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Mechanical Properties of Symmetric Tilt Grain Boundaries in Silicon and Silicon Carbide: A Molecular Dynamics Study

机译:硅和碳化硅中对称倾斜晶界的力学性质:分子动力学研究

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摘要

The mechanical properties of polycrystalline materials are governed by the underlying microstructure. In this context, in this dissertation, the role of grain boundaries on the mechanical response of two technologically important materials namely silicon and silicon carbide are examined. In particular, the dynamics of silicon carbide and silicon symmetric tilt bicrystals under shear load are characterized via molecular dynamics simulations. Cubic silicon carbide bicrystals with low-angle grain boundaries exhibit stick-slip behavior due to athermal climb of edge dislocations along the grain boundary at low temperatures. With increasing temperature, stick-slip becomes less pronounced due to competing dislocation glide, and at high-temperatures, structural disordering of the low-angle grain boundary inhibits stick-slip. In contrast, structural disordering of the high-angle grain boundary is induced under shear even at low temperatures, resulting in a significantly dampened stick-slip behavior. When a single layer graphene sheet is introduced at the grain boundary of the symmetric tilt silicon-carbide bicrystals, the resultant shear response is dictated by the orientation of the graphene sheet. Specifically, when the graphene layer is oriented perpendicular to the gain boundary, stick-slip behavior displayed by the low-angle grain boundaries is inhibited, though both low-angle and high-angle grain boundaries exhibit displacement along crystallographic planes parallel with the applied shear direction. On the other hand, when the graphene sheet is parallel to the grain boundary, shear deformation at the grain boundary for both low-angle and high-angle bicrystals is diminished. In silicon bicrystals, high-angle grain boundaries demonstrate coupled motion, characterized by an additional normal motion of the grain boundary. Interestingly, this phenomenon was observed previously in metallic materials. Further, the grain boundary coupling factor, which is ratio of the grain boundary normal velocity to the grain translation velocity, matches the predicted geometric value. The underlying atomic scale mechanisms that govern the grain boundary coupled motion consists of concerted rotations of silicon tetrahedra within the grain boundary. For low-angle grain boundaries in silicon, the activation of dislocation glide along the predicted slip-plane takes precedence and no grain boundary coupling is observed. This behavior is similar to that of silicon carbide seen at high-temperatures but for silicon it occurs for a large temperature window.
机译:多晶材料的机械性能由下面的微观结构控制。在此背景下,本文研究了晶界对两种技术上重要的材料即硅和碳化硅的机械响应的作用。特别地,通过分子动力学模拟表征了碳化硅和对称对称倾斜双晶硅在剪切载荷下的动力学。具有低角度晶界的立方碳化硅双晶由于在低温下沿晶界的边缘位错的无热爬升而表现出粘滑行为。随着温度的升高,由于竞争性位错滑动,粘滑变得不那么明显,并且在高温下,低角度晶界的结构紊乱抑制了粘滑。相反,即使在低温下,在剪切作用下也会引起高角度晶界的结构紊乱,从而大大降低了粘滑性能。当在对称倾斜碳化硅双晶的晶界处引入单层石墨烯片时,所得的剪切响应取决于石墨烯片的取向。具体而言,当石墨烯层垂直于增益边界取向时,尽管低角度和高角度晶界均沿平行于所施加剪切力的晶体平面表现出位移,但低角度晶界所显示的粘滑行为被抑制。方向。另一方面,当石墨烯片平行于晶界时,对于低角度和高角度的双晶,在晶界处的剪切变形都减小了。在双晶硅中,高角度晶界表现出耦合运动,其特征在于晶界的附加法向运动。有趣的是,这种现象以前是在金属材料中观察到的。此外,作为晶界法向速度与晶粒平移速度之比的晶界耦合因子与预测的几何值匹配。控制晶界耦合运动的基本原子尺度机制由晶界内四面体硅的协调旋转组成。对于硅中的低角度晶界,位错滑动沿预测滑移面的激活优先,并且未观察到晶界耦合。这种行为类似于在高温下观察到的碳化硅的行为,但是对于硅,它的发生在较大的温度范围内。

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    Bringuier Stefan;

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  • 年度 2015
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