-Ca-Mn alloy plate: Experiments and crystal plasticity studies

摘要

Mechanical anisotropics of an extruded Mg-Al-Ca-Mn alloy plate were investigated under both monotonic and cyclic conditions through a combination of experiments and crystal plasticity finite element simulations. To this end, monotonic tensile, compression as well as fully-reversed strain-controlled and load-controlled fatigue tests were conducted on specimens aligned either in the extrusion or transverse direction. Plastic activities of the deformation mechanisms for the different loading conditions were quantitatively analyzed based on the results of numerical simulations. The asymmetrical mechanical response between tension and compression along the two loading directions was found to be associated with the lack of axi-symmetry of the relatively strong basal texture. In addition, the peculiar and lower tensile flow stress in the transverse direction was explained by the activation of deformation twinning in grains with their c-axis aligned along the loading direction. Under low-cycle fatigue conditions, a lower asymmetry of stress-strain hysteresis curves was observed in the transverse direction due to the occurrence of twinning/detwinning in both the tensile and compressive portions. High-cycle fatigue experiments revealed lower fatigue lives in the transverse direction than in the extrusion direction but similar fatigue limits. Crystallographic analyses of multiple secondary cracks revealed crack initiation from basal slip, twin bands, grain boundaries and non-metallic inclusions. The evaluation of slip-induced and twin-induced fatigue criteria from the results of fatigue simulations of explicitly modeled polycrystals suggested that the most detrimental crack initiation mechanism was crack initiation from basal slip as it predicted a significantly lower initiation life in the transverse direction than in the extrusion direction, as observed experimentally.