Strengthening and toughening mechanisms in CoCrFeMnNi high-entropy alloys with grain size-twin thickness dual-gradient structures revealed by molecular dynamics simulations

WZ Qiang and LC Long and CQ Dang and Q Wu, MATERIALS TODAY NANO, 32, 100720 (2025).

DOI: 10.1016/j.mtnano.2025.100720

High-entropy alloys (HEAs) have emerged as promising structural materials due to their unique compositional design and exceptional mechanical performance. While compositional tuning has long been a primary strategy for enhancing HEA properties, recent advances have demonstrated that gradient microstructural design, such as bimodal grain size distributions and twin structures, can further optimize the strength-toughness balance. However, atomic-scale mechanisms underlying such synergistic strengthening in dual-gradient systems remain poorly understood. Here, we employ large-scale molecular dynamics (MD) simulations to investigate the tensile and compressive deformation behaviors of CoCrFeMnNi-HEAs with three representative microstructures: homogeneous, bimodal-grain-gradient, and grain size-twin thickness dual- gradient structures. In the homogeneous and bimodal-grain-gradient structures, deformation proceeds via alternating Shockley partial slip and sessile dislocation formation under tension, and via more synchronized slip under compression, resulting in fluctuating tensile responses and more stable compressive behavior. The bimodal-grain- gradient structure exhibits enhanced yield strength due to the development of strain gradients during the elastic-plastic transition. Furthermore, the dual-gradient structure introduces twin boundaries (TBs) that facilitate initial dislocation nucleation while impeding continued slip, thus contributing to both strengthening and toughening. The tailored combination of twin thicknesses and grain size reveals a spatially coordinated deformation mechanism: high-density TBs in hard domains enhance structural stability, while low-density TBs in soft domains promote Shockley partial dislocation slip. This positively correlated dual-gradient architecture promotes uniform deformation, mitigates strain localization, and introduces secondary strengthening through effective load partitioning among the face-centered cubic (FCC) phase, hexagonal close-packed (HCP) phase, and amorphous phase. These findings deepen our understanding of atomistic deformation mechanisms in gradient-structured HEAs and offer design strategies for high- performance structural materials.

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