Gradient structures have excellent mechanical properties, such as synergetic strength and ductility. It is desirable to reveal the connection between the gradient structure and mechanical properties. However, few studies have been conducted for materials with heterogeneous dislocation distribution. In the present study, we use the discrete dislocation dynamics (DDD) method to investigate the effect of dislocation density gradient on the elastoplastic behavior of single crystals controlled by source activation. In contrast to the intuitive expectation that gradient structure affects the mechanical properties, the DDD simulations show that the elastic moduli and yield stresses of three gradient samples (i.e., no gradient, low gradient, and high gradient) are almost identical. Different from the progressive elastic–plastic transition in the samples controlled by Taylor hardening (i.e., the mutual interaction of dislocation segments), the flow stresses of source activation ones enter into a stage of nearly ideal plasticity (serrated flow) immediately after yielding. The microstructure evolution demonstrates that the mean dislocation spacing is relatively large. Thus, there are only a few or even one dislocation source activated during the plastic flow. The intermittent operation of sources leads to intensive fluctuation of stress and dislocation density, as well as a stair-like evolution of plastic strain. The present work reveals that the effect of dislocation density gradient on the mechanical response of crystals depends on the underlying dislocation mechanisms controlling the plastic deformation of materials.

中文翻译：

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位错机理对异质位错晶体晶体弹塑性行为的影响

渐变结构具有出色的机械性能，例如协同强度和延展性。希望揭示梯度结构和机械性能之间的联系。但是，对于位错分布不均的材料进行的研究很少。在本研究中，我们使用离散位错动力学（DDD）方法来研究位错密度梯度对受源激活控制的单晶弹塑性行为的影响。与对梯度结构影响机械性能的直观期望相反，DDD仿真表明，三个梯度样品（即无梯度，低梯度和高梯度）的弹性模量和屈服应力几乎相同。与泰勒硬化控制的样品中逐渐的弹塑性转变（即位错节段的相互作用）不同，源激活剂的流动应力在屈服后立即进入接近理想的可塑性阶段（锯齿状流动）。显微组织演变表明，平均位错间距相对较大。因此，在塑性流动期间仅激活了少数甚至一个位错源。源的间歇运行导致应力和位错密度的剧烈波动，以及塑性应变的阶梯状演变。目前的工作表明，位错密度梯度对晶体机械响应的影响取决于控制材料塑性变形的位错机制。