Electron Backscatter Diffraction Data Research Articles

A Lie-algebra-based description of disclination densities and the quantification of partial disclinations in deformed polycrystalline metals

Crystalline defects (e.g., dislocations, disclinations and grain boundaries) play a critical role in determining the mechanical and functional properties of metallic materials. From a physical point of view, dislocations and disclinations are the two fundamental types of topological defects in crystals, which originate from the breaking of translational symmetry and rotational symmetry, respectively. In spite of extensive studies on dislocations in the literature, the effects of disclinations on mechanical properties of metals have not been well recognized, partially due to the lack of an appropriate description for the rotational nature of disclinations. Here we suggest a Lie-algebra-based method to quantify the rotational properties of disclinations, which leads to a convenient way to determine disclination density distribution directly from Electron Backscatter Diffraction data. Through quasi-in-situ Electron Backscatter Diffraction characterizations, three major formation mechanisms of disclinations have been identified in deformed polycrystalline Mg alloys, i.e., dislocation-grain boundary interaction, dislocation redistribution and twin-twin reaction, all of which can be treated as topological reactions among various types of defects under a general framework. Our work not only suggests a new mathematical tool to investigate the interactions and reactions among multiple types of crystalline defects, but also provides a new insight to understand the deformation behaviors of metals and alloys based on dislocation/disclination theory.

A Denoising Autoencoder for Improved Kikuchi Pattern Quality and Indexing in Electron Backscatter Diffraction

The rapid collection and indexing of electron diffraction patterns as produced via electron backscatter diffraction (EBSD) has enabled crystallographic orientation and structural determination, as well as additional property-determining strain and dislocation density information with increasing speed, resolution, and efficiency. Pattern indexing quality is reliant on the noise of the collected electron diffraction patterns, which is often convoluted by sample preparation and data collection parameters. EBSD acquisition is sensitive to many factors and thus can result in low confidence index (CI), poor image quality (IQ), and improper minimization of fit, which can result in noisy datasets and misrepresent the microstructure. In an attempt to enable both higher speed EBSD data collection and enable greater orientation fit accuracy with noisy datasets, an image denoising autoencoder was implemented to improve pattern quality. We show that EBSD data processed through the autoencoder results in a higher CI, IQ, and a more accurate degree of fit. In addition, using denoised datasets in HR-EBSD cross correlative strain analysis can result in reduced phantom strain from erroneous calculations due to the increased indexing accuracy and improved correspondence between collected and simulated patterns.

An extraordinary improvement in cryogenic toughness of K-TIG welded 9Ni steel joint assisted by alternating axial magnetic field

A keyhole-TIG welding method assisted with alternating axial magnetic field (MF) was proposed to obtain qualified cryogenic toughness joints. The results show that the MF can refine prior austenite grains and establish distinct Ni-rich grain boundaries. Furthermore, the electron back scatter diffraction data demonstrate that the martensite grains have also been refined, and there is a higher density high angle grain boundary (HAGB) in the MF-assisted joint than in the ordinary joint. Under the cryogenic (−196 °C) Charpy V-notch impact test, the MF-assisted joint express a 37 J absorbed energy, which is 300% higher than the 11.7 J of the ordinary joint. This extraordinary increasing in toughness results from the refinement of grains and high density distribution of HAGB, which act as obstacles to crack propagation in cleavage fracture and form small facets on the fracture surface. The MF-assisted welding method has been proven to obtain qualified 9Ni steel joints.

