Gradient nano-grained (GNG) metals have shown a better strength-ductility combination compared to their homogeneous counterparts. In this paper, the mechanical properties and the related deformation mechanisms of GNG aluminum were investigated using three-dimensional multiscale discrete dislocation dynamics (DDD). GNG polycrystalline models and uniform nano-grained (UNG) counterparts were constructed within a multiscale DDD framework. A dislocation-penetrable grain boundary model based on a coarse-graining approach was adopted to handle the interaction between dislocations and grain boundaries. The simulation results show that the yield stress and strain hardening of the GNG sample is larger than the value calculated by the rule of mixtures, indicating a synergetic strengthening induced by the gradient structure. The associated microstructure evolution demonstrates that dislocations initially activate and glide in the larger grains and then gradually propagate into the smaller grains in the GNG sample, this sequential yielding of layers with different grain sizes generates stress and strain gradients, which is accommodated by geometrically necessary dislocations (GNDs). Moreover, we found that the Bauschinger effect in GNG sample is stronger than those in component UNG samples, suggesting a significant back stress strengthening in the GNG sample during plastic deformation. Finally, a theoretical model is established which successfully describes the Bauschinger effect of GNG and corresponding UNG samples according to the features of dislocation evolution upon unloading. The present study provides insights into the outstanding mechanical property of GNG metals from the view of dislocation dynamics at the submicron scale and offers theoretical guidance for designing strong-and-ductile metals.

与均质金属相比，梯度纳米晶粒 (GNG) 金属表现出更好的强度-延展性组合。在本文中，使用三维多尺度离散位错动力学（DDD）研究了 GNG 铝的力学性能和相关的变形机制。GNG 多晶模型和均匀纳米晶粒 (UNG) 对应物是在多尺度 DDD 框架内构建的。采用基于粗晶粒方法的位错可穿透晶界模型来处理位错与晶界之间的相互作用。模拟结果表明，GNG试样的屈服应力和应变硬化大于混合物规则计算的值，表明梯度结构引起了协同强化。相关的微观结构演变表明，位错最初在较大晶粒中激活并滑动，然后逐渐传播到 GNG 样品中的较小晶粒中，这种具有不同晶粒尺寸的层的顺序屈服产生应力和应变梯度，这由几何上必要的位错适应（接地）。此外，我们发现 GNG 样品中的包辛格效应比组件 UNG 样品中的强，这表明 GNG 样品在塑性变形过程中存在显着的背应力强化。最后，根据卸载时位错演化的特征，建立了一个理论模型，成功地描述了GNG和相应UNG样品的包辛格效应。