一种新的扫描正电子微束对晶体缺陷的成像与检测
Defect Imaging and Detection of Precipitates Using a New Scanning Positron Microbeam
摘要：
我们报道了一种新研制的基于高强度正电子源NEPOMUC正电子三重慢化的扫描正电子微束。为了提高亮度，应用具有$100\,$ nm薄Ni（100）箔和9.6%效率的再减载单元以将束斑的面积减小60倍。以这种方式，在$19\times19\,$ mm $^{2}$的大扫描范围内实现了具有$33\,μ$ m的横向分辨率的缺陷光谱。此外，使用多普勒展宽光谱（DBS）的2D缺陷成像被证明是在不到两分钟的异常短的测量时间内进行的$1\times1$毫米$^{2}$（$100\times100\,μ$ m $^{2}$）的区域与$250\,μ$ m（$50\,μ$ m）的分辨率。采用空间分辨率高的同步DBS技术，研究了高强时效铝合金（AlCu 6 Mn，EN AW-2219 T87）激光焊缝中的缺陷结构。缺陷分布的可视化显示了原材料和焊接区之间的急剧过渡以及非常小的热影响区。在接收态材料中检测到空位状缺陷和富Cu沉淀物，并且在较小程度上在焊缝的过渡区中检测到空位状缺陷和富Cu沉淀物。最值得注意的是，在焊缝的中心没有形成Cu-空位复合物的空位，并且可以清楚地识别晶格中的Cu原子的溶解，即形成过饱和溶液。
英文摘要：
We report on a newly developed scanning positron microbeam based on threefold moderation of positrons provided by the high intensity positron source NEPOMUC. For brightness enhancement a remoderation unit with a $100\,$nm thin Ni(100) foil and 9.6% efficiency is applied to reduce the area of the beam spot by a factor of 60. In this way, defect spectroscopy is enabled with a lateral resolution of $33\,μ$m over a large scanning range of $19\times19\,$mm$^{2}$. Moreover, 2D defect imaging using Doppler broadening spectroscopy (DBS) is demonstrated to be performed within exceptional short measurement times of less than two minutes for an area of $1\times1$mm$^{2}$ ($100\times100\,μ$m$^{2}$) with a resolution of $250\,μ$m ($50\,μ$m). We studied the defect structure in laser beam welds of the high-strength agehardened Al alloy (AlCu6Mn, EN AW-2219 T87) by applying (coincident) DBS with unprecedented spatial resolution. The visualization of the defect distribution revealed a sharp transition between the raw material and the welded zone as well as a very small heat affected zone. Vacancy-like defects and Cu rich precipitates are detected in the as-received material and, to a lesser extent, in the transition zone of the weld. Most notably, in the center of the weld vacancies without forming Cu-vacancy complexes, and the dissolution of the Cu atoms in the crystal lattice, i.e. formation of a supersaturated solution, could be clearly identified.
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