Analysis of the mechanism of orientations evolution during hot rolling and mechanical properties of TiBw/TA15 composites based on crystal plasticity finite element model

An in-depth understanding of the crystal orientation evolution during hot rolling of TiB whisker (TiBw) /TA15 composites and the anisotropy of the as-rolled plates can help fully utilize the material properties. In this paper, the crystal plasticity finite element models of high-temperature (HT) β-phase and room-temperature (RT) α-phase were constructed from electron backscattering diffraction data. Based on this, the orientation evolution during hot rolling in the single-phase region and the effects of the matrix texture on the mechanical properties of the as-rolled plates were analyzed. The effect of TiBw on the anisotropy was studied by the composites finite element model. Results showed that the α-fiber texture of the β-phase was formed during HT rolling. This texture was converted to the T-texture of the α-phase at RT during cooling according to the Burgers orientation relationships. The TiBw had little effect on the matrix texture composition. The TiBw and matrix texture caused the matrix to have higher strength along the rolling direction and the transverse direction, respectively. The matrix texture dominated the difference in mechanical properties because its effect exceeded that of TiBw. The effect of the matrix on the mechanical properties was caused by the Schmid factors (SFs) and the critical resolved shear stress (CRSS) of the slip system together. The slip mode was influenced by SFs determined by the angular relationship between the crystal orientation and the loading direction. The CRSS of the activated slip system determined the yield strength.

Modelling of fatigue damage in granitic rock by piezoelectric effect in quartz phase due to alternating current excitation

This paper considers numerical modelling of hypothetical fatigue damage in granitic rock by alternating current (AC) excitation of piezoelectric properties of Quartz. For this end, a numerical method consisting of a rock mineral mesostructure model, an implicit time stepping scheme to solve the piezoelectro-mechanical problem, and a fatigue damage model was developed. The rock material was assumed to be heterogenous linear elastic and isotropic, save the Quartz piezoelectric properties, which are anisotropic. An evolution equation-based continuum scalar damage model based on an evolving back stress tensor and a moving Drucker–Prager type of endurance surface was applied to compute the damage inflicted by the AC excitation. The damage was computed in a post-processed mode, i.e., un-coupled to the material model, at this stage of investigations. Some preliminary axisymmetric simulations are presented with a rock mesotructure based on electron backscatter diffraction data. These simulations corroborate the hypothesis that fatigue damage can be induced on granitic rock by converse piezoelectric effect in the Quartz phase by sinusoidal alternating current. More specifically, fatigue damage was induced on a disc-shaped numerical rock sample at a voltage of 15 kV with 2.5 kHz of frequency.

On the dependence of creep-induced dislocation configurations on crystallographic orientation in pure Al and Al-Mg.

The peak broadening in neutron diffraction experiments on tensile specimens of pure Al (99.8%) and an Al-Mg alloy pre-deformed at different creep strains is analysed. These results are combined with the kernel angular misorientation of electron backscatter diffraction data from the creep-deformed microstructures. It is found that differently oriented grains possess different microstrains. These microstrains vary with creep strain in pure Al, but not in the Al-Mg alloy. It is proposed that this behaviour can explain the power-law breakdown in pure Al and the large creep strain observed in Al-Mg. The present findings further corroborate a description of the creep-induced dislocation structure as a fractal, predicated on previous work.

Extracting quartz deformation fabrics from polymineralic rocks

Quartz dynamic recrystallization fabrics are the record of deformation temperatures, strain rates, and fluids, and can be qualitatively and quantitatively evaluated using optical microscopy and electron backscatter diffraction (EBSD). Characterization of quartz deformation fabrics have been largely guided by textures in homogenous monomineralic samples. However, in nature such lithological homogeneity is uncommon, and mica and feldspar that interrupt the interconnectedness of quartz inhibit the exchange of material across quartz grain boundaries, effectively pinning these grains and preventing dislocation creep.We present two EBSD methods of extracting quartz domains within polyphase mylonites to investigate the effects of discontinuous quartz distribution and remove isolated quartz grains pinned by other phases. These samples provide textural evidence for grain boundary migration in quartz at temperatures exceeding 500 °C. Quartz domains vary in size from cm thick foliation parallel continuous ribbons, to isolated quartz grains separated by networks of mica and feldspar. We quantify contrasting textural responses to deformation between connected and isolated quartz grains using existing measures of EBSD data analysis, including M-index, grain orientation spread, kernel averaged misorientation, and grain size. We find that removing isolated quartz grains yields pole figures that are more representative of the regional deformation.

