Nanocrystalline (NC) metals often show transition from Hall–Petch (HP) strengthening to inverse HP softening as the average grain size decreases below a critical value. Compared to the HP behavior whose mechanism is well understood as the dislocation pile-up at grain boundaries (GBs), several hypotheses have been proposed to explain the veiled inverse HP behavior and no consensus has been reached yet. In this work, we propose a size-dependent model considering the influence of grain size on the intragranular dislocation storage ability and unify the HP and inverse HP relations for NC metals. The reduction of the intragranular dislocation storage ability with decreasing grain size is revealed as the underlying mechanism of the breakdown of the HP behavior in NC. Prediction of the critical grain size of $26.9\mathrm{nm}$ for the HP-inverse HP transition of NC copper agrees well with experimental results. Numerical results suggest that the harmonized deformation of GB and grain interior (GI) dominates the remarkable ductility enhancement of NC metals in the inverse HP region. Moreover, our results suggest that the formation of dimple structures spanning several grains at fracture surfaces of NC metals is attributed to the coalescence of inter- and intra-granular microcracks and microvoids in clustered grains with Goss texture in local shear bands. As the GB strength increases, NC metals show enhancement in yield strength and delay in occurrence of the inverse HP behavior. Our studies give new insight into the contribution of GB sliding to the plastic behaviors of NC metals, and provide valuable guidance for the rational design of NC metals with high ductility and high strength.

随着平均晶粒尺寸减小到临界值以下，纳米晶 (NC) 金属通常表现出从霍尔-佩奇 (HP) 强化到逆 HP 软化的转变。与其机制被很好地理解为晶界（GBs）位错堆积的 HP 行为相比，已经提出了几种假设来解释隐蔽的逆 HP 行为，但尚未达成共识。在这项工作中，我们提出了一个尺寸依赖模型，考虑了晶粒尺寸对晶内位错存储能力的影响，并统一了 NC 金属的 HP 和反 HP 关系。随着晶粒尺寸的减小，晶内位错存储能力的降低被揭示为 NC 中 HP 行为破坏的潜在机制。临界晶粒尺寸的预测$26.9\mathrm{纳米}$NC 铜的 HP 反 HP 跃迁与实验结果非常吻合。数值结果表明，GB 和晶粒内部 (GI) 的协调变形主导了逆 HP 区域中 NC 金属显着的延展性增强。此外，我们的结果表明，在 NC 金属的断裂表面形成跨越多个晶粒的凹坑结构归因于局部剪切带中具有 Goss 纹理的簇状晶粒中的晶粒间和晶粒内微裂纹和微孔洞的聚结。随着 GB 强度的增加，NC 金属表现出屈服强度的增强和逆 HP 行为发生的延迟。我们的研究为 GB 滑动对 NC 金属塑性行为的贡献提供了新的见解，