Unique yielding behavior and pseudo-Lüders deformation in harmonic-structured low-carbon steel

This study investigates the deformation behavior of harmonic-structured (HS) low-carbon steel fabricated by mechanical milling (MM) followed by spark plasma sintering (SPS). Mechanical milling generated a nanocrystalline surface layer on the powder particles, which formed an interconnected fine-grained Shell network surrounding coarse-grained Core regions after sintering. The resulting HS compact consisted of a Shell region with an average grain size of 3.1 μm and a Core region with an average grain size of 26.3 μm. Tensile testing revealed that the HS material exhibited an upper yield stress approximately twice that of the homogeneous (Homo) material while maintaining superior tensile toughness. The HS material also showed a pronounced yield drop and a stress-instability region accompanied by serrated flow. Surface observations demonstrated that localized deformation propagated gradually through the specimen, with a characteristic length scale corresponding to that of the Core–Shell units. This deformation mode, termed “pseudo-Lüders deformation,” was distinct from conventional Lüders deformation. Microhardness measurements and transmission electron microscopy revealed preferential hardening and dislocation accumulation in the Shell region, indicating heterogeneous strain partitioning during deformation. The resulting strain incompatibility between the Shell and Core regions is suggested to be responsible for the observed heterogeneous strain hardening. Furthermore, pre-straining eliminated both the yield drop and pseudo-Lüders deformation, resulting in continuous yielding behavior. These findings demonstrate that harmonic structural heterogeneity significantly modifies the yielding behavior of low-carbon steel and provides new insight into the control of strain localization in heterostructured metallic materials.