Tool for automatic macrozone characterization from EBSD data sets of titanium alloys.

Microtexture heterogeneities are commonly found in titanium forgings because of the thermomechanical processing. Also known as macrozones, these regions can reach millimetres in length, with grains sharing a similar crystallographic orientation leading to less resistance to crack propagation. Since the link between macrozones and the reduction of cold-dwell-fatigue performance on rotative components in gas turbine engines was established, efforts have been put into macrozone definition and characterization. The electron backscatter diffraction (EBSD) technique, widely used for texture analysis, allows for a qualitative macrozone characterization; however, further processing is required to define the boundaries and disorientation spread of each macrozone. Current approaches often use c-axis misorientation criteria, but this can sometimes lead to a large disorientation spread within a macrozone. This article describes the development and application of a computational tool implemented in MATLAB for automatic macrozone identification from EBSD data sets on the basis of a more conservative approach where both the c-axis tilting and rotation are considered. The tool allows for detection of macrozones according to the disorientation angle and density-fraction criteria. The clustering efficiency is validated by pole-figure plots, and the effects of the key parameters defining the macrozone clustering (disorientation and fraction) are discussed. In addition, this tool was successfully applied to both fully equiaxed and bimodal microstructures of titanium forgings.

Investigation of the tensile deformation behaviour in Ni-based superalloy inconel alloy 617 using EBSD-based finite element simulations and optical flow method

To improve component design, the fundamental understanding of the fatigue behaviour of gas turbine materials is essential. Since Ni-alloys exhibit pronounced elastic anisotropy, the local grain orientation strongly affects the stress and strain distribution in the material under mechanical loadings. This work addresses the characterisation of anisotropic elastic–plastic deformation and its consequences for crack initiation of nickel-base superalloy IN617 under tensile loading. Samples were loaded in situ in a scanning electron microscope (SEM) to correlate the deformation behaviour with the grain structure and the grain orientation determined by electron backscatter diffraction (EBSD) measurements. To calculate the resulting stresses and strains, the EBSD data were used to develop a model by finite element method (FEM) considering the grain structure and orientation. The results of the elastic–plastic finite element (FE) simulation were compared with the theories of the E⋅m model based on the Schmidt factor (m) and anisotropic Young’s modulus (E). A mathematical image registration method called “optical flow method” (OFM), which is capable to calculate the transformation of EBSD measuring points during deformation, was applied to the EBSD data. The strains calculated by the optical flow method and by FE simulation were compared for two samples. The findings revealed large strains in the later crack initiation area found in both the OFM and FEM. The developed FEM model was verified by the successful correlation of hypotheses of the E·m model with the simulated mechanical behaviour. Furthermore, the impact of the microstructural neighbourhood on the mechanical behaviour was emphasised.Graphical

Orientation Relationship of FeNiC and FeNiCSi from Variant Detection in EBSD Data

The determination of orientation relationships in dual or multi-phase materials is very important in the field of interface engineering for the design of materials with tailored properties. In this work, a code is developed for the automated and statistical analysis of the orientation relationship of electron backscatter diffraction data. The code is applied to the example of Fe-Ni-(Si)-C alloys containing lenticular martensite and retained austenite, and it is shown that the orientation relationship (OR) corresponds to the Greninger–Troiano OR and that a statistically reliable investigation of the OR between the retained austenite and the related martensite variants is feasible using the code developed in this study.

The Effect of Secondary‐Phase Fraction on the Deformation of Olivine + Ferropericlase Aggregates: 1. Microstructural Evolution

