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Micro-channel plate (MCP) detectors, when used at pulsed-neutron-source instruments, offer the possibility of high spatial resolution and high contrast imaging with pixel-level spectroscopic information. Here we demonstrate the possibility of multimodal analysis including total neutron cross-section spectra measurements, quantitative material differentiation imaging and texture-sensitive in-line phase imaging, from a single exposure using an MCP detector. This multimodal approach operates in full-field imaging mode, with the neutron transmission spectra acquired at each individual detector pixel. Due to the polychromatic nature of the beam and spectroscopic resolving capability of the detector, no energy scanning is required. Good agreement with the library reference data is demonstrated for neutron cross-section spectra measurements. Two different images corresponding to two selected energy bandwidths are used for elemental differentiation imaging. Moreover, the presence of changes in texture, i.e., preferred grain orientation, in the sample is identified from our phase-retrieval imaging results., Comment: Revised manuscript resubmitted to Physical Review A

The present report describes a big data numerical study of crystal plasticity finite element (CPFE) modelling using static and grain-based meshing to investigate the dependence of the results on the discretization approach. Static mesh refers to the integration point-based representation of the microstructure in which the integration points (IPs) within a finite element may belong to different grains, while in the grain-based meshing the minimum discretization unit is an element that may only belong to one grain. The crystal plasticity constitutive law was coded using UMAT subroutine within commercial finite element software Abaqus. Multiple sets of RVEs were investigated under strain-controlled loading and periodic boundary conditions. The stress and strain contour maps obtained from RVEs with static mesh and grain-based mesh were compared. The simulation results reveal that both discretization methods provide reliable predictions of the stress-strain curves and the stress/strain localization points in polycrystalline alloys. Static mesh tends to smooth the stress/strain profile at the grain boundary, whilst stress/strain discontinuities are present in the grain-based mesh results. The above findings remain valid when the number of grains within an RVE increases from 34 to 1250. To quantify the difference between static and grain-based meshing, a relative measure of deviation is defined. The deviations of global stress were found to be relatively small, within 0.5%, while local deviations were significant up to 50%. Static mesh has the advantage of reducing both the preprocessing procedures and computational time compared to grain-based mesh. It is concluded that static mesh is preferred when investigating the material's macroscopic behaviour, whilst grain-based mesh is recommended for the study of the local response using CPFEM., Comment: 28pages, 17 figures

Understanding the creep mechanisms and deformation response at different stresses and temperatures is crucial for design using nickel-base superalloys for high-temperature applications. In this study, the creep behaviour of a newly designed superalloy (nominated Alloy 11) at different stress and temperature was systematically investigated using SEM, STEM, EBSD and synchrotron X-ray diffraction. Material properties and mechanical response were analysed by considering such properties as lattice parameters and misfit, phase volume fraction, microstrain in reference and crept samples. Detwinning and dynamic recrystallization were observed for the crept samples. STEM characterization of deformed materials reveals that multiple deformation mechanisms and defects could be identified in crept samples, namely, dislocations in the gamma matrix channels and in gamma prime precipitates, along with continuous and isolated stacking faults. The highest values of lattice misfit, microstrain and lattice parameter change were observed at the centre of dogbone-shaped crept samples. This was associated with the hump shaped temperature distribution profile during creep testing. The significant spatial variation detected in terms of lattice parameters, misfit and microstrain supports the conclusion that it is important to perform spatially resolved measurements instead of considering each sample as a single measurement point when investigating creep response. It is also worth noting that the statistics of the lattice-parameter distribution for the gamma and gamma prime phases obey a gaussian distribution. In conclusion, a discussion is given for the meaning and implications of these findings for future research., Comment: 40 pages, 19 figures

Gibbs-Thompson effect is the general term referring to the influence of interfaces on the course of phase transformations such as precipitation or solidification. Whilst attention is most often focused on the Gibbs-Thomson effect on nucleation, growth and coarsening, the present study considers the reverse process of precipitate dissolution in a nickel-base superalloy during in situ TEM observation. The presence of several distinct populations of gamma-prime precipitates (primary, secondary, tertiary and grain boundary) allows the differences due to particle size to be quantified and interpreted. Important implications arise for the selection of heat treatment schedules for nickel-base superalloys and other alloy systems., Comment: 9 pages, 4 Figures

The stress-strain curves of most metallic alloys are often described using the relatively simple Ramberg-Osgood relationship. Whilst this description captures the overall stress-strain curve under monotonic tensile loading with reasonable overall accuracy, it often presents significant errors in the immediate post-yield region where the interplay between the elastic and plastic strains is particularly significant. This study proposes and develops a new approach to the description of the tensile stress-strain curve based on the Yield Volume Fraction (YVF) function. The YVF description provides an excellent match to experimental stress-strain curves based on a physically meaningful parameter that corresponds to the cumulative volume fraction of the polycrystal that undergoes yielding during monotonic deformation. The statistical nature of the polycrystal yield phenomenon is highlighted by the fact that the YVF model achieves good agreement with observations when the lognormal and extreme value distributions are employed to express the cumulative density function for the total yield volume fraction, and the probability density function for the incremental yield volume fraction, respectively. This proposed approach is compared with crystal plasticity finite element (CPFE) simulations and the Ramberg-Osgood model, along with experimental observations. The results highlight the potential of more extensive use of statistical methods in the description of material deformation response for improved design., Comment: 15 pages, 6 figures

Point by point strain scanning is often used to map the residual stress (strain) in engineering materials and components. However, the gauge volume and hence spatial resolution is limited by the beam defining apertures and can be anisotropic for very low and high diffraction (scattering) angles. Alternatively, wavelength resolved neutron transmission imaging has a potential to retrieve information tomographically about residual strain induced within materials through measurement in transmission of Bragg edges - crystallographic fingerprints whose locations and shapes depend on microstructure and strain distribution. In such a case the spatial resolution is determined by the geometrical blurring of the measurement setup and the detector point spread function. Mathematically, reconstruction of strain tensor field is described by the longitudinal ray transform; this transform has a non-trivial null-space, making direct inversion impossible. A combination of the longitudinal ray transform with physical constraints was used to reconstruct strain tensor fields in convex objects. To relax physical constraints and generalise reconstruction, a recently introduced concept of histogram tomography can be employed. Histogram tomography relies on our ability to resolve the distribution of strain in the beam direction, as we discuss in the paper. More specifically, Bragg edge strain tomography requires extraction of the second moment (variance about zero) of the strain distribution which has not yet been demonstrated in practice. In this paper we verify experimentally that the second moment can be reliably measured for a previously well characterised aluminium ring and plug sample. We compare experimental measurements against numerical calculation and further support our conclusions by rigorous uncertainty quantification of the estimated mean and variance of the strain distribution.

This study concerns the phase transformation kinetics and pertains to the processes of crystallisation of amorphous polymers, ceramics, and bulk metallic glasses (BMG), as well as solid state transformations by the nucleation and growth mechanism. It is also likely to be relevant to the description of dissolution and precipitation processes, and more generally chemical reaction kinetics, as well as biochemical processes such as ligand binding, haemoglobin oxygenation, etc. The mathematical framework is examined of the of the 'knee' function, or K-function, and its implications are explored., Comment: 6 pages, 5 figures