Applications of electron backscatter diffraction to materials science: status in 2009

Over the last two decades electron backscatter diffraction (EBSD) in the scanning electron microscope has become a powerful tool for the characterisation of crystalline materials. Via an in-depth analysis of published work in 2008 compared with 2003, this article captures the present contribution that EBSD is making to materials science. From the analysis it is shown that some aspects of EBSD application have increased greatly in recent years, particularly the range of materials analysed, microtexture determination, general microstructure characterisation, application to interfaces and combinations of EBSD with other applications such as modelling or materials testing. On the other hand some other applications of EBSD are still emerging, such as macrotexture determination, true phase identification and three-dimensional EBSD.     

Phase identification of oxide scale on low carbon steel

Abstract

The classical model of iron oxide scale layers; wüstite, magnetite and hematite, is more complicated in reality and its properties change with the factors that affect their development, such as time, temperature, alloying elements in the steel and oxidation environment. An understanding of the oxide scale formation and its properties can only be achieved by careful examination of the scale microstructure. Throughout this study, electron back-scatter diffraction (EBSD) has proved to be a powerful technique for verifying the individual phases in the oxide scale accurately. The results show that EBSD can be used to give a complete characterisation of the oxide scale formed on low carbon steel surface. Microstructural features such as grain size, shape and grain boundary characteristics have been successfully determined and analysed.     

3D reconstruction and characterization of polycrystalline microstructures using a FIB–SEM system

A novel methodology is described in this paper which is a step towards three-dimensional representation of grain structures for microstructure characterization and processing microstructural data for subsequent computational analysis. It facilitates evaluation of stereological parameters of grain structures from a series of two-dimensional (2D) electron backscatter diffraction (EBSD) maps. Crystallographic orientation maps of consecutive serial sections of a micron-size specimen are collected in an automated manner using a dual-beam focused ion beam–scanning electron microscope (FIB–SEM) outfitted with an EBSD system. Analysis of the serial-sectioning data is accomplished using a special purpose software program called “Micro-Imager”. Micro-Imager is able to output characterization parameters such as the distribution of grain size, number of neighboring grains, and grain orientation and misorientation for every 2D section. Some of these data can be compared with results from stereological exercises. Stacking the 2D statistical information obtained from the analysis of the serial-sectioning data provides a means to quantify the variability of grain structure in 3D.     

A procedure for indirect and automatic measurement of prior austenite grain size in bainite/martensite microstructures

An alternative procedure for indirect and automatic measurement of the prior austenite grain size (PAGS) in bainite/martensite is proposed in this work. It consists in the determination of an effective grain size by means of statistical post-processing of electron backscatter diffraction (EBSD) data. The algorithm developed for that purpose, which is available on-line, has been applied to simulated EBSD maps as well as to both nanocrystalline bainitic steel and commercial hot-rolled air-cooled steel with a granular bainitic microstructure. The new proposed method has been proven to be robust, and results are in good agreement with conventional PAGS measurements. The added value of the procedure comes from its simplicity, as no parent reconstruction is involved during the process, and its suitability for low-magnification EBSD maps, thus allowing a large step size and coverage of a substantially broader area of the sample than the previous methods reported.     

EBSD – orientation analysis of monocrystalline diamonds used for diamond metal composites – Influence of sample preparation

This paper focuses on a new field of application for the EBSD‐technique. Generally, EBSD‐mappings are performed on different metal alloys used for quality assurance and to get information about the microstructure regarding grain orientation, grain size and distribution. In contrast, the orientation determination of monocrystalline diamond grains with an EBSD system is not a conventional method. Thus, this work describes the EBSD testing sequence in detail and illustrates the preparation of orientation data for a statistical design. Furthermore, dependencies of the sample preparation, alignment to the detector, and the analyzed position on the diamond on the quality of the Kikuchi‐patterns, respectively on the indexing rates, have been scrutinized. Finally, the orientation obtained of each tested diamond sample has been utilized in a statistical design to show a direct influence of the crystal orientation on the wear behavior of the diamond grains.     

Review Grain and subgrain characterisation by electron backscatter diffraction

The application of automated Electron Backscatter Diffraction (EBSD) in the scanning electron microscope, to the quantitative analysis of grain and subgrain structures is discussed and compared with conventional methods of quantitative metallography. It is shown that the technique has reached a state of maturity such that linescans and maps can routinely be obtained and analysed using commercially available equipment and that EBSD in a Field Emission SEM (FEGSEM) allows quantitative analysis of grain/subgrains as small as 0.2 m. EBSD can often give more accurate measurements of grain and subgrain size than conventional imaging methods, often in comparable times. Subgrain/cell measurements may be made more easily than in the TEM although the limited angular resolution of EBSD may be problematic in some cases. Additional information available from EBSD and not from conventional microscopy, gives a new dimension to quantitative metallography. Texture and its correlation with grain or subgrain size, shape and position are readily measured. Boundary misorientations, which are readily obtainable from EBSD, enable the distribution of boundary types to be determined and CSL boundaries can be identified and measured. The spatial distribution of Stored Energy in a sample and the amount of Recrystallization may also be measured by EBSD methods.     