AbstractTo study the microstructural evolution of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (fPer) between 0.1 and 0.8. Additionally, single‐phase samples of both Ol and Per were deformed under the same experimental conditions to facilitate comparison of the microstructures in two‐phase and single‐phase materials. Each sample was deformed in torsion at T = 1523 K, P = 300 MPa at a constant strain rate up to a final shear strain of γ = 6 to 7. Microstructural developments, analyzed via electron backscatter diffraction (EBSD), indicate differences in both grain size and crystalline texture between single‐ and two‐phase samples. During deformation, grain size approximately doubled in our single‐phase samples of Ol and Per but remained unchanged or decreased in two‐phase samples. Zener‐pinning relationships fit to the mean grain sizes in each phase for samples with 0.1 ≤ fPer ≤ 0.5 and for those with 0.8 ≥ fPer ≥ 0.5 demonstrate that the grain size of the primary phase is controlled by phase‐boundary pinning. Crystallographic preferred orientations, determined for both phases from EBSD data, are significantly weaker in the two‐phase materials than in the single‐phase materials.

Evidence for multiple early impacts on the H chondrite parent body from electron backscatter diffraction analysis

AbstractWe examined H4 chondrites Beaver Creek, Forest Vale, Quenggouk, Ste. Marguerite, and Sena with the electron backscatter diffraction (EBSD) techniques of Ruzicka and Hugo (2018) to determine if there is evidence for shock metamorphism consistent with the previously inferred histories of their early impact excavation or lack thereof. We find that all samples have EBSD data consistent with a history of synmetamorphic impact shock (i.e., shock during thermal metamorphism), followed by postshock annealing. Petrographic analysis of Sena, Quenggouk, and Ste. Marguerite found exsolved Cu and irregular troilite within Fe metal, features consistent with shock metamorphism. All samples have a spatial variability in grain deformation consistent with shock processes, though Forest Vale, Quenggouk, and Ste. Marguerite may have relict signatures of accretional deformation as indicated by variability in their olivine deformation metrics. Within the context of previous workers' geochemical observations, a more complex history is inferred for each sample. The “slow‐cooled” samples, Quenggouk and Sena, were subject to synmetamorphic shock without excavation and annealed at depth. The same is true of the “fast‐cooled” samples, Beaver Creek, Forest Vale, and Ste. Marguerite. However, after annealing, these rocks were excavated by a secondary impact or impacts around 5.2–6.5 Ma post‐CAI formation and were left to cool rapidly on the surface of the H chondrite parent body. These interpreted histories are best compatible with a model of an impact‐battered but intact onion shell for the earliest history of the H parent body. However, the EBSD evidence does not preclude a parent body disruption after 7 Ma post‐CAI formation.

Deformation-induced martensite transformation and variant selection in AISI 316L austenitic stainless steel during uniaxial tensile deformation

Present work has investigated the orientation dependence of deformation-induced martensite (DIM) transformation and contemporaneous variant selection in AISI 316L austenitic stainless steel during uniaxial tensile loading. The combined effect of slip and deformation-induced martensite transformation have been analysed comprehensively using experimental macrotexture analysis, visco-plastic self-consistent (VPSC) simulation and reconstruction of prior-austenite orientations from electron backscatter diffraction (EBSD) data. It has been shown that when the polycrystalline austenitic material is deformed, grains with orientations like Brass, Rot-Goss, Rot-Cu undergo plastic deformation by slip and rotate towards {110}<111> orientation. Grains of this orientation eventually undergo DIM transformation at sufficiently higher engineering strain (≥69%) levels. Variant selection during DIM transformation from {110}<111> component of austenite has been critically examined using a model developed based on the phenomenological theory of martensite crystallography (PTMC). It has been observed that in general, variants with higher interaction energies are preferred over the others. The results in turn indicate that Patel-Cohen theory, which is believed to be valid for stress-induced martensite (i.e. during elastic deformation), holds equally good for strain-induced martensite (i.e. during plastic deformation) as well.

