The effect of stress state and strain partition mode on the damage behavior of a Mg-Ca alloy

• The central hole tension has a greater resistance to fracture than uniaxial tension. • Grain boundary cracking is caused by deformation inhomogeneity and large strain gradient. • A localized deformation mode is beneficial than a diffuse one for the dilute Mg-Ca alloy. • The effect of stress state and strain partition mode on fracture was analyzed and identified. • Forming method with large strain requirement at local region was designed to improve formability of the alloy. The ductility of magnesium alloys is greatly affected by damage formation and accumulation. Due to its low-symmetry close-packed hexagonal structure, magnesium alloy's damage behaviors are different from most cubic crystal metals. A comprehensive study was conducted to uncover the underlying damage mechanism of a Mg-0.5%wtCa alloy by using mechanical tests with different stress states, microstructure characterization and the crystal plasticity finite element method (CPFEM). It is found that the central hole tension has a greater resistance to damage than uniaxial tension, as confirmed by the results of digital image correlation (DIC) measurement and X-ray micro-computed tomography (XCT) characterization. This unusual phenomenon is related to the variation of fracture mechanisms in the two scenarios. For the uniaxial tension, plentiful grain boundary cracks are formed and the fractography shows an intergranular feature; while for the central hole tension, the propensity of intergranular fracture is less evident. Through combining the SEM observations of the boundary cracks and the strain distribution determined by the CPFEM simulation, it is revealed that the boundary cracking is caused by the intense deformation heterogeneity and the large strain gradient. Uniaxial tension has a diffuse deformation mode with the randomly distributed ″soft″ grains and ″hard″ grains. This strain partition mode results in a number of spots with high local strain gradient; these spots serve as the nucleation sites of the grain boundary cracks. The results of the notched tests with different notch radii also show the fact that the localized deformation mode is beneficial than the diffuse one. Therefore, in addition to the stress state, the strain partition mode also plays a vital role in the fracture of the dilute Mg-Ca alloy. Further corroboration was done by a three-bending test with gradient deformation mode. A large fracture strain was obtained in the test, indicating that the magnesium alloy has a potentially good formability when the forming process is reasonably designed.