Sandra Cabeza

Monochromator and analyzer systems that rely on bent single crystals are in use throughout the neutron scattering community. An adequate component for the simulation of such crystals was missing in the widely used neutron simulation software package McStas. The newly developed component Monochromator_bent, which fills this gap, is introduced. It can serve as a model for crystal monochromators and analyzers of various kinds, including the bent perfect crystals, mosaic crystals, and crystals combining mosaicity with bending. The performance of the component is tested at several configurations and compared with the results of another simulation program, SIMRES. Validation is carried out using analytical calculations and the McStas NCrystal_sample component for the case of unbent crystals. Excellent agreement in all tests and good performance in terms of computing speed has been found. The component has been included in the present distribution of McStas 3.5.

Residual stresses have been characterized in four Wire Arc Additive Manufacturing specimens with neutron diffraction technique. Firstly, two methods are investigated for obtaining the reference diffracted angle θ₀ that is required for the computation of micro-strains and, thus, the stresses. θ₀ was obtained using two approaches. The first one required a strain-free specimen in order to get directly the reference diffracted angles θ₀ in three directions. The second one is based on the plane stress assumption to get θ₀ indirectly by imposing that the normal stress was equal to zero. Both methods led to similar residual stress profiles for the 1-layer specimen what validated this approach for all specimens that did not have a strain-free specimen available. The second part of this work focused on the effect of addition of a new layer on residual stresses. The measurements showed that the longitudinal stress was tensile in the Heat Affected Zone (HAZ) and Fusion Zone (FZ) with a maximum value located at the parent material - layers interface where the thermal loadings were applied. A decrease of this maximum value from 257 MPa to 199 MPa appeared after deposition of a new layer which is due to some stress relaxation effect. Inside the parent material, a large zone presents compressive longitudinal stress up to -170 MPa. The bottom part of the parent material is under tensile stress likely due to its upward bending following the thermal contraction of the deposited layers during cooling to ambient temperature.

For the duplex stainless steel X2CrNiMoN22-5-3, phase-specific strain pole figures (strain PFs) for the phases ferrite (bcc) and austenite (fcc) were analysed under uniaxial tensile loading for various loading states in purely elastic and elasto-plastic regimes. Experimentally, strain PFs were determined by means of in situ neutron diffraction strain measurements under defined uniaxial loading. These experimental results were compared with strain PFs calculated using elasto-plastic self-consistent (EPSC) modelling. The comparison was performed for two different {hkl} planes per phase. While classic load stress and load partitioning analyses for multi-phase materials are often limited to the load direction and a selected direction transverse to it, the results illustrate the added value of determining a strain PF, especially when a phase-specific texture is present. The comparison with experimental data shows how well the load partitioning behaviour can be predicted using common EPSC models, using the example of a duplex stainless steel. The EPSC model used was validated with the software ISODEC in its elastic range. Based on the results of the EPSC model, and taking into account the local phase-specific crystallographic texture, a prediction can be made as to what extent intergranular stresses and phase-specific textures could affect the results of a (residual) stress analysis by means of the diffraction method. This makes it possible to assess whether, for technical applications, meaningful residual stress results can be expected in certain component directions.

In the present work, neutronographic in situ diffraction stress analyses during uniaxial loading and subsequent unloading were carried out on the two duplex stainless steels X2CrNiMoN22-5-3 and X3CrNiMoN27-5-2 with nominal phase fractions for ferrite:austenite of 50:50% and 70:30%, respectively. In addition to the different phase fractions, the two steels also differed in their phase-specific crystallographic texture. The load-partitioning behaviour and the phase-specific micro (residual) stress evolution for total strains up to about 9% were investigated. The results indicated that for both materials under load, the phase-specific stress in the ferrite phase was significantly higher than in the austenite phase, while no texture development through the plastic deformation could be observed.