Primary hot working characteristics and microstructural evolution of as-cast and homogenized Ti-4Al-2.5V-1.5Fe alloy

Hot deformation characteristics of Ti-4Al-2.5V-1.5Fe-0.25O alloy in as-cast and homogenized condition was studied using isothermal hot compression tests in both β and α + β fields at strain rates ranging from 3 × 10−4 s−1 to 1 s−1. To understand the hot deformation behaviour and to identify the optimum regime for the primary breakdown of titanium alloy ingot, processing map based on strain rate sensitivity (m), kinetic analysis, and microstructural characterization were used. Micro-mechanisms of deformation in the β-field were analyzed with the help of reconstruction of parent β grain orientations using electron backscatter diffraction data. Kinetic analysis and microstructural characterization of samples deformed at low strain rates (ε̇<10−2 s−1) in β-field confirmed dynamic recovery assisted by dislocation glide/climb. In comparison, the samples deformed at higher strain rates (ε̇>10‐2s−1) in β-field exhibited deformation bands and recrystallized grains formed through static recrystallization. In the α + β field, kinetic parameters at low strain rates (ε̇<10−2 s−1) indicated dynamic globurization, which were corroborated by microstructural analysis. Samples deformed at high strain rates (ε̇>10−2 s−1) in the α + β field exhibited adiabatic shear bands, strain-induced porosity and lamellar α-kinking. The optimum regime for thermomechanical processing (900°C to 1150°C and 3ⅹ10−4 s−1 to 3ⅹ10−2 s−1) is arrived at by combining the strain rate sensitivity map and microstructural analysis. Further, constitutive equations have been developed to predict the steady-state flow stress values in β and α + β fields.

Optimized method for the identification of slip directions in Mg alloy based on intragranular misorientation axes analysis

In this work, an optimized method was proposed to identify the activated slip directions (i.e., gliding direction of dislocations) based on intragranular misorientation axes (IGMA) analysis only using electron backscattered diffraction (EBSD) data. By using MTEX toolbox in MATLAB software, the figures which showed the slip directions and the change of rotation angles (i.e, the grain reference orientation deviation (GROD) angles) in each grain, were plotted based on the EBSD data. By comparing the directions in which the GROD angle increases with the slip directions in the grain, the potential activated prismatic slip directions can be determined. Taking two grains in the extruded Mg–6Zn–1Y-0.5Zr alloy as examples. Two different prismatic slip directions belonging to two prismatic slip systems, [1-210] (GROD value: 0.64 → 0.75) and [-12-10] (GROD value: 0.34 → 0.74), were activated in the grain 1. Meanwhile, only one prismatic slip system, (10-10) [-12-10] (GROD value: 0.29 → 0.8), was initiated in the grain 2. Moreover, this method was further applied to determine the basal slip systems. Two basal slip systems, (0001) [-12-10] (GROD value: 2.7 → 4) and (0001) [-2110] (GROD value: 3.2 → 4.2), were activated in the selected grains, demonstrating that this method is also suitable for identifying the basal slip systems except for the prismatic systems. In summary, a novel method to determine the active slip directions in Mg alloys was successfully developed and experimentally validated in Mg alloys, which was simpler and more efficient compared to other widely utilized methods.

Microscale Spatial Variations in Coseismic Temperature Rise on Hematite Fault Mirrors in the Wasatch Fault Damage Zone

AbstractCoseismic temperature rise activates fault dynamic weakening that promotes earthquake rupture propagation. The spatial scales over which peak temperatures vary on slip surfaces are challenging to identify in the rock record. We present microstructural observations and electron backscatter diffraction data from three small‐displacement hematite‐coated fault mirrors (FMs) in the Wasatch fault damage zone, Utah, to evaluate relations between fault properties, strain localization, temperature rise, and weakening mechanisms during FM development. Millimeter‐ to cm‐thick, matrix‐supported, hematite‐cemented breccia is cut by ∼25–200 μm‐thick, texturally heterogeneous veins that form the hematite FM volume (FMV). Grain morphologies and textures vary with FMV thickness over μm to mm lengthscales. Cataclasite grades to ultracataclasite where FMV thickness is greatest. Thinner FMVs and geometric asperities are characterized by particles with subgrains, serrated grain boundaries, and(or) low‐strain polygonal grains that increase in size with proximity to the FM surface. Comparison to prior hematite deformation experiments suggests FM temperatures broadly range from ≥400°C to ≥800–1100°C, compatible with observed coeval brittle and plastic deformation mechanisms, over sub‐mm scales on individual slip surfaces during seismic slip. We present a model of FM development by episodic hematite precipitation, fault reactivation, and strain localization, where the thickness of hematite veins controls the width of the deforming zones during subsequent fault slip, facilitating temperature rise and thermally activated weakening. Our data document intrasample coseismic temperatures, resultant deformation and dynamic weakening mechanisms, and the length scales over which these vary on slip surfaces.

