The microstructures, mechanical properties, and deformation substructures of gradient Mo_0.3NiCoCr medium-entropy alloys (MEAs) with very coarse grain size created by pre-torsion have been investigated. The strength of MEAs increases with the increase of torsion angle, while the tensile elongation nearly remains the same, suggesting the enhanced strength-ductility synergy. The initial dislocation density gradient structure after torsion and the following deformation substructure under tension are uncovered by means of electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and transmission electron microscopy (TEM). The crystal plasticity finite element method (CPFEM) is employed to quantitively evaluate the evolution of dislocation densities and mechanical twinning volume fraction. The combination of experimental characterization and theoretical modeling enables to clarify the underlying strengthening and strain hardening mechanisms. The gradient distribution of dislocations created by the torsion leads to the rise of yield strength. Moreover, the high order of microbands, which arise from the activation of multiple slip systems during torsion, and additional mechanical twinning form in the gradient MEAs upon loading, constituting multiple level gradient structures. As the plastic strain goes on, the microbands can propagate and refine continuously, along with the interactions with the nano twins, in these MEAs with very coarse grain size up to ∼500 µm, which produce progressively high strain hardening and stabilize the plastic deformation over the whole deformation regime. This study thus offers guidance for optimizing the mechanical performance of structural materials via tuning the design of gradient structure.

_{梯度Mo 0.3}的显微组织、力学性能和变形子结构已经研究了通过预扭转产生的具有非常粗晶粒尺寸的 NiCoCr 中熵合金 (MEA)。MEA 的强度随着扭转角的增加而增加，而拉伸伸长率几乎保持不变，表明增强的强度-延展性协同作用。通过电子背散射衍射 (EBSD)、电子通道对比成像 (ECCI) 和透射电子显微镜 (TEM) 揭示了扭转后的初始位错密度梯度结构和随后的拉伸变形子结构。采用晶体塑性有限元法 (CPFEM) 定量评估位错密度和机械孪晶体积分数的演变。实验表征和理论建模的结合能够阐明潜在的强化和应变硬化机制。扭转产生的位错梯度分布导致屈服强度升高。此外，在扭转过程中由多个滑移系统的激活产生的高阶微带，以及在加载时在梯度 MEA 中形成的额外机械孪晶，构成了多级梯度结构。随着塑性应变的继续，微带可以不断传播和细化，以及与纳米孪晶的相互作用，在这些具有非常粗晶粒尺寸高达 ~ 500 µm 的 MEA 中，产生逐渐高的应变硬化并稳定塑性变形整个变形机制。