Comparison of quartz crystallographic preferred orientations identified with optical fabric analysis,electron backscatter and neutron diffraction techniques

Three techniques are used to measure crystallographic preferred orientations (CPO) in a naturally deformed quartz mylonite: transmitted light cross‐polarized microscopy using an automated fabric analyser, electron backscatter diffraction (EBSD) and neutron diffraction. Pole figure densities attributable to crystal‐plastic deformation are variably recognizable across the techniques, particularly between fabric analyser and diffraction instruments. Although fabric analyser techniques offer rapid acquisition with minimal sample preparation, difficulties may exist when gathering orientation data parallel with the incident beam. Overall, we have found that EBSD and fabric analyser techniques are best suited for studying CPO distributions at the grain scale, where individual orientations can be linked to their source grain or nearest neighbours. Neutron diffraction serves as the best qualitative and quantitative means of estimating the bulk CPO, due to its three‐dimensional data acquisition, greater sample area coverage, and larger sample size. However, a number of sampling methods can be applied to FA and EBSD data to make similar approximations.     

EBSD characterization of carbide–carbide boundaries in WC–Co composites

A sample of WC‐6wt%Co was investigated for grain boundary character distribution and occurrence of coincidence site lattice (CSL) boundaries on a statistical basis. For this purpose orientation measurements of the grains were carried out using electron back‐scattered diffraction (EBSD). The dominant misorientation relationships were determined by complementary EBSD data representation tools such as orientation maps, misorientation angle distribution histograms and the sectioned three‐dimensional misorientation space. It was found that the grain boundary character distribution of the material is nearly random and the CSL boundaries are not present in statistically significant amounts. It was also found that the amount of binder phase does not play a role in the formation of special boundaries. The paper focuses on the methodology of characterizing grain boundaries in a hexagonal material using EBSD.     

Reconstruction of grains and subgrains from electron backscatter diffraction maps

Electron backscatter diffraction maps are capable of yielding a substantial amount of quantitative information about grains, subgrains and boundaries, and the amount and quality of the data may be substantially increased if the pixels of the map are re‐analysed so as to ‘reconstruct’ complete grains or subgrains. The paper discusses the various methods of grain reconstruction and the use of such methods to obtain microstructural information correlating the parameters of dimension, position, orientation and misorientation, which cannot usually be obtained by other means. Grain reconstruction also reveals the nature, location and contacts of all the triple junctions in the microstructure, and the paper discusses two important examples of how these data may be further analysed using automated routines. Boundary connectivity and the length and direction of likely paths along which grain boundary events such as creep fracture or stress corrosion may occur can readily be determined. The overall alignment of boundaries in deformed metals, with respect to the crystallography and the deformation geometry, may be determined as a function of the length and misorientation of the boundary segments.     

The analysis of quartz c-axis fabrics using a modified optical microscope

A new fully automated microfabric analyzer (MiFA) is described that can be used for the fast collection of high‐resolution spatial c‐axis orientation data from a set of digital polarized light images. At the onset of an analysis the user is presented with an axial‐distribution diagram (AVA –‘Achsenverteilungsanalyse’) of a thin section. It is then a simple matter to build‐up c‐axis pole figures from selected areas of interest. The c‐axis inclination and colatitudes at any pixel site is immediately available to create bulk fabric diagrams or to select measurements in individual areas. The system supports both the interactive selection of c‐axis measurement sites and grid array selection. A verification process allows the operator to exclude dubious measurements due to impurities, grain boundaries or bubbles. We present a comparison of bulk and individual c‐axis MiFA measurements to pole figures measured with an X‐ray texture goniometer and to data collected from a scanning electron microscope furnished with electron backscatter diffraction (EBSD) facility. A second sample, an experimentally deformed quartzite, illustrates that crystal orientations can be precisely linked to any location within an individual grain.     

