In the endeavors of working with microstructures in polycrystalline metals for better strength and ductility, grain boundaries (GBs) are placed at the front burner for their pivotal roles in plastic deformation. Often the mechanical properties of polycrystalline metals are governed by mutual interactions among GBs and dislocations. A thorough comprehension of GB deformation is therefore critical for the design of metals of superb performance. In this research, we investigated the mechanical behavior of symmetric tilt grain boundaries in face-centered cubic (F.C.C.) nickel, which may be subject to tension, shearing, and mixing-mode load using molecular dynamics simulations. We observed that (1) there exist four types of micro deformation mechanisms in GBs, and illustrate at the atomistic scale their distinctions and their dependence on the activation of lattice slip in the crystal; (2) GBs are intrinsically brittle under tension but exhibit ductile behavior during shearing. Shifting from pure tension with increasing shear component during mixing-mode load leads to GB toughening; and (3) there lacks conceivable dependence of GB tensile strength on tilted GBs, in contrast to a relatively rough trend of greater shear strength in GBs of large misorientation. GB energy shows no direct connection with GB strength, as broadly reported in existing literature. This research enhances our mechanistic understanding of GB plasticity in crystalline metals, and points to a potential way of making strong-yet-tough polycrystalline metals through GB engineering: in addition to GB structure manipulation, tuning the loading mode of GBs may open another avenue for their better performance.

中文翻译：

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倾斜晶界的本征拉伸脆性及其剪切增韧


在努力研究多晶金属的微观结构以提高强度和延展性的过程中，晶界 (GB) 因其在塑性变形中的关键作用而被置于最前面。多晶金属的机械性能通常由晶界和位错之间的相互作用决定。因此，彻底理解 GB 变形对于设计具有卓越性能的金属至关重要。在这项研究中，我们利用分子动力学模拟研究了面心立方 (FCC) 镍中对称倾斜晶界的机械行为，该晶界可能会受到张力、剪切和混合模式载荷的影响。我们观察到：（1）晶界中存在四种类型的微变形机制，并在原子尺度上说明了它们的区别以及它们对晶体中晶格滑移激活的依赖性； (2) 晶界在张力下本质上是脆性的，但在剪切过程中表现出延性行为。在混合模式负载期间，从纯张力转变为增加剪切分量，导致 GB 增韧； (3) 晶界拉伸强度不存在对倾斜晶界的可想象的依赖性，这与大偏差晶界中较大的剪切强度的相对粗略的趋势形成对比。正如现有文献中广泛报道的那样，GB 能量与 GB 强度没有直接关系。这项研究增强了我们对晶界金属中晶界可塑性的机械理解，并指出了通过晶界工程制造坚固而坚韧的多晶金属的潜在方法：除了晶界结构操纵之外，调整晶界的加载模式可能会开辟另一条途径。他们更好的表现。