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1. New Methodologies for Grain Boundary Detection in EBSD Data of Microstructures

   https://doi.org/10.2514/6.2022-1424  

   Catania, Richard K.; Senthilnathan, Arulmurugan; Sions, John; Snyder, Kyle; Al-Ghaib, Huda; Zimmerman, Ben; Acar, Pinar (December 2021, New Methodologies for Grain Boundary Detection in EBSD Data of Microstructures)

   This work discusses new methodologies for identifying the grain boundaries in color images of metallic microstructures and the quantification of their grain topology. Grain boundaries have a large impact on the macro-scale material properties. Particularly, this work employs the experimental microstructure data of Titanium-Aluminum alloys, which can be used for various aerospace components owing to their outstanding mechanical performance in elevated temperatures. The grain topology of these metallic microstructures is quantified using the concept of shape moment invariants. In order to capture the grains using the shape moment invariants, it is necessary to identify the grain boundaries and separate them from their respective grains. We present two methodologies to detect the grain boundaries. The first method is the tolerance-based neighbor analysis. The second method focuses on creating three-dimensional space of pixel intensity values based on the three color channels and measuring the Euclidean distance to separate different grains. Additionally, since the grain boundaries may not possess the same material properties as the grain itself, this work investigates the effect of including the grain boundaries when determining the homogenized material properties of the given microstructure. To generate adequate statistical information, microstructures are reconstructed from the experimental data using the Markov Random Field (MRF) method. Upon separating the grains, we use the shape moment invariants to quantify the shapes of different grains. Using the shape moment invariants and the experimental material property values, three neural network functions are developed to investigate the effects of grain boundaries on material property predictions. 

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2. Crustal Structure of Southern California from Velocity and Attenuation Tomography

   https://doi.org/10.4401/ag-9149  

   Jordan, Thomas H (January 2024, Annals of Geophysics)

   Recent studies of Southern California have employed velocity and attenuation tomography to elucidate two aspects of crustal structure. Low-frequency velocity tomography reveals large-scale heterogeneity that can be approximated by a discrete set (𝐾 ≤ 10) of tectonic regions, each characterized by an isostatically balanced lithospheric column reflecting the composition and tectonic history of the region. The boundaries between tectonic regions are typically localized structures expressed at the surface by major faults, topographic fronts, and geochemical transitions. The efficacy with which tomographic regionalization (TR) represents the subsurface geography of major tectonic units in Southern California indicates that the TR idealization works surprisingly well in this part of the Pacific‐North America plate boundary. High‐frequency attenuation tomography in Southern California supports the hypothesis that the decay of P and S waves propagating through the upper and middle crust is dominated by elastic scattering from small-scale heterogeneities of high fractal dimension, rather than by anelastic dissipation. The amplitude of the scattering varies systematically among the tectonic regions and correlates with the degree of recent faulting and deformation within a region. These results motivate speculation that small-scale crustal heterogeneities responsible for high-frequency attenuation are generated within the seismogenic zone through a dynamic interplay between distributed deformation and localized fracturing. 

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3. Spurious Rayleigh-wave apparent anisotropy near major structural boundaries: a numerical and theoretical investigation

   https://doi.org/10.1093/gji/ggae305  

   Zeng, Qicheng; Lin, Fan-Chi; Tsai, Victor C (September 2024, Geophysical Journal International)

