Laue pattern analysis for 2D strain mapping in light ion implanted polycristals
Résumé
The swelling of nuclear fuel (uranium dioxide, UO2) in storage conditions is partly due to the alpha decay (helium production) of radioactive fission products. In order to study the behaviour of spent fuel, helium ions are implanted at the surface of fresh UO2 polycrystalline disks. This technique allows the study of nuclear fuel in storage conditions while dealing with low radioactive samples.
In such implanted samples, a thin layer close to the surface is deformed. X-ray micro-diffraction in Laue mode is used to measure the induced strain. Experiments were done on the BM32 beamline at the European Synchrotron Radiation Facility (Grenoble, France) with a 1×2 µm2 beam size, much smaller than the grain size (about 18 µm).
In the resulting Laue patterns, the diffraction spots are observed to split forming double spots, one corresponding to the non-deformed substrate and the other to the deformed (implanted) layer.
- In a first study, the spots (substrate or deformed layer) were detected using Gaussian shape functions. This has enabled to characterise the strain tensor, and its dependence on grain orientation, which has been confirmed by mechanical modelling. An experimental noise of about 5×10-4 has been observed on the strain components.
- To reduce this noise, a specific image analysis has been developed. It includes the use of bi-Gaussian shape functions to improve spot detections and several numerical tools (conditioning, goodness-of-fit, hat matrix, etc.), based on least squares techniques and statistics, for finding incorrect data and estimating the accuracy of the result. The precision obtained by these methods allows characterising slight strain variations as those measured at grain boundaries.
The use of these tools is not restricted to the study of ion implanted polycrystals but could also help characterising polycrystals whose surface has been either optimised (for example by nitriding) or deteriorated (by corrosion, oxidation, tribology, polishing or ion beam milling).