Energy selective neutron imaging for the characterization of polycrystalline materials

This multipart dissertation focuses on the development and evaluation of advanced methods for material testing and characterization using neutron diffraction and imaging techniques. A major focus is on exploiting diffraction contrast in energy selective neutron imaging (often referred to as Bragg edge imaging) for strain and phase mapping of crystalline materials. The dissertation also evaluates the use of neutron diffraction to study the effect of multi-axial loading, in particular the role of applying directly shear strains from the application of torsion. A portable tension-torsion-tomography loading system has been developed for in-situ measurements and integrated at major user facilities around the world. Promising applications for the Bragg edge technique are implemented at the neutron imaging facility CONRAD at the reactor source BER-II as well as at neutron time of flight instruments. Strain mapping is successfully demonstrated for all cases to yield quantifiable results, but is limited in practicality due to limitations in choice of the scattering vector (direction of probed strain tensor component) and the gauge volume selection. The use of Bragg edge imaging for crystalline phase mapping was explored and appears to be a very promising technique. The extension to three-dimensionally resolved tomography is presented for samples exhibiting the TRansfomation Induced Plasticity (TRIP) effect, while challenges with characterizing textured samples are discussed. Individual crystallites within a polycrystalline material exhibit elastic anisotropy which is significant as that can lead to stress concentrations and inhomogeneities during plastic deformation. Characterization of elastic anisotropy is important to understand the effects of texture on the macroscopic mechanical properties. Diffraction methods can do this, by probing the response of individual lattice planes to externally applied mechanical stress. Past experimental data using diffraction based methods have largely been limited to uni‑axial tensile and/or compressive loading conditions, even though shear dominates most common failure mechanisms for structural materials. Within this dissertation, experimental techniques have been established for the measurement of lattice strains under applied torsion (pure shear) and lattice specific shear moduli are reported. This is accomplished using a (traditional) neutron diffractometer instrument, in conjunction with special alignment procedures and the specifically designed axial-torsional loading system

Wetting Behavior of Polymer Melts with Refractory Coatings at High Temperature

Within the scope of this thesis, an experimental system has been designed, developed and manufactured for the determination of the wetting behavior of liquids and polymer melts with solid surfaces (coated and uncoated) at high temperatures (\u3e 200 ºC). The measurement system incorporates a modified Wilhelmy plate technique, using a precision weighing module, a vertical linear stage, custom developed application software using LabView with suitable hardware and a high temperature furnace with thermocouple feedback control. Experiments have been performed and are reported to evaluate the performance of the testing system, using liquids of known wetting properties. A suitable testing procedure based on dynamic Wilhelmy plate theory is proposed, involving investigation of advancing and receding liquid-probe interactive forces and hysteresis loops. Interfacial wetting and wicking behavior of polystyrene melt with clay based refractory coatings, as used in the lost-foam casting (LFC) process, are presented a function of temperature using this measurement system. Experiments of particular interest were performed for two different types of refractory coating and for polymer melts at processing temperatures between 220°C and 300°C, where they show pronounced viscoelastic behavior. Different variables, obtained from the hysteresis loops, were utilized as quantitative indicators for comparison, including the area under the loop from contact onwards, the slope of advancing and receding lines in the force-displacement domain, the force hysteresis at zero displacement and Fast Fourier Transform (FFT) analysis of the hysteresis loop

A multiscale study of hot-extruded CoNiGa ferromagnetic shape-memory alloys

Ferromagnetic shape-memory CoNiGa alloys have attracted much scientific interest due to their potential alternative use as high-temperature shape-memory alloys, bearing a high prospect for actuation and damping applications at elevated temperatures. Yet, polycrystalline CoNiGa, due to strong orientation dependence of transformation strains, suffers from intergranular fracture. Here, two multi-grain CoNiGa samples were prepared by a novel hot extrusion process that can promote favourable grain-boundary orientation distribution and improve the material's mechanical behaviour. The samples were investigated by multiple methods and their microstructural, magnetic, and mechanical properties are reported. It is found that a post-extrusion solutionising heat treatment leads to the formation of a two-phase oligocrystalline homogeneous microstructure consisting of an austenitic parent B2 phase and γ-CoNiGa precipitates. Reconstruction of the full 3D grain morphology revealed large, nearly spherical grains with no low-angle grain boundaries throughout the entire sample volume. The presence of γ precipitation affects the transformation behaviour of the samples, by lowering the martensitic transformation temperature, while, in conjunction with the oligocrystalline microstructure, it improves the ductility. Controlling the composition of the B2 matrix, as well as the phase fraction of the γ phase, is thus crucial for the optimal behaviour of the alloys

