Mapping the strain-localization evolution of grain boundary and its interactions with slip/twin at the microscale

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.