In real crystals and at finite temperatures point defects are inevitable. Under shear their dynamics severely influence the mechanical properties of these crystals, giving rise to non-linear effects, such as ductility. In an effort to elucidate the complex behavior of crystals under plastic deformation it is crucial to explore and to understand the interplay between the timescale related to the equilibrium point-defect diffusion and the shear-induced timescale. Based on extensive non-equilibrium molecular dynamics simulations we present a detailed investigation on the yielding behavior of cluster crystals, an archetypical model for a defect-rich crystal: in such a system clusters of overlapping particles occupy the lattice sites of a regular (FCC) structure. In equilibrium particles diffuse via site-to-site hopping while maintaining the crystalline structure intact. We investigate these cluster crystals at a fixed density and at different temperatures where the system remains in the FCC structure: temperature allows us to vary the diffusion timescale appropriately. We then expose the crystal to shear, thereby choosing shear rates which cover timescales that are both higher and lower than the equilibrium diffusion timescales. We investigate the macroscopic and microscopic response of our cluster crystal to shear and find that the yielding scenario of such a system does not rely on the diffusion of the particles – it is rather related to the plastic deformation of the underlying crystalline structure. The local bond order parameters and the measurement of local angles between neighboring clusters confirm the cooperative movement of the clusters close to the yield point. Performing complementary, related simulations for an FCC crystal formed by harshly repulsive particles reveals similarities in the yielding behavior between both systems. Still we find that the diffusion of particles does influence characteristic features in the cluster crystal, such as a less prominent increase of order parameters close to the yield point. Our simulations provide for the first time an insight into the role of the diffusion of defects in the yielding behavior of a defect-rich crystal under shear. These observations will thus be helpful in the development of theories for the plastic deformation of defect-rich crystals.

中文翻译：

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在剪切下产生富含点缺陷的模型晶体：来自分子动力学模拟的见解

在实际晶体和有限温度下，点缺陷是不可避免的。在剪切下，它们的动力学会严重影响这些晶体的机械性能，从而产生非线性效应，例如延展性。为了阐明晶体在塑性变形下的复杂行为，探索和理解与平衡点缺陷扩散相关的时间尺度与剪切诱导的时间尺度之间的相互作用是至关重要的。基于广泛的非平衡分子动力学模拟，我们对团簇晶体的屈服行为进行了详细研究，这是一种富含缺陷晶体的原型模型：在这样的系统中，重叠粒子的团簇占据规则 (FCC) 的晶格位置结构体。在平衡粒子扩散通过点到点跳跃，同时保持晶体结构完整。我们以固定的密度和系统保持在 FCC 结构中的不同温度研究这些簇晶体：温度允许我们适当地改变扩散时间尺度。然后我们将晶体暴露于剪切力，从而选择覆盖比平衡扩散时间尺度更高和更低的时间尺度的剪切速率。我们研究了我们的簇晶体对剪切的宏观和微观响应，发现这种系统的屈服情景不依赖于粒子的扩散——它与底层晶体结构的塑性变形有关。局部键序参数和相邻簇之间局部角度的测量证实了靠近屈服点的簇的协同运动。对由强烈排斥粒子形成的 FCC 晶体进行互补的相关模拟揭示了两种系统之间屈服行为的相似性。我们仍然发现粒子的扩散确实影响了团簇晶体中的特征，例如接近屈服点的有序参数的增加不太明显。我们的模拟首次提供了对缺陷扩散在剪切下富缺陷晶体屈服行为中的作用的洞察。因此，这些观察结果将有助于发展富缺陷晶体塑性变形的理论。对由强烈排斥粒子形成的 FCC 晶体进行互补的相关模拟，揭示了两种系统之间屈服行为的相似性。我们仍然发现粒子的扩散确实影响了团簇晶体中的特征，例如接近屈服点的有序参数的增加不太明显。我们的模拟首次提供了对缺陷扩散在剪切下富缺陷晶体屈服行为中的作用的洞察。因此，这些观察结果将有助于发展富缺陷晶体塑性变形的理论。对由强烈排斥粒子形成的 FCC 晶体进行互补的相关模拟，揭示了两种系统之间屈服行为的相似性。我们仍然发现粒子的扩散确实影响了团簇晶体中的特征，例如接近屈服点的有序参数的增加不太明显。我们的模拟首次提供了对缺陷扩散在剪切下富缺陷晶体屈服行为中的作用的洞察。因此，这些观察结果将有助于发展富缺陷晶体塑性变形的理论。我们仍然发现粒子的扩散确实影响了团簇晶体中的特征，例如接近屈服点的有序参数的增加不太明显。我们的模拟首次提供了对缺陷扩散在剪切下富缺陷晶体屈服行为中的作用的洞察。因此，这些观察结果将有助于发展富缺陷晶体塑性变形的理论。我们仍然发现粒子的扩散确实影响了团簇晶体中的特征，例如接近屈服点的有序参数的增加不太明显。我们的模拟首次提供了对缺陷扩散在剪切下富缺陷晶体屈服行为中的作用的洞察。因此，这些观察结果将有助于发展富缺陷晶体塑性变形的理论。