Strengthening mechanisms in a heterostructured Fe34Mn20Co20Cr20Ni6 high-entropy alloy: Roles of grain interior, grain boundary, and hetero boundary

This study systematically investigates the strengthening mechanisms in a heterostructured Fe 34 Mn 20 Co 20 Cr 20 Ni 6 high-entropy alloy (HEA), with a focus on the distinct roles of the grain interior, grain boundary, and hetero boundary. The novelty of this work lies in its controlled, multiscale methodology that successfully decouples these contributions. Nanoindentation tests combined with post-mortem electron backscatter diffraction (EBSD) characterizations reveal that fine grain (FG) regions exhibit higher nanohardness than coarse grain (CG) regions, due to their lower pre-existing dislocation density, which requires higher stress for dislocation nucleation. The enhanced strength-ductility balance is attributed to synergistic effects: i) Hetero boundaries demonstrate a superior strengthening effect due to driven by intensified strain and stress concentrations and hetero-deformation induced (HDI) hardening from coupled forward/back stresses. ii) A grain-size-dependent deformation mechanism is identified, with FG regions showing smaller activation volume compared to CG, suggesting a transition in dominant deformation mechanisms from dislocation slip to deformation twinning.