Grain size effects on strain localization and fracture behavior in ferrite–martensite dual-phase steels characterized by digital image correlation analysis
Myeong‐heom Park, Yuichi Tagusari, Akinobu Shibata, Nobuhiro Tsuji
Abstract
Dual-phase (DP) steels, consisting of soft ferrite and hard martensite phases, are widely utilized for their favorable balance of strength and ductility. Grain refinement in DP steels has been recognized as an effective strategy to enhance strength without sacrificing ductility, particularly in the post-uniform elongation regime. In the present study, the underlying mechanism responsible for improved post-uniform elongation through grain refinement was investigated using high-resolution digital image correlation (DIC) and detailed microstructural analysis of micro-void evolution. Two DP microstructures with ferrite grain sizes of 28.9 μm and 11.8 μm were fabricated, exhibiting markedly different local deformation behaviors. The fine-grained DP specimen (11.8 μm) exhibited more homogeneous deformation, attributed to a greater contribution of martensite to plastic deformation. Numerous micro-voids were observed in the fine-grained DP specimen, most of which remained small even under large deformation. In contrast, the coarse-grained specimen (28.9 μm) exhibited fewer micro-voids, several of which showed notable growth, potentially acting as fracture initiation sites, and contributing to limited post-uniform elongation. DIC analysis at the fracture stage successfully captured the strain distribution, revealing that micro-voids and cracks were predominantly located in regions with high strain gradients. These observations suggest that grain refinement may promote ductility by mitigating strain localization and reducing the propensity for void growth.