High Angular Resolution Electron Backscatter Diffraction Research Articles

Weighted Burgers Vector analysis of orientation fields from high-angular resolution electron backscatter diffraction

The Weighted Burgers Vector (WBV) method can extract information about dislocation types and densities present in distorted crystalline materials from electron backscatter diffraction (EBSD) maps, using no assumptions about which slip systems might be present. Furthermore, high-angular resolution EBSD (HR-EBSD) uses a cross-correlation procedure to increase the angular precision of EBSD measurements by an order of magnitude compared to conventional EBSD. However, the WBV technique has not previously been applied to HR-EBSD data and therefore it remains unclear as to which low-angle substructures can be reliably characterised by WBV analysis of conventional EBSD data and which require additional HR-EBSD processing. To establish some practical examples that can be used to guide future data-acquisition strategies, we compare the output of the WBV method when applied to conventional EBSD data and HR-EBSD data collected from the most common minerals in Earth's lower crust (plagioclase feldspar) and upper mantle (olivine). The results demonstrate that HR-EBSD and WBV processing are complementary techniques. The increase in angular precision achieved with HR-EBSD processing allows low-angle (on the order of 0.1°) structures, which are obscured by noise in conventional EBSD data, to be analyzed quantitatively using the WBV method. Combining the WBV and HR-EBSD methods increases the precision of calculated WBV directions, which is essential when using information about active slip systems to infer likely deformation mechanisms from naturally deformed microstructures. This increase in precision is particularly important for low-symmetry crystals, such as plagioclase, that have a wide range of available slip systems that vary in relative activity with changing pressure, temperature and differential stress. Because WBV directions are calculated using no assumptions about which slip systems may be present, combining this technique with HR-EBSD to refine the precision of lattice orientation gradients is ideal for investigating complex natural materials with unknown deformation histories.

Investigation of fine-scale dislocation distributions at complex geometrical structures by using HR-EBSD and a comparison with conventional EBSD

Dislocation distribution is one of the elementary factors that directly influence deformation processes in structural components. Electron backscatter diffraction (EBSD) analysis is one of the experimental techniques that allow for a semi-quantitative evaluation of the Geometrically Necessary Dislocation (GND) distribution in polycrystals. Owing to its noise floor the accuracy of conventional Hough transformed-based GND analysis is restricted to relatively large plastic strains. This problem can be overcome by using the cross-correlation-based high angular resolution electron backscatter diffraction (HR-EBSD) method. The present study demonstrates the improvements in characterizing localized dislocation distribution using the HR-EBSD method compared to the conventional approach. To demonstrate the efficacy, we investigated two more extreme examples of deformation conditions in polycrystalline zirconium (Zr), 1) a plastically deformed crack tip in presence of applied strain, and 2) a misfit plastic field around a hydride precipitate. The edge, screw and total GND components are compared for both cracked and hydride specimens. Dissimilarities in the identification of screw dislocations and subtle features with misorientation uncertainties are reported using HR-EBSD and conventional EBSD. A direct correlation between slip bands and the HR-EBSD estimated GNDs is presented, enhancing the scope of this approach to identify individual slip bands, which is traditionally challenging for any misorientation-based characterization technique.

Exploring structural origins responsible for the exceptional mechanical property of additively manufactured 316L stainless steel via in-situ and comparative investigations

The laser beam powder-bed-fusion (PBF-LB) technique offers the possibility to fabricate metallic materials with unparalleled mechanical properties. However, due to the complexity of nonequilibrium solidification structures, untangling the role of specific structural features in influencing mechanical properties through post-mortem analysis remains a significant challenge. Here, we tracked the complete evolution of deformation microstructures in PBF-LB 316L stainless steel (SS) and conventionally manufactured (CM) counterparts using in-situ electron channeling contrast imaging (ECCI) in conjunction with high-angular resolution electron backscatter diffraction (HR-EBSD). Our findings underscore the importance of the intrinsic dislocation configuration, i.e., faulted dipole, in modulating the plastic deformation behaviors of the PBF-LB sample. The periodic arrangement of faulted dipoles triggers a widespread activation of stacking faults (SFs) with extremely fine spacing at the yielding stage, resulting in a high passing stress (∼433 MPa) that is responsible for the high yield strength. Meanwhile, such configuration facilitates uniform plastic deformation and mediates twinning propensity, thereby alleviating stress concentrations and enhancing ductility. Moreover, by correlating the mechanical response with microstructural evolution, we establish a modified model for multiple strengthening mechanisms in the PBF-LB sample. Additionally, we examine the effect of nonequilibrium solidification structures on the strain rate sensitivity of PBF-LB samples. Taking 316L SS as an exemplar, our results extend the current understanding of the structural origins responsible for the exceptional mechanical properties of metals manufactured by PBF-LB.