Investigation of the microstructure and mesotexture formed during thermomechanical controlled rolling in microalloyed steels

Thermomechanical controlled rolling (TMCR) has been widely used as an effective method to improve the properties of steels. In the present paper, two commercial TMCR microalloyed steels were investigated. The grain size distributions and grain boundary misorientation angles were measured using electron backscattered diffraction patterns obtained using orientation imaging microscopy (OIM). The equivalent grain diameters were also measured using optical microscopy. Mixed coarse‐ and fine‐grained regions were observed and the microhardness values were measured for both areas. Grain boundary misorientation angle distributions showed that the magnitude of mesotexture developed in the steel is dependent upon the rolling passes, the reduction ratio and the rolling temperatures including the finish rolling temperature. The surface layer, up to 2 mm depth, in Com‐A steel had about 55% of grain boundaries with a misorientation angle below 12°, much higher than in the central area (～30%), whereas Com‐B steel showed similar distributions from the rolling surface to the centre. Misorientation results obtained from laboratory rolled steel plates with various TMCR procedures are also discussed as are OIM results from Charpy impact brittle fracture surfaces examining the effect of misorientation on crack propagation.     

Three-dimensional EBSD study on the relationship between triple junctions and columnar grains in electrodeposited Co–Ni films

Electrodeposited nanocrystalline materials are expected to have a homogeneous grain size and a narrow grain size distribution. In Co–Ni electrodeposited films, however, under certain conditions an undesired columnar grain structure is formed. Fully automated three‐dimensional (3D) orientation microscopy, consisting of a combination of precise material removal by focussed ion beam and subsequent electron backscatter diffraction (EBSD) analysis, was applied to fully characterize the grain boundaries of these columnar grains in order to gain further understanding on their formation mechanisms. Two‐dimensional orientation microscopy on these films indicated that the development of columnar grains could be related to the formation of low‐energy triple junctions. 3D EBSD allowed us to verify this suggestion and to determine the boundary planes of these triples. The triplets are formed by grain boundaries of different quality, a coherent twin on the {} plane, an incoherent twin and a large‐angle grain boundary. These three boundaries are related to each other by a rotation about the 〈〉 direction. A second particularity of the columnar grains is the occurrence of characteristic orientation gradients created by regular defects in the grain. Transmission electron microscopy was applied to investigate the character of the defects. For this purpose, a sample was prepared with the focussed ion beam from the last slice of the 3D EBSD investigation. From the TEM and 3D EBSD observations, a growth mechanism of the columnar grains is proposed.     

The influence of low-strain thermo-mechanical processing on grain boundary network characteristics in type 304 austenitic stainless steel

Grain boundary engineering of austenitic stainless steel, through the introduction of plastic strain and thermal annealing, can be used to develop microstructures with improved resistance to inter‐granular degradation. The influence of low‐strain thermo‐mechanical processing on grain boundary network development, with systematic variations of annealing treatments, has been investigated. Three stages of the microstructure development during grain boundary engineering in low‐strain processing conditions are identified, and correlated with changes in grain boundary character and deviation distributions. Low‐energy connected length segments at triple junctions, which have been proposed to be responsible for crack bridging during inter‐granular stress corrosion cracking, can be influenced by the choice of the annealing treatment parameters. The development of individual grain boundary length segments of different character showed consistent trends with increasing grain size. Crack length predictions are consistent with the beneficial effect of designing microstructures with high fractions of twin grain boundaries and smaller grain size.     

Extraction and parametrization of grain boundary networks in glacier ice,using a dedicated method of automatic image analysis

Microstructure analysis of polar ice cores is vital to understand the processes controlling the flow of polar ice on the microscale. This paper presents an automatic image processing framework for extraction and parametrization of grain boundary networks from images of the NEEM deep ice core. As cross‐section images are acquired using controlled surface sublimation, grain boundaries and air inclusions appear dark, whereas the inside of grains appears grey. The initial segmentation step of the software is to separate possible boundaries of grains and air inclusions from background. A Machine learning approach is utilized to gain automatic, reliable classification, which is required for processing large data sets along deep ice cores. The second step is to compose the perimeter of section profiles of grains by planar sections of the grain surface between triple points. Ultimately, grain areas, grain boundaries and triple junctions of the later are diversely parametrized. High resolution is achieved, so that small grain sizes and local curvatures of grain boundaries can systematically be investigated.     

