Neutron diffraction and diffraction contrast imaging for mapping the TRIP effect under load path change

The transformation induced plasticity (TRIP) effect is investigated during a load path change using a cruciform sample. The transformation properties are followed by in-situ neutron diffraction derived from the central area of the cruciform sample. Additionally, the spatial distribution of the TRIP effect triggered by stress concentrations is visualized using neutron Bragg edge imaging including, e.g., weak positions of the cruciform geometry. The results demonstrate that neutron diffraction contrast imaging offers the possibility to capture the TRIP effect in objects with complex geometries under complex stress states.Fil: Polatidis, Efthymios. Paul Scherrer Institute; SuizaFil: Morgano, Manuel. Paul Scherrer Institute; SuizaFil: Malamud, Florencia. Comision Nacional de Energia Atomica. Gerencia D/area Invest y Aplicaciones No Nucleares. Departamento Haces de Neutrones del Ra10 - Cab.; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Bacak, Michael. Paul Scherrer Institute; SuizaFil: Panzner, Tobias. Paul Scherrer Institute; Suiza. Swissneutronics; SuizaFil: Van Swygenhoven, Helena. Paul Scherrer Institute; Suiza. École Polytechnique Fédérale de Lausanne; SuizaFil: Strobl, Markus. Paul Scherrer Institute; Suiz

Overcoming High Energy Backgrounds at Pulsed Spallation Sources

Instrument backgrounds at neutron scattering facilities directly affect the quality and the efficiency of the scientific measurements that users perform. Part of the background at pulsed spallation neutron sources is caused by, and time-correlated with, the emission of high energy particles when the proton beam strikes the spallation target. This prompt pulse ultimately produces a signal, which can be highly problematic for a subset of instruments and measurements due to the time-correlated properties, and different to that from reactor sources. Measurements of this background have been made at both SNS (ORNL, Oak Ridge, TN, USA) and SINQ (PSI, Villigen, Switzerland). The background levels were generally found to be low compared to natural background. However, very low intensities of high-energy particles have been found to be detrimental to instrument performance in some conditions. Given that instrument performance is typically characterised by S/N, improvements in backgrounds can both improve instrument performance whilst at the same time delivering significant cost savings. A systematic holistic approach is suggested in this contribution to increase the effectiveness of this. Instrument performance should subsequently benefit.Comment: 12 pages, 8 figures. Proceedings of ICANS XXI (International Collaboration on Advanced Neutron Sources), Mito, Japan. 201

The phylogeography and incidence of multi-drug resistant typhoid fever in sub-Saharan Africa.

There is paucity of data regarding the geographical distribution, incidence, and phylogenetics of multi-drug resistant (MDR) Salmonella Typhi in sub-Saharan Africa. Here we present a phylogenetic reconstruction of whole genome sequenced 249 contemporaneous S. Typhi isolated between 2008-2015 in 11 sub-Saharan African countries, in context of the 2,057 global S. Typhi genomic framework. Despite the broad genetic diversity, the majority of organisms (225/249; 90%) belong to only three genotypes, 4.3.1 (H58) (99/249; 40%), 3.1.1 (97/249; 39%), and 2.3.2 (29/249; 12%). Genotypes 4.3.1 and 3.1.1 are confined within East and West Africa, respectively. MDR phenotype is found in over 50% of organisms restricted within these dominant genotypes. High incidences of MDR S. Typhi are calculated in locations with a high burden of typhoid, specifically in children aged <15 years. Antimicrobial stewardship, MDR surveillance, and the introduction of typhoid conjugate vaccines will be critical for the control of MDR typhoid in Africa

Microstructure evolution of stainless steel subjected to biaxial load path changes: In-situ neutron diffraction and multi-scale modeling

The lattice strain and intensity evolution obtained from in-situ neutron diffraction experiments of 316L cruciform samples subjected to 45 degrees and 90 degrees load path changes are presented and predicted using the multi-scale modeling approach proposed in Upadhyay et al., IJP 108 (2018) 144-168. At the macroscale, the multi-scale approach uses the implementation of the viscoplastic self-consistent polycrystalline model as a user-material into ABAQUS finite element framework to predict the non-linearly coupled gauge stresses of the cruciform geometry. The predicted gauge stresses are then used to drive the elasto-viscoplastic fast Fourier transform polycrystalline model to predict the lattice strain and intensity evolutions. Both models use the same dislocation density based hardening law suitable for load path changes. The predicted lattice strain and intensity evolutions match well with the experimental measurements for all reflections studied. The simulation results are analyzed in detail to understand the role of elastic anisotropy, plastic slip, grain neighborhood interactions and cruciform geometry on the microstructural evolution during biaxial load path changes

A crystallographic extension to the Olson-Cohen model for predicting strain path dependence of martensitic transformation

A modification to the empirical Olson-Cohen strain-induced austenite to martensite transformation kinetic model is proposed. The proposed kinetic model accounts for the stress state at the grain level and the crystallography of the transformation mechanism. Two transformation mechanisms sensitive to the local stress state are incorporated in the model. First, the resolved shear stress on a slip plane in the direction perpendicular to the Burgers vector determines the stacking fault width (SFW) which in turn determines the potential nucleation sites. Second, the stress triaxiality governs the probability of the structural alpha'-martensite formation at a nucleation site. The kinetic model is implemented in the elastoplastic self-consistent (EPSC) crystal plasticity model to study the stress state and texture dependence of the strain-induced alpha'-martensite transformation and the mechanical response of metastable austenitic steels. The simulations are compared with experimental mechanical and phase fraction data from different austenitic steels subjected to simple tension, plane strain tension, equibiaxial tension, simple compression, and torsion. It is demonstrated that the appropriate modeling of alpha'-martensite phase fractions allows capturing the experimentally measured mechanical response. The implementation and insights from these predictions, including the role of texture evolution on martensite transformation, are discussed in this paper. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved