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

9614562卡斯纳对金属疲劳现象知之甚少，部分原因是没有很好地描述反向变形的位错动力学。当施加的拉应力或剪应力相反时，由于薄片的屈曲，直接观察循环变形过程中的位错(如在高压电子显微镜上进行原位循环或反向塑性变形试验)是困难的。最近的实验表明，垂直方向的交替张应力可以避免箔的屈曲，并可以逆转位错的运动。为了便于加载，设计了一个新的试件工作台，并在NSF探索性研究小额拨款(SGER)下在高压电子显微镜中进行了测试。在显微镜下成功地对铝单晶进行了一系列的反向形变和疲劳实验，这些单晶在预饱和前循环变形，形成了密集的边缘位错偶极子束，其中的螺位错横跨沟道。研究发现，虽然刃口和螺杆对应变幅值的贡献相同，但螺杆位错在反转过程中移动较早。偶极脉中位错环的扩展似乎是塑性应变幅值的主要来源。偶极子翻转似乎对可逆应变没有显著贡献。在这个项目中，使用新技术检验了这些初步结论。通过提高显微图像的质量来扩展这些实验。该方法考虑了持续滑移带形成的重要情况，因为滑移带产生的挤压和侵入可能是疲劳裂纹形核的场所。在承受交变载荷的应用中，疲劳失效是金属合金部件的主要失效机制。这个项目提供了对反向加载情况下的疲劳机制的深入了解。*_

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. *** _