Time-of-flight 3D Neutron Diffraction for Multigrain Crystallography

This thesis presents a new technique for measuring spatially resolved microstructures in crystalline materials using pulsed neutron beams. The method, called Time-of-Flight Three Dimensional Neutron Diffraction (ToF 3DND), identifies the position, shape and crystallographic orientation of the individual grains within the sample.

The experiments were conducted at the single crystal diffractometer SENJU at the Japanese neutron source J-PARC. The choice of this instrument was motivated by its large coverage of the reciprocal space. The instrument had to undergo modifications to enable ToF 3DND measurements: a time resolved imaging detector, developed at University of Berkeley, California, was fitted in the interior of SENJU in order to record the extinction spots in the transmitted beam (i.e. areas with missing intensity due to Bragg diffraction of the individual grains).

The arrangement of the two detector systems facilitated two versions of ToF 3DND. The first version, restricted to imaging data, enables reconstruction of the position and shape of the individual grains without the use of crystallography. Afterwards, the orientations of the individual grains are indexed.

The algorithms for both steps have been developed in this study. The work also shows that based on the transmission data alone the orientations are not uniquely determined; however, it is possible to find an unique solution by including diffraction signals from the imaging detector. The second version uses an existing method for indexing SENJU data, which then serves as prior information to restrict the extinction spots that belong to the same grain.

The ToF 3DND methods are verified through the study of two different samples: an Iron rod and a shape memory alloy (SMA) CoNiGa bi-crystal. Part of this verification involves comparison with electron backscatter diffraction (EBSD). In the Iron rod, 107 grains were indexed from the SENJU data. As a comparison, 108 grains were reconstructed from the nearfield data alone.

This constitutes roughly 10 times as many grains as previously reported using a continuous neutron source. Thanks to the penetrating properties of the neutron, ToF 3DND complements existing X-ray techniques in those cases where X-rays cannot penetrate the sample. Besides, with ToF 3DND it is also much easier to detect light elements such as those in energy materials.

With a future availability of an energy dispersive x-ray detector with high energy resolution, the methods reported here can be directly transferred to synchrotron and white beam based studies.