Strain localization induced by closely spaced lamellae structure in a Mg alloy containing long period stacking ordered structure

Abstract

The Mg alloys containing the long period stacking ordered (LPSO) phase are a major research focus for their desirable mechanical properties, yet the role of the LPSO phase in the form of intragranular lamellae in tensile ductility has been elusive. Here, we investigate strain localization induced by LPSO lamellae in a Mg-Gd-Y-Zn-Zr alloy using correlated microstructure characterization, high-resolution full-field strain measurements and crystal plasticity simulations. The aged alloy containing lamellae exhibits reduced ductility due to intragranular microcracks along basal planes, contrasting with grain boundary cracks in the lamellae-free counterpart. Heaviside-digital image correlation (DIC) analyses reveal that the lamellar structure intensifies basal slip activity, leading to enhanced strain localization compared to the same magnesium alloy without this structural feature. Site-specific characterizations aided by atomic force microscopy (AFM) and transmission electron microscopy (TEM) identified that severe slip steps occur in the Mg matrix between closely spaced lamellae. Parameterized crystal plasticity simulations further revealed that the strain localization on the basal plane originates from the elevated local stress that is raised by the adjacent lamellae of high stiffness, and is exacerbated by high lamellae thickness-to-spacing ratios or volume fractions. Moreover, the promotion of the easy basal slip and suppression of the hard yet desired non-basal slip by lamellae make the deformation mode more restrictive, which is against the requirement for homogeneous deformation. These findings elucidate the potential detrimental effect of lamellae on ductility, thereby providing valuable insights for future design of Mg alloys containing LPSO phases.