In situ observation of intergranular crack nucleation in a grain boundary controlled austenitic stainless steel

Grain boundary engineering has been proposed to increase the lifetime performance of sensitized austenitic stainless steel in aggressive environments. Increased microstructure resistance is typically associated with higher fractions of twin (Σ3) grain boundaries, but there is uncertainty about the properties and role of other boundaries. To develop predictive models for stress corrosion crack nucleation, more information is required about how grain boundary crystallography and the orientations of the grain boundary plane and its surrounding grains affect crack development. Digital image correlation combined with electron backscatter diffraction has been used to characterize the microstructure and to observe, in situ , the nucleation and propagation of short stress corrosion cracks in thermo-mechanically processed type 304 stainless steel. The crack path and its growth rate have been determined and are found to be influenced by the microstructure.     

Orientation independent retardation imaging using quantitative polarized phase microscopy

Simultaneous optical phase and retardation measurement of a birefringent specimen is demonstrated independently of a priori knowledge of the optic axis orientation. The two‐dimensional retardation distribution in both magnitude and angle of the fast axis orientation is uniquely determined from transverse phase images recorded with a bright field transmission microscope using light polarized at a minimum of three different polarization orientations. This approach opens a new possibility for stain‐free phase and orientation‐independent retardation characterization of samples using only one polarizer without needing other additional optical elements traditionally used in polarimetric measurements. Microsc. Res. Tech. 2012. © 2012 Wiley Periodicals, Inc.     

Grain boundary character distributions in Ni-16Cr-9Fe using selected area channeling patterns: Methodology and results

Selected area channeling patterns imaged on an SEM are digitized and displayed on the screen of a Macintosh computer, on which the user selects channeling bands that are measured to determine orientation. Grain boundary misorientations are found using the orientation information for pairs of grains adjacent at grain boundaries, and the boundaries are classified as low angle boundaries (LABs), coincident site lattice boundaries (CSLBs), or general boundaries (GHABs) based on the misorientation information. The technique was implemented to analyze the grain boundary character distributions (GBCDs) in Ni-16Cr-9Fe. The GBCDs of solution annealed material were similar to those expected in an aggregate of randomly oriented polycrystals. However, sequential thermomechanical treatments (5% tensile strain + 945°C:75 min + 2% tensile strain + 890°C:15 h + 3% tensile strain + 890°C:20 h or 9% compressive strain + 890°C:20 h + 9% compressive strain + 890°C:20 h + 3% compressive strain + 890°C:15 h) applied after the solution anneal lowered the proportions of GHABs in the GBCDs from 76–79% to 47–64%. The CSL-enhanced GBCDs of both the tensile-deformed samples and the compression-deformed sample appear to have evolved mainly through impingement of twin and twin-related boundaries during recrystallization; the CSL-enhanced GBCD of a compression-deformed sample appears to have been influenced by grain rotation processes to a greater degree than were the tensile-deformed samples. The CSL boundaries in the CSL-enhanced GBCDs were, in general, closer to the exact CSL misorientations than were those in the near-random GBCDs of the solution annealed material. An analysis of the distribution of misorientation axes did not indicate any correlation between grain misorientation texture and GBCD evolution.     

The use of combined cathodoluminescence and EBSD analysis: a case study investigating grain boundary migration mechanisms in quartz

Grain boundary migration is an important mechanism of microstructural modification both in rocks and in metals. Combining detailed cathodoluminescence (CL) and electron backscatter diffraction (EBSD) analysis offers the opportunity to relate directly changes in crystallographic orientation to migrating boundaries. We observe the following features in naturally heated quartz grains from the thermal aureole of the Ballachulish Igneous Complex (Scotland, U.K.): (a) propagation of substructures and twin boundaries in swept areas both parallel and at an angle to the growth direction, (b) development of slightly different crystallographic orientations and new twin boundaries at both the growth interfaces and within the swept area and (c) a gradual change in crystallographic orientation in the direction of growth. All these features are compatible with a growth mechanism in which single atoms are attached and detached both at random and at preferential sites, i.e. crystallographically controlled sites or kinks in boundary ledges. Additionally, strain fields caused by defects and/or trace element incorporation may facilitate nucleation sites for new crystallographic orientations at distinct growth interfaces but also at continuously migrating boundaries. This study illustrates the usefulness of combined CL and EBSD in microprocess analysis. Further work in this direction may provide detailed insight into both the mechanism of static grain growth and the energies and mobilities of boundaries in terms of misorientation and grain boundary plane orientation.     

