Tran, Khanh Van: Neutron bragg edge imaging of phase transformations in steel. , Dissertation Technische Universität Berlin, 2023
10.14279/depositonce-16993
Open Access Version
Abstract:
Imaging techniques using neutron beams are developing very fast and nowadays are applied as non-destructive analyzing tools in many research fields. The neutrons differ strongly from electrons, protons or X-rays in terms of their interaction with matter: they have good penetration in most common metallic materials while they have a high sensitivity to light elements such as hydrogen, hydrogenous substances or lithium. This makes neutrons ideally suited probes for research fields related to energy and engineering applications, e.g., batteries, hydrogen storage, embrittlement of steels and phase transformations in metals. Moreover, the wave properties of neutrons can be exploited to perform diffraction, phase-contrast and dark-field imaging experiments. This dissertation focuses on exploiting neutron wavelength-selective imaging for strain and phase mapping of crystalline materials using diffraction (Bragg-edge) contrast. Neutron wavelength-selective is based on diffraction contrast and is used for detecting the morphological changes of battery electrodes, water distribution in fuel cells, texture, strain and phase distribution in crystalline materials and many other applications such as magnetic phase transitions, cultural heritage object, geo materials and so on. In this study, complex martensitic phase distribution owing to the transformation induced plasticity effect was reported for the first time for a 304L steel with a rectangular cross-section under torsional loading using neutron Bragg-edge imaging technique. The extension to 4D spectral wavelength-resolved tomography was also reported for plastically deformed metastable stainless steel. This technique was then applied to quantify crystalline phase fractions. Furthermore, characterizations of preferred crystal orientations (texture) were also evaluated based on neutron wavelength-selective imaging technique. The Bragg-edge transmission profiles showed the different preferred orientations for two samples with the same specifications but originated from two different manufacturers. This characterization technique is very useful for controling manufacturing processes and better understanding of fracture behaviors of materials. The main experiments presented in the dissertation were performed at the neutron imaging facility CONRAD-2 at the research reactor BER-II (steady-state source), Germany and at the engineering diffractometer ENGIN-X at the spallation source ISIS (pulsed source) at the Rutherford Appleton Laboratory, UK.