Role of disclinations around kink bands on deformation behavior in Mg–Zn–Y alloys with a long-period stacking ordered phase

Mg–Zn–Y alloys that contain an Mg-based long-period stacking ordered (LPSO) phase exhibit excellent mechanical properties because of the kink bands formed by plastic deformation. Such “kink-band strengthening” has attracted significant attention. Herein, we conducted deformation tests on directionally solidified single-phase LPSO Mg85Zn6Y9 alloys to investigate the kink-band strengthening mechanism. High-angular resolution electron backscatter diffraction was performed to detect misorientation between the matrices on either side of the kink bands. The misorientations corresponded to the magnitudes of the Frank vectors of the disclinations around the kink, and closely matched the estimations from geometric analysis, which supported the existence of disclinations. Moreover, the Frank vector of the kink bands increased after shear deformation, which indicates that shear deformation introduced new disclinations around the kinks. Scanning electron microscopy demonstrated that the kinks clearly obstructed basal ⟨a⟩ slip. Furthermore, scanning transmission electron microscopy of a sheared kink band revealed dislocation pile-ups on both sides of the kink/matrix interface, as well as the formation of secondary kinks that stabilized the structure against shear deformation (i.e., self-accommodation) and bend contours caused by elastic stress fields. The results suggest that disclinations behave as long-range obstacles to dislocation motion (e.g., by reducing the increase in kink-band self-energy or by the elastic stress field of disclinations). We believe that this study will play a key role in identifying the factors responsible for kink-band strengthening in LPSO-phase Mg alloys and in understanding the phenomena underlying the strengthening mechanism.

HR-EBSD analysis of in situ stable crack growth at the micron scale

Understanding the local fracture resistance of microstructural features, such as brittle inclusions, coatings, and interfaces, at the microscale is critical for microstructure-informed design of materials. In this study, a novel approach has been formulated to decompose the J-integral evaluation of the elastic energy release rate to the three-dimensional stress intensity factors directly from experimental measurements of the elastic deformation gradient tensors of the crack field by in situ high (angular) resolution electron backscatter diffraction (HR-EBSD). An exemplar study is presented of a quasi-static crack, inclined to the observed surface, propagating on low index {hkl} planes in a (001) single crystal silicon wafer.

Characterization of a nanopipe dislocation in GaN by means of HR-EBSD and field dislocation mechanics analysis

A nanopipe threading screw dislocation in a GaN layer is studied. A recently developed high-angular resolution electron backscatter diffraction technique, relying on a global image registration of Kikuchi patterns, is used to assess elastic strain and rotation fields in the defected area. The characterization is complemented with predictions from a piezoelectric field dislocation mechanics model of a threading screw dislocation line. In plane elastic fields are obtained at the free surface, which arise from the cancellation of the dislocation bulk shear stress field. The experimental and simulated long range fields agree qualitatively well and correspond to a screw dislocation with Burgers vector magnitude 3c = 1.56 nm.

In-situ diffraction based observations of slip near phase boundaries in titanium through micropillar compression

We investigate local deformation within a dual-phase Ti6242 alloy by considering the interaction between α orientation (i.e., slip system) and the presence of β phase (i.e., fraction and morphology), and the corresponding evolution of stored dislocations. In-situ micropillar compression combined with micro-Laue diffraction and high angular resolution electron backscatter diffraction (HR-EBSD) shows that the high rate of geometrically necessary dislocation (GND) accumulation was observed in basal-slip oriented pillar, while easy formation of glide step with minimal GND accumulation was found in prism-slip oriented pillar. The difference is explained due to the role of the phase boundary for these slip configurations, where local stress concentrations can arise even in pillar with easy slip orientation (i.e., prism slip) if the phase boundary acts as a barrier to slip and results in inhomogeneity of local deformation. This study reveals significance of β phase in potentially modulating the local dislocation based hardening and hence local stress states within Ti polycrystalline aggregate.

Effect of sample thinning on strains and lattice rotations measured from Transmission Kikuchi diffraction in the SEM

Cross correlation based high angular resolution EBSD (or HR-EBSD) has been developed for measurement of elastic strains, lattice rotations (and estimating GND density). Recent advances in Transmission Kikuchi diffraction (TKD), especially the on-axis geometry allows the possibility of acquiring patterns at higher spatial resolution. However, some controversy remains as to whether stresses/strains measured after the sample thinning process are still representative of the bulk sample. In this paper, we explore a way of applying the HR-EBSD method to study strains and lattice rotations in an initially bulk sample, that is then progressively thinned down until a similar analysis can be performed on thin (and electron transparent) samples. Thus, HR-TKD will be compared as a possible alternative to HR-EBSD, in scenarios when it is not always possible to perform EBSD on the surface of the sample. An estimate of strain relaxation in the sample as a result of sample thinning is presented.

