Tension-compression asymmetry of an AM magnesium alloy unveiled by in-situ synchrotron X-ray diffraction

Magnesium (Mg) alloys typically exhibit anisotropic mechanical behaviors due to their hexagonal close-packed (hcp) crystal structures, often leading to tension-compression asymmetries. Understanding of the asymmetrical and related deformation mechanisms is crucial for their structural applications, particularly in the lightweight transportation industries. Nevertheless, the underlying deformation mechanisms (e.g., slip versus twinning) at each deformation stage during tension and compression have not been fully understood. In this study, we employed tensile and compressive tests on extruded Al and Mn containing Mg alloy, i.e., an AM alloy Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca, during the synchrotron X-ray diffraction. Our results show that distinct deformation behaviors and mechanisms in tension and compression are associated with the strong texture in the extruded samples: (i) The tensile deformation is dominated by dislocation slips, with activation of non-basal 〈a〉 and 〈c + a〉 slip, but deformation twinning is suppressed. (ii) The compressive deformation shows early-stage tensile twinning, followed by dislocation slips. Twinning induces grain reorientation, leading to significant lattice strain evolution aligned with the texture. The pronounced tension-compression asymmetry is attributed to the favorable shear stress direction formed in the twinning system during compression, which facilitates the activation of tensile twins. During tension, the strain hardening rate (SHR) drops significantly after yielding due to limited activated slip systems. In contrast, the samples under compression exhibit significant increases in SHR after yielding. During compression, dislocation multiplication dominates the initial strain hardening, while twinning progressively contributes more significantly than dislocation slip at higher strains. This study improves our understanding of the tension-compression and strain hardening asymmetries in extruded AM Mg alloys.