Abstract
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 μm over a large scanning range of 19 × 19 mm2. 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 × 1 mm2 (100 × 100 μm2) with a resolution of 250 μm (50 μm). We studied the defect structure in laser beam welds of the high-strength age-hardened 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.
Highlights
Crystal defects such as dislocations, precipitates and different species of point defects highly influence or even significantly determine the macroscopic physical properties of all kind of materials
In order to enable (C)DB spectroscopy with a spatial resolution well below 100 μm, we have developed a new positron microbeam providing high enough intensity and reasonable measurement times
A new positron microbeam for the investigation and 2D imaging of defects on an atomic scale using DB spectroscopy has been successfully put into operation
Summary
Defect imaging and detection of precipitates using a new scanning positron microbeam. Any further distribution of this work must maintain remoderation unit with a 100 nm thin Ni(100) foil and 9.6% efficiency is applied to reduce the area of attribution to the the beam spot by a factor of 60. In this way, defect spectroscopy is enabled with a lateral resolution of author(s) and the title of the work, journal citation. We studied the defect structure in laser beam welds of the high-strength age-hardened Al alloy (AlCu6Mn, EN AW-2219 T87) by applying (coincident) DBS with unprecedented spatial resolution.
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