Slip band interactions and GND latent hardening in a galling resistant stainless steel

Slip activation, slip band interactions, and GND densities in iron-base, galling resistant alloy Nitronic 60 have been characterised at the grain length scale using small-scale mechanical testing with high resolution digital image correlation and high-angular resolution electron backscatter diffraction. By correlating the two measurement techniques, new insight into slip band interactions, the generation of lattice curvature and the corresponding accumulation of geometrically necessary dislocations (GNDs) is provided. Multiple discrete slip bands are typically active within single grains, resulting in significant slip band interactions. Crossing slip bands were found to generate accumulations of GNDs. Regions where slip bands block other slip bands were associated with the highest GND densities, in excess of three time the densities of crossing slip bands. Representative crystal plasticity modelling investigations have demonstrated that discrete slip blocking events are responsible for locally elevated GND density. This behaviour is rationalised in terms of lattice curvature associated with the differing levels of constraint provided by the crossing or blocking-type behaviours. Ferrite grains are also found to contribute to the generation of GNDs. Together, these two effects provide significant work hardening mechanisms, likely to be key to the development of future iron-base hard facing alloys.

3D HR-EBSD Characterization of the plastic zone around crack tips in tungsten single crystals at the micron scale

High angular resolution electron backscatter diffraction (HR-EBSD) was coupled with focused ion beam (FIB) slicing to characterize the shape of the plastic zone in terms of geometrically necessary dislocations (GNDs) in W single crystal in 3 dimensions. Cantilevers of similar size with a notch were fabricated by FIB and were deformed inside a scanning electron microscope at different temperatures (21 ∘C, 100 ∘C and 200 ∘C) just above the micro-scale brittle-to-ductile transition (BDT). J-integral testing was performed to analyse crack growth and determine the fracture toughness. At all three temperatures the plastic zone was found to be larger close to the free surface than inside the specimen, similar to macro-scale tension tests. However, at higher temperature, the 3D shape of the plastic zone changes from being localized in front of the crack tip to a butterfly-like distribution, shielding more efficiently the crack tip and inhibiting crack propagation. A comparison was made between two identically deformed samples, which were FIB-sliced from two different directions, to evaluate the reliability of the GND density estimation by HR-EBSD. The analysis of the distribution of the Nye tensor components was used to differentiate between the types of GNDs nucleated in the sample. The role of different types of dislocations in the plastic zone is discussed and we confirm earlier findings that the micro-scale BDT of W is mainly controlled by the nucleation of screw dislocations in front of the crack tip.

Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength

The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, strain hardening may cause significant evolution of strength in the colder load-bearing portion of the lithosphere. Recent rheological data from low-temperature deformation experiments on olivine suggest that strain hardening occurs due to the presence of temperature-independent back stresses generated by long-range elastic interactions among dislocations. These interpretations provided the basis for a flow law that incorporates hardening by the development of back stress. Here, we test this dislocation-interaction hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of ≤4.3GPa or in a nanoindenter at applied contact stresses of ≥10.2GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 1014m−2 and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned in slip bands and interacting with dislocations of other types that act as obstacles. The resulting accumulations of dislocations in their slip planes, and associated stress heterogeneities, are consistent with strain hardening resulting from long-range back-stresses acting among dislocations and thereby support the form of the flow law for low-temperature plasticity. Based on these observations, we predict that back stresses among dislocations will impart significant mechanical anisotropy to deformed lithosphere by enhancing or reducing the effective stress. Therefore, strain history, with associated microstructural and micromechanical evolution, is an important consideration for models of lithospheric strength. The microstructural observations also provide new criteria for identifying the operation of back-stress induced strain hardening in natural samples and therefore provide a means to test the applicability of the flow law for low-temperature plasticity.