Friction Stir Welding of AA5754-H24: Impact of Tool Pin Eccentricity and Welding Speed on Grain Structure, Crystallographic Texture, and Mechanical Properties.

This study investigates the effect of tool pin eccentricity and welding speed on the grain structure, crystallographic texture, and mechanical properties of friction stir welded (FSWed) AA5754-H24. Three tool pin eccentricities of 0, 0.2, and 0.8 mm at different welding speeds ranging from 100 mm/min to 500 mm/min and a constant tool rotation rate of 600 rpm were investigated. High-resolution electron backscattering diffraction (EBSD) data were acquired from each weld's center of the nugget zone (NG) and processed to analyze the grain structure and texture. In terms of mechanical properties, both hardness and tensile properties were investigated. The grain structure in the NG of the joints produced at 100 mm/min, 600 rpm, and different tool pin eccentricities showed significant grain refining due to dynamic recrystallization with average grain sizes of 18, 15, and 18 µm at 0, 0.2, and 0.8 mm pin eccentricities, respectively. Increasing the welding speed from 100 to 500 mm/min further reduced the average grain size of the NG zone to 12.4, 10, and 11 µm at 0, 0.2, and 0.8 mm eccentricity, respectively. The simple shear texture dominates the crystallographic texture with both B¯/B texture component with the C component at their ideal positions after rotating the data to align the shear reference frame with the FSW reference frame in both the PFs and ODF sections. The tensile properties of the welded joints were slightly lower than the base material due to the hardness reduction in the weld zone. However, the ultimate tensile strength and the yield stress for all welded joints increased by increasing the friction stir welding (FSW) speed from 100 to 500 mm/min. Welding using the pin eccentricity of 0.2 mm resulted in the highest tensile strength; at a welding speed of 500 mm/min, it reached 97% of the base material strength. The hardness profile showed the typical W shape with a reduction in the hardness of the weld zone and a slight recovery of the hardness in the NG zone.

Correlated structure viscoplastic self-consistent polycrystal plasticity: Application to modeling strain rate sensitive deformation of Ti-6Al-4 V

This paper presents a multi-level viscoplastic self-consistent (VPSC) model for correlated structures (CS-VPSC). A polycrystalline aggregate response is homogenized over inclusions, which are linearized using the generalized affine scheme. Response of the inclusions is that of either a single grain or a correlated structure of multiple single crystals. The single crystal response is a strain rate adjusted power-law viscoplastic formulation, which considers not only the driving force in the glide direction but also the two orthogonal shear and three normal components of the driving force. The resistance to slip is grain size sensitive and based on the evolution of dislocation density. Furthermore, the resistances of prismatic and basal slip systems are adjusted based on the geometry of slip transfer between adjacent correlated structures. The overall model linking the single crystal micro-scale to a correlated structure meso‑scale to an aggregate macro-scale response is also coupled with the implicit finite elements (FE-CS-VPSC). The model is used for modeling strain rate sensitive mechanical response and microstructural evolution of additively manufactured Ti-6Al-4 V (Ti64) alloy in two conditions. As-built and stress-relived (SR) microstructure consisted of correlated α-lath/lamellar structures, while thermo-hydrogen refined microstructure (THRM) consisted primarily of α- and β-globular structures and some correlated α-lath/lamellar structures. Electron backscatter diffraction data, neutron diffraction data, and mechanical data were used to initialize, calibrate, and validate the model. To this end, the flow stress and texture evolution of the materials in tension and compression under quasi-static and high strain rate deformations were predicted using CS-VPSC, while the texture evolution during rolling and impact was simulated using FE-CS-VPSC. Good predictions of the texture evolution and mechanical response including the tension/compression asymmetry allowed us to discuss the roles of correlated structures, slip system activation stresses, size effects, strain partitioning between phases, and slip activities in such predictions.