Segregation-induced hole drilling at grain boundaries

Grain boundaries in metals provide preferential sites for impurity atoms to segregate, forming an area of the metal with different chemical and physical properties compared to the bulk. This also means that the electron beam damage characteristics of the grain boundary might be different from that of the bulk. Segregation can be expected to make grain boundaries particularly vulnerable to electron-beaminduced hole drilling and beam damage, if the segregation produces a near two-dimensional compound with increased ionicity. Here we report preferential hole drilling at grain boundaries in an Fe-0–4 wt%P alloy. Such a phenomenon confirms evidence from energy-loss spectroscopy that charge transfer occurs between P and Fe at the grain boundary. It also explains why it is easier to detect phosphorus using EDX compared to PEELS, and suggests that the electron beam damage may be a major limiting factor in grain-boundary studies.     

AFM analysis of anisotropic dissolution in dense hydroxyapatite

Dense, polycrystalline hydroxyapatite (HA) ceramics prepared by microwave sintering process were immersed for 1-60s in an acidic solution of pH 3, similar to the circumstance of osteoclastic resorption. Dissolution behavior on the surface of HA immersed in the solution was investigated. The surface topography was evaluated by using atomic force microscopy (AFM) with tapping mode. The erosion of grain boundaries in the dense HA was observed and the grain boundary dissolution occurred to a great extent forming a wide gap (50-100nm) between grains. Notably, anisotropic dissolution was observed on the HA surface showing two topographically different regions; grains (Grain 1) with relatively intact surface, and grains (Grain 2) involving formation of subgrain boundaries and aligned crystallites in a certain direction. The difference in RMS surface roughness evidenced the distinct dissolution feature of ceramic grains. The surface roughness of Grain 1 increased from 1.45+/-0.30 to 3.11+/-0.63nm, whereas the roughness of Grain 2 increased from 2.36+/-0.11 to 5.27+/-1.34nm.     

'Five-parameter' analysis of grain boundary networks by electron backscatter diffraction

This paper describes state‐of‐the‐art analysis of grain boundary populations by EBSD, with particular emphasis on advanced, nonstandard analysis. Data processing based both on misorientation alone and customised additions which include the boundary planes are reviewed. Although commercial EBSD packages offer comprehensive data processing options for interfaces, it is clear that there is a wealth of more in‐depth data that can be gleaned from further analysis. In particular, determination of all five degrees of freedom of the boundary population provides an exciting opportunity to study grain boundaries by EBSD in a depth that was hitherto impossible. In this presentation we show ‘five‐parameter’ data from 50 000 boundary segments in grain boundary engineered brass. This is the first time that the distribution of boundary planes has been revealed in a grain boundary engineered material.     

EBSD and TEM investigation of the hot deformation substructure characteristics of a type 316L austenitic stainless steel

The evolution of crystallographic texture and deformation substructure was studied in a type 316L austenitic stainless steel, deformed in rolling at 900 °C to true strain levels of about 0.3 and 0.7. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. At the lower strain level, the deformation substructure observed by EBSD appeared to be rather poorly developed. There was considerable evidence of a rotation of the pre‐existing twin boundaries from their original orientation relationship, as well as the formation of highly distorted grain boundary regions. In TEM, at this strain level, the substructure was more clearly revealed, although it appeared rather inhomogeneously developed from grain to grain. The subgrains were frequently elongated and their boundaries often approximated to traces of {111} slip planes. The corresponding misorientations were small and largely displayed a non‐cumulative character. At the larger strain, the substructure within most grains became well developed and the corresponding misorientations increased. This resulted in better detection of sub‐boundaries by EBSD, although the percentage of indexing slightly decreased. TEM revealed splitting of some sub‐boundaries to form fine microbands, as well as the localized formation of microshear bands. The substructural characteristics observed by EBSD, in particular at the larger strain, generally appeared to compare well with those obtained using TEM. With increased strain level, the mean subgrain size became finer, the corresponding mean misorientation angle increased and both these characteristics became less dependent on a particular grain orientation. The statistically representative data obtained will assist in the development of physically based models of microstructural evolution during thermomechanical processing of austenitic stainless steels.     

