The elastoplastic transition of a metastable β-Ti alloy, Timetal-18, is studied using in-situ high energy synchrotron X-ray diffraction microscopy (HEDM). The measured evolutions of the complete elastic strain (and stress) tensor(s), resolved shear stress, lattice rotation and rotation of the stress state of the grains are compared with the predictions of the elasto-viscoplastic Micromechanical Analysis of Stress-Strain Inhomogeneities with fast Fourier transform (MASSIF) code instantiated with an experimentally measured microstructure which matched that of the sample. The preferred glide plane of dislocations with ½<111> Burgers vectors of the BCC alloy was explored. It was found that the polycrystalline stress-strain response could be equally well described by any of the candidate glide planes or combinations thereof (i.e., pencil glide). However, simulations involving slip on 112 planes yielded a marginally better description of the individual grain-level responses, as compared to the simulation involving only the 110 planes. The small (typically <1$\,^\circ$) crystallographic reorientations that the grains undergo during the elastoplastic transition, are insufficient to permit discrimination between candidate slip modes. The resolved shear stress (RSS) distributions showed a sharp increase in skewness around macroscopic yield and it was found that the hardening during the elastoplastic transition is primarily due to intergranular interactions. Analysis of hard and soft grains suggests non-Schmid effects may be present, even in these low strain rate, room temperature experiments. Finally, examination of the individual responses revealed strain softening in some of the grains. Intragranular heterogeneity in the orientation and stress state are highlighted as important areas for future investigations, which may reveal answers to unresolved questions in this research.

The efficacy of an elasto-viscoplastic fast Fourier transform (EVPFFT) code was assessed based on blind predictions of micromechanical fields in a sample of Inconel 625 produced with additive manufacturing (AM) and experimentally characterized with high-energy X-ray diffraction microscopy during an in situ tensile test. The blind predictions were made in the context of Challenge 4 in the AFRL AM Modeling Challenge Series, which required predictions of grain-averaged elastic strain tensors for 28 unique target (Challenge) grains at six target stress states given a 3D microstructural image, initial elastic strains of Challenge grains, and macroscopic stress&mdash;strain response. Among all submissions, the EVPFFT-based submission presented in this work achieved the lowest total error in comparison with experimental results and received the award for Top Performer. A post-Challenge investigation by the authors revealed that predictions could be further improved, by over 25\% compared to the Challenge-submission model, through several model modifications that required no additional information beyond what was initially provided for the Challenge. These modifications included a material parameter optimization scheme to improve model bias and the incorporation of the initial strain field through both superposition and eigenstrain methods. For the first time with respect to EVPFFT modeling, an ellipsoidal-grain-shape Eshelby approximation was tested and shown to improve predictive capability compared to previously used spherical-grain-shape assumptions. Lessons learned for predicting full-field micromechanical response using the EVPFFT modeling method are discussed.

The simulation and modeling of part-level distortion and residual stress in diverse metal additive manufacturing (AM) geometries has great potential to enable the rapid adoption of this technology in engineering design. Moreover, the use of additive manufacturing component libraries (CLs) offer a computationally efficient means of quantifying these part-level defects resultant from AM processing. We report on how the individual simulations of simple shapes, potential entries in a CL, can be superimposed to provide an indication of distortion and residual stresses in complex geometries. Laser powder bed fusion AM was used to construct test geometries of varied shapes and their combinations in the form of CLs in an effort to characterize location-dependent and feature-dependent distortion distributions. Blue light scanning was used to experimentally measure 3D distortions in order to investigate the interaction between the component shapes and local boundary conditions. Overall, part-level distortions were highly dependent on test component geometry, local boundary conditions, and shape combination. Commercial finite element software was used to verify experimental trends and to make predictions of distortion. The use of CLs resulted in over 20 times savings in computational cost while reproducing overall trends in distortion for test geometry assemblies. Therefore, it is anticipated that the use of CLs for L-PBF AM geometries has demonstrated potential to facilitate efficient simulations of full component AM assemblies, thereby reducing the need for costly trial-and-error-type experimental analysis.

The influence of thermomechanical processing (TMP) of a Ti-6Al-4V alloy on the transformation texture and intervariant boundary network were investigated by conventional EBSD mapping along with the five-parameter boundary analysis approach. The texture characteristics of Ti-6Al-4V subjected to deformation in the beta regime followed by the beta-+alpha martensitic transformation were examined using visco-plastic self-consistent simulation and forward calculation of the transformation texture. Comparison of the simulated and experimental texture characterisitcs revealed that the transformed alpha texture was dominated by the variant selection associated with substructure development in the beta parent phase and the occurrence of specific self-accommodating alpha variants in the microstructure, promoting the quadrilateral and/or V shape variant arrangement. This resulted in a progressive increase in the 63.26 degrees/$[$10553$]$ intervariant boundaries with the strain increment, at the expense of 60 degrees/ $[$1 1 20$]$. Moreover, the grain boundary network for all conditions was dominated by the triple junctions (grain boundary network) terminating on 63.26 degrees/$[$10553$]$ and 60 degrees/$[$1 1 20$]$ intervariants. It is shown that the elastic interactions among the variants during the martensitic transformation is the dominant parameter affecting the grain boundary network, despite the presence of dislocation based variant selection.