Nondestructive quantification of single crystal elasticity for additively manufactured SB-CoNi-10C, IN625, and Ti64

Resonant ultrasound spectroscopy (RUS) is capable of determining the single crystal elastic constants from polycrystalline specimens with known crystallographic texture. However, the calculated single crystal elastic constants vary with the measured texture, resulting in inconsistent estimates for additively manufactured (AM) specimens with heterogeneous texture regions. In this work, the accuracy of the determined single crystal elastic constants is improved by incorporating the uncertainty of the texture in the determination of single crystal elasticity, and requiring only small quantities of electron backscatter diffraction data (EBSD) to do so. The single crystal elastic constants are determined by Bayesian inference with parallelized sequential Monte Carlo (SMC), enabling an order of magnitude reduction in computational cost. AM specimens of a cobalt-nickel-base superalloy (SB-CoNi-10C) demonstrate that the incorporation of texture variability enables the single crystal elastic constants to converge to the reported literature values within one standard deviation, avoiding any dependence on the initial texture values. The single crystal elastic constants of nickel-base-superalloy Inconel 625 (IN625) and Ti-6Al-4V (Ti64) are determined from AM specimens, using only RUS and EBSD data. The determined single crystal elastic constants of IN625 agree between two different texture conditions (induced by AM raster strategy), as well as with the literature values, within one standard deviation. The single crystal elastic constants determined from three AM Ti64 specimens, printed with different beam powers, agree with the range of literature values within two standard deviations but demonstrate variability between AM specimens, indicating that the frequencies may be susceptible to the effects of secondary phases.

Crystallographic preferred orientations and microtexture of the Himalayan eclogites revealing records of syn-deformation peak metamorphic stage and subsequent exhumation

Electron backscattered diffraction (EBSD) and crystallographic preferred orientations (CPO) studies on Himalayan high- and ultrahigh-pressure eclogites are reported. Subduction-related peak eclogite facies (omphacite, garnet, quartz, rutile) and exhumation-related post-eclogite facies (hornblende, epidote, titanite) minerals were investigated to understand subduction and exhumation-related deformation regimes. Petrographic features from the studied eclogite samples show preferably shape-oriented omphacites coexisting with random to weakly shape-oriented garnets, and overgrown by hornblende and symplectites. Quartz grains are also randomly shape-oriented in the eclogite matrix whereas epidote grains display shape-preferred orientations. Based on EBSD data, an L-type CPO was dominant in omphacites in the studied eclogites, exhibited by the maximum density of [001] axes parallel to the lineation and the high density of poles to the (010) and (110) planes forming girdles perpendicular to the foliation. This CPO likely developed in a constrictional strain regime during the peak eclogite-facies conditions due to dislocation creep and is associated with an intra-crystalline (110) [001] and (110) <110> slip systems. EBSD data of garnets exhibit complex fabric, suggesting they most likely behaved like rigid objects in the presence of omphacite, indicating insignificant plastic deformation. EBSD data of quartz in high-pressure eclogites show irregular CPO but in ultrahigh-pressure eclogites the fabric is relatively strong. EBSD data of rutile show CPO of [001] axes strongly aligned along the lineation, suggesting its deformation was coeval to the omphacite. Titanite, mainly overgrown around rutile, indicates a random CPO different from rutile, suggesting its growth during late-stages. Hornblende in both types of eclogites shows identical fabric to that of omphacite, displaying CPO of [001] axes parallel to the lineation and poles of (010) and (110) planes form girdles perpendicular to the foliation, suggesting topotactic replacement over omphacite during exhumation. The EBSD data, combined with textural features, indicate syn-deformation peak metamorphism as well as preserve records of subsequent exhumation.