Atomistic Study of Fracture of Nano-Crystalline Materials

在具有极细（纳米尺寸）晶粒的晶体材料中，形成晶界的原子百分比相当大。由于晶界结构的随机性和准稳定性，使得晶界结构呈现出一些独特的性质。为了弄清控制纳米晶材料强度性能的动态结构因素，进行了大规模的计算机模拟，得到了以下结果：(1)发现了Hall-Petch逆关系，表明材料的软化随晶粒尺寸的减小而减小。这种关系表示为强度与晶体缺陷分数的关系，表明强度主要由边界原子结构的动态重排决定。(2)不仅是晶体缺陷，而且位错源如Frank-Read源也不能在晶粒中稳定存在。但由于扩展位错的宽度与晶粒尺寸相当，层错可以穿过晶粒存在，并产生各向异性，从而导致应变和循环硬化。(3)在纳米线材或纳米厚度的薄板中，对于层错能高的材料，晶界滑动会导致边界内断裂。但在层错能较低的情况下，晶间滑动占主导地位，抑制了二次晶间滑动，提高了延展性。(4)在非晶材料中，在局部温度升高的辅助下，发生了局部颈缩等密集局部形变，形变诱导的再结晶出现，纳米级晶粒形核长大。

In a crystalline materials with very fine (nanometer-size) grain, percentage of the atoms which form the grain boundary is fairly large. Because of randomness and quasi-stableness of the grain boundary structure, certain unique properties appear in it. Under the purpose to make clear the dynamical structural factors controlling the strength properties of the nano-crystalline materials, large scale computer simulation was performed and the following results have been obtained.(1) Inverse Hall-Petch relation which states softening of the materials with decease of the grain size, is found. This relation is expressed as the relationship between the strength and the fraction of the crystal defect, which suggests the strength mainly determined by the dynamical rearrangement in the boundary atomic structure.(2) Not only the crystal defect but also the source of dislocation such as the Frank-Read source cannot exist stationarily in the grain. But because of width of the extended dislocation becomes comparable to size of the grain, the stacking fault able to be present through the grain and it brings about the anisotropic properties, which causes the strain and cyclic hardening.(3) In nano-wire or thin plate with nano-thickness, the intra-boundary rapture is bought about due to grain boundary slide for materials with high stacking fault energy. But the inter-grain slid becomes dominate for the case that the stacking fault energy is low, which raise the ductility due to the suppression of the secondary inter-grain slide.(4) In an amorphous materials, an intensively localized deformation is taken place such as a localized necking, deformation induced recrystalization appears and nano-size crystal grain nucleates and grows, with the assist of the local temperature rise.