Precise measurement of strain accommodation in a Mg-Gd-Y-Zn alloy using cross-correlation-based high resolution EBSD

Mg-RE alloys show improved ductility compared to other classical magnesium alloys at room temperature. The present study focused on the strain accommodation mechanism in a wrought Mg-5.4Gd-1.8Y-1.5Zn alloy at the microstructural scale. For this purpose, a tensile specimen was uniaxially strained in-situ in tension up to 6%, and the lattice rotation tensor and geometrically necessary dislocation (GND) density were measured using cross-correlation-based high angular resolution electron backscatter diffraction (HR-EBSD) technique. The strain incompatibility within the Mg-Gd-Y-Zn alloy was accommodated by basal slip together with pyramidal dislocation. The GND measurements via HR-EBSD showed a high occurrence of pyramidal <c + a> dislocation even in well-aligned grains. The orientation dependence of basal <a> and pyramidal <c + a> dislocation was also investigated. It was found that the soft oriented grains had a higher basal <a> density, whereas the average GND density of <c + a> slip appeared to decrease with the Schmid factor of pyramidal dislocation. In addition, slip transfer process of the basal-basal and basal-pyramidal type was observed with high m' factors in this study.

The effect of δ-hydride on the micromechanical deformation of a Zr alloy studied by in situ high angular resolution electron backscatter diffraction

Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%) is used as nuclear fuel cladding materials and hydride embrittlement is a major failure mechanism. To explore the effect of δ-hydride on plastic deformation and performance of Zircaloy-4, in situ high angular resolution electron backscatter diffraction (HR-EBSD) was used to quantify stress and geometrically necessary dislocation (GND) density during bending tests of hydride-free and hydride-containing single crystal Zircaloy-4 microcantilevers. Results suggest that while the stress applied was accommodated by plastic slip in the hydride-free cantilever, the hydride-containing cantilever showed precipitation-induced GND pile-up at hydride-matrix interface pre-deformation, and considerable locally-increasing GND density under tensile stress upon plastic deformation.

High‐Angular Resolution Electron Backscatter Diffraction as a New Tool for Mapping Lattice Distortion in Geological Minerals

AbstractAnalysis of distortions of the crystal lattice within individual mineral grains is central to the investigation of microscale processes that control and record tectonic events. These distortions are generally combinations of lattice rotations and elastic strains, but a lack of suitable observational techniques has prevented these components being mapped simultaneously and routinely in earth science laboratories. However, the technique of high‐angular resolution electron backscatter diffraction (HR‐EBSD) provides the opportunity to simultaneously map lattice rotation and elastic strain gradients with exceptional precision, on the order of 0.01° for rotations and 10−4 in strain, using a scanning electron microscope. Importantly, these rotations and lattice strains relate to densities of geometrically necessary dislocations and residual stresses. Recent works have begun to apply and adapt HR‐EBSD to geological minerals, highlighting the potential of the technique to provide new insights into the microphysics of rock deformation. Therefore, the purpose of this review is to provide a summary of the technique, to identify caveats and targets for further development, and to suggest areas where it offers potential for major advances. In particular, HR‐EBSD is well suited to characterizing the roles of different dislocation types during crystal plastic deformation and to mapping heterogeneous internal stress fields associated with specific deformation mechanisms/microstructures or changes in temperature, confining pressure, or macroscopic deviatoric stress. These capabilities make HR‐EBSD a particularly powerful new technique for analyzing the microstructures of deformed geological materials.

Evaluation of growth sector orientation changes of high B doped high pressure and high temperature diamond by high resolution electron backscatter diffraction study

The orientation of growth sectors of highly B doped p+ high pressure and high temperature (HPHT) substrates were investigated by high angular resolution electron backscatter diffraction (HR-EBSD). The lattice rotation mapping images of the areas across the 〈111〉 growth sector boundaries were measured. The crystal orientation was found to be inclined approximately 0.03° for [001] to [-1-10], compared with the [-1-11] growth sector area at a given location. Other orientation inclinations were observed such as 0.02° for [001] to [110] and 0.025° for [001] to [1-10]. The same phenomena were confirmed for the other two crystals. Conventionally, it has been recognized that a HPHT-grown crystal is a perfect single crystal, however, our results indicate that “twin boundaries” are formed between the growth sectors for p+ HPHT substrates. To meet the requirements for the power device wafer specifications, the establishment of growth technology is desirable in the near future.

The role of β-titanium ligaments in the deformation of dual phase titanium alloys

Multiphase titanium alloys are critical materials in high value engineering components, for instance in aero engines. Microstructural complexity is exploited through interface engineering during mechanical processing to realise significant improvements in fatigue and fracture resistance and strength. In this work, we explore the role of select interfaces using in-situ micromechanical testing with concurrent observations from high angular resolution electron backscatter diffraction (HR-EBSD). Our results are supported with post mortem transmission electron microscopy (TEM). Using micro-pillar compression, we performed in-depth analysis of the role of select β-titanium (body centred cubic) ligaments which separate neighbouring α-titanium (hexagonal close packed) regions and inhibit the dislocation motion and impact strength during mechanical deformation. These results shed light on the strengthening mechanisms and those that can lead to strain localisation during fatigue and failure.

