Abstract The gradient nanograined metals have the unexpected combination of high strength and high ductility. However, the detailed dynamic and continuous process of deformation mechanism at nanoscale need to be further understood. Herein, we report the deformation behavior and microstructure evolution of gradient-nanograined body-centred cubic alloys using molecular dynamics simulations. An analytical model is also established to predict the strength for nanograined alloys. The results show that the alloying can soften the gradient-grained and random-nanograined Fe-based alloys. With the increasing Ni concentration, dislocation density decreases, due to the fact that the Ni solute increases stacking fault energy and the deformation twinning acts as a strong obstacle to suppress dislocation motion. The gradient distributions of strain and stress in gradient-nanograined metals occur due to the plastic incompatibilities, resulting in the inhomogeneous deformation of gradient-nanograined Fe and alloys. In addition, the inverse gradient-nanograined distribution, including large grain in the top surface and small grain in the central region, generates in random-nanograined alloys. It is owing to the grain growth in surface layer, resulting in the gradient strain and stress. Interestingly, the nucleated dislocation network and the deformation twinning considered as a reduction of dislocation mean free path decline rapidly with the increasing grain size for gradient-nanograined structure. The theoretical modeling determines the total strength derived from the individual contributions of different strengthening mechanisms, revealing that the grain boundaries dominate the yield strength and the back stress contributes the most to the straining hardening in nanograined materials.

中文翻译：

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梯度纳米结构体心立方合金中强化-软化权衡的起源

摘要 梯度纳米晶粒金属具有意想不到的高强度和高延展性。然而，纳米尺度变形机制的详细动态和连续过程需要进一步了解。在此，我们使用分子动力学模拟报告了梯度纳米晶粒体心立方合金的变形行为和微观结构演变。还建立了一个分析模型来预测纳米晶粒合金的强度。结果表明，合金化可以软化梯度晶粒和随机纳米晶粒的铁基合金。随着Ni浓度的增加，位错密度降低，这是因为Ni溶质增加了层错能，变形孪晶是抑制位错运动的强大障碍。梯度纳米晶粒金属的应变和应力梯度分布由于塑性不相容性而发生，导致梯度纳米晶粒铁和合金的不均匀变形。此外，在随机纳米晶粒合金中产生逆梯度纳米晶粒分布，包括顶部表面的大晶粒和中心区域的小晶粒。这是由于表层晶粒长大，产生梯度应变和应力。有趣的是，随着梯度纳米晶粒结构晶粒尺寸的增加，被视为位错减少的形核位错网络和变形孪晶的平均自由程迅速下降。理论模型确定了来自不同强化机制的个体贡献的总强度，