Texture analysis implementation at the neutron strain diffractometer POLDI

This study demonstrates the capabilities of the proposed data analysis strategy to perform quantitative crystallographic texture analysis at POLDI. Moreover, the CAD-modeler nature of POLTex allows to perform virtual experiments enabling to improve the design of each experiment by evaluating the feasibility and estimating the counting time for each sample size, geometry, and orientation. These virtual experiments are benefic to optimize the allocated beam time. The present methodology will benefit from the POLDI upgrade through the addition of a second detector bank, among others. This will allow a better coverage of the pole figure for a given measurement time or vice versa.

Neutron diffraction is a powerful tool for determining the crystallographic orientation distribution of polycrystalline materials. Due to the deep penetration of thermal neutrons in most materials, neutron diffraction is considered the most effective technique for volume texture analysis, offering superior texture data quality compared to other methods. Their high penetration allows for bulk texture measurements with minimal sample preparation, a crucial benefit for studying irradiated specimens after extended service periods or characterizing manufacturing routes in museum-quality cultural heritage objects. Moreover, by employing different types of sample environments, neutron diffractometers enabled the monitoring of texture changes during thermomechanical treatments. Characterization of crystallographic texture using neutron diffractometers can be facilitated with the use of polychromatic neutrons beam among others. These approaches allow to obtain the diffraction spectra including multiple Bragg peaks in a single measurement, thus reducing the need for extensive sample rotations, in addition to a precise control of the volume investigated within the sample.

These capabilities are exploited at POLDI, the time-of-flight pulse overlap diffractometer at SINQ, Paul Scherrer Institute. A novel two pronged numerical and experimental analysis routine is implemented with the POLTex toolbox, specifically designed to perform spatially resolved texture analysis in bulk specimens at POLDI. The proposed methodology is based on splitting the solid angle covered by the POLDI detector (which covers an angular range of 30°) into several smaller units of higher angular resolution and reconstructing the orientation distribution function from several incomplete experimental pole figures. The POLTex toolbox allows, among others, to calculate the neutron flight paths of each unit as a function of sample size, geometry, and orientation of simple or complex specimens by leveraging on the SALOME platform (open-source CAD modeler), which allows inserting CAD objects into a virtual detailed PODI geometry. Finally, from the neutron flight paths, which allows to correct for the neutron attenuation, and the diffraction data of each unit for each Euler angles investigated, the experimental pole figures are reconstructed, and the orientation distribution functions are calculated using MTEX. This novel methodology available at POLDI is demonstrated through the characterization of two specimens and validated with independent characterization techniques with success. The specimens investigated are a stainless steel additively manufactured sample and a hot-rolled and annealed Zircaloy-4 plate.