   SUMMARY The recent developments in array-based surface-wave tomography have made it possible to directly measure apparent phase velocities through wave front tracking. While directionally dependent measurements have been used to infer intrinsic $$2\psi $$ azimuthal anisotropy (with a 180° periodicity), a few studies have also demonstrated strong but spurious $$1\psi $$ azimuthal anisotropy (360° periodicity) near major structure boundaries particularly for long period surface waves. In such observations, Rayleigh waves propagating in the direction perpendicular to the boundary from the slow to the fast side persistently show a higher apparent velocity compared to waves propagating in the opposite direction. In this study, we conduct numerical and theoretical investigations to explore the effect of scattering on the apparent Rayleigh-wave phase velocity measurement. Using 2-D spectral-element numerical wavefield simulations, we first reproduce the observation that waves propagating in opposite directions show different apparent phase velocities when passing through a major velocity contrast. Based on mode coupling theory and the locked mode approximation, we then investigate the effect of the scattered fundamental-mode Rayleigh wave and body waves interfering with the incident Rayleigh wave separately. We show that scattered fundamental-mode Rayleigh waves, while dominating the scattered wavefield, mostly cause short wavelength apparent phase velocity variations that could only be studied if the station spacing is less than about one tenth of the surface wave wavelength. Scattered body waves, on the other hand, cause longer wavelength velocity variations that correspond to the existing real data observations. Because of the sensitivity of the $$1\psi $$ apparent anisotropy to velocity contrasts, incorporating such measurements in surface wave tomography could improve the resolution and sharpen the structural boundaries of the inverted model. 

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4. Impact of the plastic deformation microstructure in metals on the kinetics of recrystallization: A phase-field study

   https://doi.org/doi.org/10.1016/j.actamat.2022.118332  

   Hamed, Ahmed; Rayaprolu, Sreekar; Winther, Grethe; El-Azab, Anter (January 2022, Acta materialia)

   The sensitivity of recrystallization kinetics in metals to the heterogeneity of microstructure and deformation history is a widely accepted experimental fact. However, most of the available recrystallization models employ either a mean field approach or use grain-averaged parameters, and thus neglecting the mesoscopic heterogeneity induced by prior deformation. In the present study, we investigate the impact of deformation-induced dislocation (subgrain) structure on the kinetics of recrystallization in metals using the phase-field approach. The primary focus here is upon the role of dislocation cell boundaries. The free energy formulation of the phase-field model accounts for the heterogeneity of the microstructure by assigning localized energy to the resulting dislocation microstructure realizations generated from experimental data. These microstructure realizations are created using the universal scaling laws for the spacing and the misorientation angles of both the geometrically necessary and incidental dislocation boundaries. The resulting free energy is used into an Allen-Cahn based model of recrystallization kinetics, which are solved using the finite element method. The solutions thus obtained shed light on the critical role of the spatial heterogeneity of deformation in the non-smooth growth of recrystallization nuclei and on the final grain structure. The results showed that, in agreement with experiment, the morphology of recrystallization front exhibits protrusions and retrusions. By resolving the subgrain structure, the presented algorithm paves the way for developing predictive kinetic models that fully account for the deformed state of recrystallizing metals. 

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5. Physics-assisted data-driven stochastic reduced-order models for attribution of heterogeneous stress distributions in low-grain polycrystals

   https://doi.org/10.1098/rspa.2024.0898  

   Zhang, Yinling; Dunham, Samuel D; Bronkhorst, Curt A; Chen, Nan (January 2025, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   Understanding stress distributions at grain boundaries in polycrystalline materials is crucial for predicting damaged nucleation sites. In high-purity materials, voids often nucleate at grain boundaries due to high stress from granular interactions and weakened atomic ordering. While traditional crystal plasticity models simulate grain-level mechanics, their high computational cost often prevents systematic identification of critical microstructural features and efficient forecast of extreme damage events. This paper addresses these challenges by developing a computationally efficient physics-assisted statistical modelling framework. The method starts by leveraging physical knowledge to hypothesize a broad set of microstructural factors influencing stress conditions. Causal inference is then applied to reveal the predominant features with physical explanations, leading to a parsimonious statistical model. A conditional Gaussian mixture model (CGMM) is employed when the identified relationship is utilized as a predictive model to quantify the uncertainty not readily explained by these features. Using body-centred cubic (BCC) tantalum as a representative material, a series of synthetic microstructures from single- to octu-crystal configurations are created. Results show that high-stress states strongly correlate with the elastic and plastic deformation capabilities and the directional misalignment of grain responses near boundaries. The statistical model achieves rapid and accurate forecasts, demonstrating its potential for analysing realistic polycrystalline materials. 

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