Small Angle Scattering in Neutron Imaging—A Review

Conventional neutron imaging utilizes the beam attenuation caused by scattering and absorption through the materials constituting an object in order to investigate its macroscopic inner structure. Small angle scattering has basically no impact on such images under the geometrical conditions applied. Nevertheless, in recent years different experimental methods have been developed in neutron imaging, which enable to not only generate contrast based on neutrons scattered to very small angles, but to map and quantify small angle scattering with the spatial resolution of neutron imaging. This enables neutron imaging to access length scales which are not directly resolved in real space and to investigate bulk structures and processes spanning multiple length scales from centimeters to tens of nanometers

Higher order correction and spectral deconvolution of wavelength-resolved neutron transmission imaging at the CONRAD-2 instrument

This paper uses a Fourier self-deconvolution method for improving the wavelength resolution in transmission experiments at continuous neutron sources utilizing a double-crystal monochromator device to probe as well as correct the generation of higher-order neutron scattering in a monochromatic neutron beam. The cold neutron radiography CONRAD-2 equipment has been utilized to resolve the steel transmission spectra of changing BCC phase and FCC phase fractions. Therefore, both low and high-spectral resolution instruments with equivalent wavelength resolution have been proposed. The primary benefit of Fourier self-deconvolution is its ability to precisely narrow individual bands without modifying their relative position or the total band area. Thus, the resolution of the transmission spectrum has been improved by a factor of 3.16, and the info that the sample material comprises two crystallographic phases has been determined by the wavelength resolution improvement employing the deconvolution approach. Additionally, the slight variation in Bragg edge position for different phase fractions and the locations of the double phase Bragg edges have also been obtained using the ray-tracing simulation tool McStas. The high resolution neutron wavelength selection experiment with the ESS test_beamline (V20) instrument employing the neutron time-of-flight detection demonstrates the precision of the resolving steel Bragg edge

Alpha texture variations in additive manufactured Ti-6Al-4V investigated with neutron diffraction

Variation of texture in Ti-6Al-4V samples produced by three different additive manufacturing (AM) processes has been studied by neutron time-of-flight (TOF) diffraction. The investigated AM processes were electron beam melting (EBM), selective laser melting (SLM) and laser metal wire deposition (LMwD). Additionally, for the LMwD material separate measurements were done on samples from the top and bottom pieces in order to detect potential texture variations between areas close to and distant from the supporting substrate in the manufacturing process. Electron backscattered diffraction (EBSD) was also performed on material parallel and perpendicular to the build direction to characterize the microstructure. Understanding the context of texture for AM processes is of significant relevance as texture can be linked to anisotropic mechanical behavior. It was found that LMwD had the strongest texture while the two powder bed fusion (PBF) processes EBM and SLM displayed comparatively weaker texture. The texture of EBM and SLM was of the same order of magnitude. These results correlate well with previous microstructural studies. Additionally, texture variations were found in the LMwD sample, where the part closest to the substrate featured stronger texture than the corresponding top part. The crystal direction of the α phase with the strongest texture component was [112¯3]

The scale of a martian hydrothermal system explored using combined neutron and x-ray tomography.

Nakhlite meteorites are igneous rocks from Mars that were aqueously altered ~630 million years ago. Hydrothermal systems on Earth are known to provide microhabitats; knowledge of the extent and duration of these systems is crucial to establish whether they could sustain life elsewhere in the Solar System. Here, we explore the three-dimensional distribution of hydrous phases within the Miller Range 03346 nakhlite meteorite using nondestructive neutron and x-ray tomography to determine whether alteration is interconnected and pervasive. The results reveal discrete clusters of hydrous phases within and surrounding olivine grains, with limited interconnectivity between clusters. This implies that the fluid was localized and originated from the melting of local subsurface ice following an impact event. Consequently, the duration of the hydrous alteration was likely short, meaning that the martian crust sampled by the nakhlites could not have provided habitable environments that could harbor any life on Mars during the Amazonian