Micromechanical characterization of grain boundary slip: Towards a deformation mechanism map

晶界滑移(GBS)是多晶材料，特别是纳米晶材料和高温下材料的一种重要变形机制。多晶材料中存在大量不同的晶界，以及其他滑移机制(如位错滑移)的发生，阻碍了对GBS过程中基本过程的定量理解。在过去的十年中，建立了聚焦离子束(FIB)球磨结构的高温纳米机械测试，并有望在今天分离、表征和量化微米级的单个晶界的GBS。我们的目标是在扫描电子显微镜(SEM)中使用小规模力学测试，以便将GBS与其他塑性变形机制分离。我们建议在微米和亚微米尺寸的压缩柱中的单个晶界上测量GBS的剪切应力-剪切应变曲线。晶界将相对于柱状加载轴线倾斜45°。使用施加的载荷和滑移几何形状(边界的面积和倾斜度、滑移方向)计算剪应力。剪切应变将由扫描电子显微镜图像计算，并通过数字图像相关分析(DIC)进行分析。一旦建立了测量GB剪切应力-剪切应变曲线的协议，我们就将就以下基本问题定量地评估GBS的机制：1.GBS的激活能是什么？2.在Rachinger类型的GBS中是否存在本征尺寸效应？3.晶界类型对GBS的影响是什么？4.应变速率对GBS的定量依赖是什么？5.GBS对于维持(宏观)多晶中的兼容性的重要性是什么？

Grain boundary sliding (GBS) is one key deformation mechanism of polycrystalline materials, particularly for nanocrystalline materials and materials at elevated service temperatures. The immense number of different grain boundaries being present in polycrystalline materials as well as the occurrence of other slip mechanisms (e.g. dislocation slip) prevented a quantitative understanding of the fundamental processes during GBS so far. During the last decade, high temperature nanomechanical testing on focused ion beam (FIB) milled structures was established, and potentially allows for the isolation, characterization and quantification of GBS of one single grain boundary at the micron scale today. It is our objective to use small scale mechanical testing inside a scanning electron microscope (SEM) in order to decouple GBS from other mechanisms of plastic deformation. We propose to measure the shear stress vs. shear strain curve for GBS on individual grain boundaries located in micron and sub-micron sized compression pillars. The grain boundaries will be inclined by 45° with respect to the pillar loading axes. The shear stress is calculated using the applied load as well as the slip geometry (area and inclination of the boundary, direction of slip). The shear strain will be calculated from SEM images, analyzed by digital image correlation (DIC). As soon as a protocol for measuring the shear stress vs. shear strain curve of a GB is established, we will quantitatively assess the GBS mechanisms with regard to the subsequent fundamental questions: 1. What is the activation energy of GBS? 2. Is there an intrinsic size effect in Rachinger-type GBS? 3. What is the influence of grain boundary type for GBS? 4. What is the strain rate dependence on GBS quantitatively? 5. What is the importance of GBS for maintaining compatibility in (macro) polycrystals?