The use of ArcGIS for determination of quartz optical axis orientation in thin section images

In this paper, a new toolbox for petrographic thin section analysis is presented. It was designed using ArcGIS ModelBuilder to extract quartz grain boundary and determine its optical axis orientation from a set of thin section images. It can perform these analyses with little or no operator intervention and convert the boundary and optical axis data into a digital database with preserved spatial relationship. In order to test the validity of toolbox's outputs, an assessment was performed using three natural quartzite thin sections. The results indicate a close relationship (>0.8) between manual and model‐base extraction of grain boundary and optical axis orientation data. This study demonstrates the feasibility and usefulness of performing thin section image analysis within the GIS framework.     

Single-section plane assessment in grain boundary engineered brass

The present paper reports a comparative analysis of Σ3 (in the coincidence site lattice notation) grain boundary types, in two grain boundary engineered brass specimens, by use of electron backscatter diffraction (EBSD) data coupled to the measurement of boundary traces in a single section. Although most of the data were analysed using the new single‐section technique, an analysis of boundary plane orientations in three dimensions was made in a subset of the data in order to validate the single section methodology. The single‐section trace analysis procedure, coupled with EBSD, is a viable and robust tool for analysis of Σ3 grain boundary planes. The procedure provides data which suggest that part of the enhanced strain‐to‐failure in specimen B compared to specimen A is the result of an increased proportion of mobile Σ3 boundaries, i.e. those which are displaced from the {111} symmetrical tilt configuration.     

A technique for recording polycrystalline structure and orientation during in situ deformation cycles of rock analogues using an automated fabric analyser

Two in situ plane-strain deformation experiments on norcamphor and natural ice using synchronous recording of crystal c-axis orientations have been performed with an automated fabric analyser and a newly developed sample press and deformation stage. Without interrupting the deformation experiment, c-axis orientations are determined for each pixel in a 5 × 5 mm sample area at a spatial resolution of 5 μm/pixel. In the case of norcamphor, changes in microstructures and associated crystallographic information, at a strain rate of ～2 × 10(-5) s(-1), were recorded for the first time during a complete in situ deformation-cycle experiment that consisted of an annealing, deformation and post-deformation annealing path. In the case of natural ice, slower external strain rates (～1 × 10(-6) s(-1)) enabled the investigation of small changes in the polycrystal aggregate's crystallography and microstructure for small amounts of strain. The technical setup and first results from the experiments are presented.     

First combined electron backscatter diffraction and transmission electron microscopy study of grain boundary structure of deformed quartzite

The structures of boundaries in a deformed and dynamically recovered and recrystallized quartz polycrystal (mylonite) were characterized by transmission electron microscopy, after the misorientation angles across the same grain boundaries had been analysed using electron backscatter diffraction in a scanning electron microscope. In this new approach, a specific sample area is mapped with electron backscatter diffraction, and the mapped area is then attached to a foil, and by the ion beam thinned for transmission electron microscopy analysis. Dislocations in grain boundaries were recognized as periodic and parallel fringes. The fringes associated with dislocations are observed in boundaries with misorientations less than 9°, whereas such fringes cannot be seen in the boundaries with misorientations larger than 17°. Some boundaries with misorientations between 9° and 17° generally have no structures associated with dislocation. One segment of a boundary with a misorientation of 13.5° has structures associated with dislocations. It is likely that the transition from low‐angle to high‐angle boundaries occurs at misorientations ranging from approximately 9° to 14°. Change in the grain boundary structure presumably influences the mobility of the boundaries. In the studied deformed quartz vein, a relative dearth of boundaries between misorientation angles of θ = 2° and θ = 15° has previously been reported, and high‐angle boundaries form cusps where they intersect low‐angle boundaries, suggesting substantial mobility of high‐angle boundaries.