Plastic strain in polycrystalline metals is highly localized in grain boundaries (GBs), slip bands (SBs) and twins. While extensive research has focused on intra-granular deformation mechanisms such as slip and twinning, strain localization at GBs has been largely overlooked. In this study, high-resolution digital image correlation (HRDIC) was employed to capture the strain distribution and its evolution during tension in an extruded pure Mg sheet. Particular attention was paid to strain localization at GBs and its governing factors. Results reveal that, at 3 % applied strain, approximately 10 % of GBs were categorized as extremely-high-strain GBs (defined as the GB where at least 20 data points have an effective shear strain (ε_eff) value exceeding the 99th percentile of the overall ε_eff distribution), and the majority (84%) of them were observed to deform at even 0.5% applied strain. This suggests that early-stage deformation plays a critical role in subsequent GB strain localization. The mean strain value and grain boundary sliding (GBS) displacement of GBs increased significantly with applied strain, with progressively accelerating increasing rates observed in most instances. Most (~62%) GBs exhibiting slip transfer showed low strain, while a small fraction (~8%) of them exhibited extremely high strain. This indicates that slip transfer can mitigate GB strain localization in most cases. However, complex local conditions are also critical, and case-by-case analysis is essential. Moreover, GBs with misorientation angles ranging from 50° to 80° were found to be more likely to exhibit extremely high strain. This work provides valuable insights into GB strain localization, which is critical for further understanding the plastic deformation of polycrystalline Mg.

The grain-scale tension-compression (T-C) asymmetric slip behavior and geometrically necessary dislocation (GND) density in an aged and twin-free Mg-10Y sheet were statistically studied using slip trace analysis and electron backscatter diffraction (EBSD) analysis. A significantly asymmetric slip activity, i.e., higher tensile slip activity and proportion of non-basal slip, was manifested. Prismatic 〈a〉 (37.1%) and basal 〈a〉 (27.6%) slips dominated the tensile deformation, followed by pyramidal Ⅱ 〈c + a〉 slip (20.0%). While during compression, basal 〈a〉 slip (61.9%) was the most active slip mode, and only 6.9% pyramidal Ⅱ 〈c + a〉 slip was observed. The critical resolved shear stress (CRSS) ratio was estimated based on ~800 sets of the identified slip traces, which suggested that the CRSS_{pyr Ⅱ}/CRSS_bas for compression was ~3 times than that of tension. The pyramidal Ⅱ 〈c + a〉 slip was more active when the slip plane was under tension than under compression, which was consistent with the calculated asymmetric CRSS_{pyr Ⅱ}/CRSS_bas. The activity of multiple slip, cross slip and slip transfer, as well as the GND density were also T-C asymmetric. This work thoughtfully demonstrated the T-C asymmetric slip behavior and plastic heterogeneity in Mg alloys which was believed to be responsible for the macroscopic T-C asymmetry when twinning was absent. The present statistical results are valuable for validating and/or facilitating crystal plasticity simulations.

Rare earth (RE) can produce excellent precipitation hardening in Mg alloys. However, when forming a solid solution, it also deteriorates formability, a problem that can usually be overcome by raising deformation temperature. Here we report an unexpected observation of high temperature brittleness in a Mg-Gd-Y-Ag alloy. As the temperature reached 500 ℃, the formability decreased drastically, leading to severe intergranular fracture under only 0.5% strain. This was caused by failure of grain boundaries, which are weakened by segregated interfacial compounds.