A set of IF steel specimens, at several stages of recrystallization, were subjected to EBSD analysis to provide data for a study of primary recrystallization. First, a criterion was defined for detecting the recrystallized grains that combines multiple parameters. The new approach pays particular attention to ensuring that reasonable recrystallized fractions are obtained in both the early and late stages of recrystallization. Using this to partition orientations maps into recrystallized and non-recrystallized grains, the distribution and density of nuclei, their correlation with EBSD parameters and stored energy, and their texture were determined. The evolution of the grain size distribution, microstructure and texture is then discussed in terms of the balance of the two main mechanisms (growth of the recrystallized grains into the deformed matrix and grain growth competition between already recrystallized grains) which occur during recrystallization.

In this paper, an original approach is proposed to compare modeling and relevant statistical experiments using β-Ti21S Body Centered Cubic (BCC) metal as a challenging benchmark. Our procedure allows the evolution of microstructural defects to be tracked in situ with excellent spatial resolution, while observing a bulk sample region sufficiently large to be statistically representative of the material. We identify multiple mechanisms such as slip transfer, slip traces, pencil glide, etc. We demonstrate that for small plastic strains (<0.25 %) under uniaxial tensile loading, the Schmid law is satisfied statistically. Under these circumstances, changes in Critical Resolved Shear Stress (CRSS) are minimal and accommodation of incompatible deformation between grains has not yet become important. The majority of the observed slip plane traces at the mesoscale corresponds to the 123 family. Fully automated while precise, the reported approach compares this data with four crystal plasticity models, and provides a methodology for similar analyses in other materials.

The elastoplastic transition of a metastable β-Ti alloy, Timetal-18, is studied using in-situ high energy synchrotron X-ray diffraction microscopy (HEDM). The measured evolutions of the complete elastic strain (and stress) tensor(s), resolved shear stress, lattice rotation and rotation of the stress state of the grains are compared with the predictions of the elasto-viscoplastic Micromechanical Analysis of Stress-Strain Inhomogeneities with fast Fourier transform (MASSIF) code instantiated with an experimentally measured microstructure which matched that of the sample. The preferred glide plane of dislocations with ½<111> Burgers vectors of the BCC alloy was explored. It was found that the polycrystalline stress-strain response could be equally well described by any of the candidate glide planes or combinations thereof (i.e., pencil glide). However, simulations involving slip on 112 planes yielded a marginally better description of the individual grain-level responses, as compared to the simulation involving only the 110 planes. The small (typically <1$\,^\circ$) crystallographic reorientations that the grains undergo during the elastoplastic transition, are insufficient to permit discrimination between candidate slip modes. The resolved shear stress (RSS) distributions showed a sharp increase in skewness around macroscopic yield and it was found that the hardening during the elastoplastic transition is primarily due to intergranular interactions. Analysis of hard and soft grains suggests non-Schmid effects may be present, even in these low strain rate, room temperature experiments. Finally, examination of the individual responses revealed strain softening in some of the grains. Intragranular heterogeneity in the orientation and stress state are highlighted as important areas for future investigations, which may reveal answers to unresolved questions in this research.

The influence of thermomechanical processing (TMP) of a Ti-6Al-4V alloy on the transformation texture and intervariant boundary network were investigated by conventional EBSD mapping along with the five-parameter boundary analysis approach. The texture characteristics of Ti-6Al-4V subjected to deformation in the beta regime followed by the beta-+alpha martensitic transformation were examined using visco-plastic self-consistent simulation and forward calculation of the transformation texture. Comparison of the simulated and experimental texture characterisitcs revealed that the transformed alpha texture was dominated by the variant selection associated with substructure development in the beta parent phase and the occurrence of specific self-accommodating alpha variants in the microstructure, promoting the quadrilateral and/or V shape variant arrangement. This resulted in a progressive increase in the 63.26 degrees/[10553] intervariant boundaries with the strain increment, at the expense of 60 degrees/ [1 1 20]. Moreover, the grain boundary network for all conditions was dominated by the triple junctions (grain boundary network) terminating on 63.26 degrees/[10553] and 60 degrees/[1 1 20] intervariants. It is shown that the elastic interactions among the variants during the martensitic transformation is the dominant parameter affecting the grain boundary network, despite the presence of dislocation based variant selection.

Bragg coherent diffraction imaging has the potential to provide significant insight into the structure-properties relationship for crystalline materials by imaging, with nanoscale resolution, three-dimensional strain fields within individual grains and nanoparticles. The capability of present-day synchrotrons to locate and measure a multiplicity of Bragg reflections from a single grain makes it possible to recover the full strain tensor with nanometer resolution. Recent methods for coupling reconstructions from several peaks to determine the strain tensor have been developed and applied to synthetic data, but have not been applied to experimental data. Here, using a coupled genetic reconstruction algorithm, we reconstruct an experimental data set and demonstrate improvements in the ability to resolve vector-valued displacement fields internal to the particle as compared to what is achieved with a noncoupled approach. The coupled approach developed in this work was also validated on simulated data sets. In both simulated and experimental data, reconstructions from our coupled Bragg peak algorithm show improvements over the noncoupled independent reconstruction method of 5\% in terms of accuracy and 53\% in terms of consistency.

Solidification or hot cracks are commonly observed defects in a number of metal alloys and may lead to deterioration of additively manufactured parts quality. In this study, ultra-high-speed x-ray radiography experiments enable the observation and characterization of bundles of hot-cracks that form in monobloc AA6061 substrate. The crack bundles are related to meltpool characteristics and pore formation. Crack propagation rate is also presented for the case of a crack that initiates from a pore. Two types of relevant pore formation are also described, namely keyhole porosity and crack-remelting porosity. The results of this study are expected to facilitate the validation of theoretical and numerical models of solidification cracking.