The impact of water on slip system activity in olivine and the formation of bimodal crystallographic preferred orientations

Crystallographic preferred orientations (CPOs) in olivine are widely used to infer the mechanisms, conditions, and kinematics of deformation of mantle rocks. Recent experiments on water-saturated olivine were the first to produce a complex CPO characterised by bimodal orientation distributions of both [100] and [001] axes and inferred to form by combined activity of (001)[100], (100)[001], and (010)[100] slip. This result potentially provides a new microstructural indicator of deformation in the presence of elevated concentrations of intracrystalline hydrous point defects and has implications for the interpretation of seismic anisotropy. Here, we document a previously unexplained natural example of this CPO type in a xenolith from Lesotho and demonstrate that it too may be explained by elevated concentrations of hydrous point defects. We test and confirm the hypothesis that combined (001)[100], (100)[001], and (010)[100] slip were responsible for formation of this CPO by (1) using high-angular resolution electron backscatter diffraction to precisely characterise the dislocation types present in both the experimental and natural samples and (2) employing visco-plastic self-consistent simulations of CPO evolution to assess the ability of these slip systems to generate the observed CPO. Finally, we utilise calculations based on effective-medium theory to predict the anisotropy of seismic wave velocities arising from the CPO of the xenolith. Maxima in S-wave velocities and anisotropy are parallel to both the shear direction and shear plane normal, whereas maxima in P-wave velocities are oblique to both, adding complexity to interpretation of deformation kinematics from seismic anisotropy.

New levels of high angular resolution EBSD performance via inverse compositional Gauss–Newton based digital image correlation

Conventional high angular resolution electron backscatter diffraction (HREBSD) uses cross-correlation to track features between diffraction patterns, which are then related to the relative elastic strain and misorientation between the diffracting volumes of material. This paper adapts inverse compositional Gauss Newton (ICGN) digital image correlation (DIC) to be compatible with HREBSD. ICGN-based works by efficiently tracking not just the shift in features, but also the change in their shape. Modeling a shape change as well as a shift results in greater accuracy. This method, ICGN-based HREBSD, is applied to a simulated data set, and its performance is compared to conventional cross-correlation HREBSD, and cross-correlation HREBSD with remapping. ICGN-based HREBSD is shown to have about half the strain error of the best cross-correlation method with a comparable computation time.

Local Strain Distribution and Microstructure of Grinding-Induced Damage Layers in SiC Wafer

The high-angular-resolution electron backscatter diffraction (HR-EBSD) technique has been utilized to evaluate the elastic strain distribution of grinding-induced damage layers in silicon carbide wafers. HR-EBSD analysis, along with transmission electron microscopy observation, revealed that the damage layers formed beneath the wafer surface when the surface was ground by diamond abrasives; the layers were classified hierarchically based on the distribution of elastic strain and lattice defects. In particular, very large elastic strain formed in a defective region of roughly 0.6 μm in thickness, just beneath the ground wafer surface, where lattice defects such as dislocations, stacking faults, and microcracks were introduced inhomogeneously by abrasive interaction and related plastic deformation and fracture. Based on this inhomogeneity, the defective region was itself classified into two types: one, a highly defective region with very large, complicated strain and high defect density; and the other, a basal plane dislocation (BPD)-glide region, with small strain and few BPDs or stacking faults. Beneath the defective region, a strain gradient region of roughly 1.8 μm in thickness, which was unambiguously identified by HR-EBSD strain analysis alone, revealed a monotonic gradient in the dominant compressive strain component, with no grinding-induced defects. Overall, HR-EBSD analysis revealed a nanometer-scale, hierarchical elastic strain distribution in the grinding-induced damage layers.

Grain Boundary Serration in Nickel-Based Superalloy Inconel 600: Generation and Effects on Mechanical Behavior

Grain boundary serration in the superalloy Inconel 600 was studied. Two microstructural variants, one with nonserrated and the other with serrated grain boundaries were generated by altering the heat-treatment conditions, while keeping other aspects of the microstructure unchanged. The effect on the creep response between 700 °C and 900 °C was measured, and the different failure modes and accumulated damage were quantified using high-angular resolution electron backscatter diffraction analysis in the scanning electron microscope and also by X-ray computed tomography. It is found that serration plays a more crucial role in the high-temperature/low-stress regime when an intergranular cracking mechanism involving cavitation is operative; here it plays a role in improving both creep life and creep ductility. Any effect of serration is less prevalent at low temperatures where transgranular failure is dominant.