In-Situ Reversed Deformation Experiments in the High-Voltage Transmission Electron Microscope

小行星9614562 对金属疲劳现象的了解很少，部分原因是反向变形的位错动力学没有得到很好的表征。 在循环变形过程中（例如在高压透射电子显微镜中进行原位循环或反向塑性变形试验），对位错的直接观察受到薄箔在施加的拉伸或剪切应力反向时的屈曲的阻碍。 最近的实验表明，箔的屈曲可以避免，位错运动可以通过在交替的垂直方向上的拉伸应力来逆转。 设计了一个新的样品台，以促进这种加载，并在美国国家科学基金会探索性研究（SGER）的小额赠款下，在高压透射电子显微镜进行测试。 在显微镜下成功地对预循环变形至预饱和的铝单晶进行了一系列反向变形和疲劳实验，形成了密集的边缘位错偶极子束，其中螺旋位错跨越通道。 结果发现，螺旋位错在反转过程中的早期移动，虽然边缘和螺钉同样有助于应变幅度。 位错环从偶极脉扩张似乎是塑性应变振幅的主要来源。 偶极翻转似乎对可逆应变没有显着贡献。 在这个项目中，这些初步结论使用新技术进行检查。 通过提高显微图像的质量来扩展这些实验。 持久滑移带形成的重要情况包括在该方法中，因为滑移带产生挤压和侵入，这可能是疲劳裂纹成核的位置。 %疲劳失效是交变载荷应用中使用的金属合金部件的主要失效机制。 该项目提供了在反向加载情况下的疲劳机制的见解。 *** _

9614562 Kassner The metal fatigue phenomenon is poorly understood, in part because the dislocation dynamics of reversed deformation is not well characterized. The direct observation of dislocations during cyclic deformation (such as with in-situ cyclic or reversed plastic deformation tests in the high voltage transmission electron microscope) is hindered by the buckling of thin foils when the applied tensile or shear stress is reversed. Recent experiments show that buckling of the foil can be avoided and dislocation movement can be reversed by tensile stressing in alternating perpendicular directions. A new specimen stage was designed to facilitate this loading and tested in the high voltage transmission electron microscope under an NSF small grant for exploratory research (SGER). A series of reversed deformation and fatigue experiments were successfully performed in the microscope on aluminum single crystals pre-cyclically deformed to presaturation, forming dense edge-dislocation dipole bundles with screw dislocations that span the channel. It was found that screw dislocations move early during reversal although edges and screws contribute equally to the strain amplitude. Dislocation loop expansion from the dipole veins appears to be a major source of plastic strain amplitude. Dipole flipping does not appear to contribute significantly to reversible strain. In this project these preliminary conclusions are examined using the new techniques. These experiments are extended by improving the quality of the microscopy images. The important case where persistent slip bands form is included in the approach because the slip bands create extrusions and intrusions which may be sites for fatigue crack nucleation. %%% Fatigue failure is a major failure mechanism in metal alloy components used in applications that see alternating loads. This project provides insights into the fatigue mechanism in reversed loading situations. *** _