The coincidence site lattice and the ‘sigma enigma’

This paper focuses on the most appropriate use of electron back-scatter diffraction (EBSD) post-acquisition analysis for categorisation of grain boundaries in polycrystals. A brief survey of the early literature shows that the most meaningful reference structure for grain boundaries in polycrystals is periodicity in the grain boundary surface, rather than the misorientation-based coincidence site lattice (CSL) and Σ notation. However, use of the CSL is convenient to the experimentalist. It is therefore suggested that wherever possible, misorientation data, obtained by EBSD mapping and classified according to CSL types, should be supplemented with other, more detailed information to aid analysis.     

Five‐Parameter Grain Boundary Analysis by 3D EBSD of an Ultra Fine Grained CuZr Alloy Processed by Equal Channel Angular Pressing

The 3D grain boundary character distribution (GBCD) of a sample subjected to equal channel angular pressing (ECAP) after eight passes and successive annealing at 650 °C for about 10 min is analyzed. The experiments are conducted using a dual beam system, which is a combination of a focused ion beam and a scanning electron microscope to collect a series of electron backscatter diffraction (EBSD) maps of the microstructure (3D EBSD). The data set was aligned and reconstructed to a 3D microstructure. The crystallographic character of the grain boundary planes was determined using three different methods, namely, the line segment method, the stereological method, and the triangular surface mesh method. The line segment and triangular surface mesh methods produce consistent data sets, both yielding approximately a 7% area fraction of coherent twins. These results starkly contrast that of the statistical stereological method, which produced a 44% area fraction of coherent twins.     

Discrimination and mapping of phase distributions by automated crystal orientation measurement

Abstract

Automated crystal orientation measurement/mapping (ACOM) in the SEM enables a fast phase discrimination and the automated acquisition of phase distribution maps at submicrometre resolution, if the phases belong to different Laue groups or if their lattice constants are at least significantly different. The database can further be used for quantitative determination of phase contents, phase boundary characterisation, and mesoscale texture analysis for each phase, such as crystal orientation mapping (COM), texture analysis (orientation density function, pole figures, misorientation distribution function), characterisation of grain and phase boundaries, and orientation stereology.     

Inhomogeneous deformation of {111}<uvw> grain in cold rolled tantalum

The microstructures of {111}<uvw> grain were characterized in detailed and systematically investigated with the aid of electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The stored energy in different regions in the grain was evaluated by the band contrast values collected from EBSD. The results show that the distribution of energy is inhomogeneous through the grain. Especially, the regions containing the least and the largest energy were extracted from the EBSD data, and then quantitatively analyzed based on the misorientation and Schmid factor. Many peaks with large misorientation appeared in the region containing larger energy, and these peaks represent the existences of micro-bands and micro-shear bands in {111} grain. The results of Schmid factor suggest that the region containing larger energy is prone to deforming ahead of the region with less stored energy, implying the more serious subdivision of the microstructure of region with larger stored energy.     

Triple junction distribution profiles as assessed by electron backscatter diffraction

The connectivity between the boundaries is very important because the defect character of triple junctions is expected to have a significant influence on the bulk properties of materials, particularly mechanical behaviour. The investigation of triple junction investigations presented here indicates that a restricted Coincidence Site Lattice (CSL) model was found to be the most relevant and practicable for the characterisation of triple junctions in Grain Boundary Engineered materials. The triple junction character distribution was measured using the automated Electron Back-Scatter Diffraction (EBSD) application of Crystal Orientation Mapping (COM) for a series of thermomechanically processed copper and alpha-brass specimens. Triple junction character statistics were determined from COM data, automatically using a custom-built computer program, utilising the CSL model. The alpha-brass data were then compared with a series of previously acquired triple junction data for a series of strain-recrystallisation copper specimens. The main aim of the investigation was to determine the relationship between the grain boundary and triple junction character distributions, which was found to be essentially a linear relationship.     

薄规格取向硅钢二次再结晶过程中Goss织构演变的Monte Carlo模拟

通过EBSD实验获取了薄规格取向硅钢(0.18 mm厚)初次再结晶样品表面晶粒组织的取向数据,并以此构建模拟的初始组织.采用Potts模型Monte Carlo方法对薄规格取向硅钢初次再结晶样品的二次再结晶过程进行了模拟仿真,研究了表面能对Goss织构演变的影响.模拟结果表明:Goss取向晶粒与相邻晶粒的表面能差是Goss取向晶粒异常长大的重要驱动力;表面能差存在一个临界值(约12％),只有当表面能差大于此临界值时才会发生表面能驱动Goss取向晶粒的异常长大.     

Investigation of small crack behaviour under cyclic loading in a dual phase steel with an FIB tomography technique

Damage accumulation is investigated in the early stage of fatigue life in a ferritic martensitic dual phase steel. Microcrack initiation and propagation are influenced by microstructure, such as grain boundaries, grain orientation and/or phase distribution morphology. The dominant crack initiation pattern is one in which microcracks are generated in a ferrite grain along slip bands inclined at a certain angle with respect to the loading direction. Subsurface observation with a focused ion beam (FIB) device and additional crystallographic characterization by means of the electron backscatter diffraction (EBSD) technique show that a slip system having a high Schmid factor value is activated and results in a crack nucleus. The FIB tomography technique with the help of EBSD measurement allows a three‐dimensional investigation of small crack behaviour to be performed.     

Experimental Determination of Deformation Induced Lattice Rotation by EBSD Technique for Slip System Analysis

A method was proposed to experimentally determine the deformation induced lattice rotation by electron backscatter diffraction(EBSD) technique,based on which the activated slip systems could be predicted.The method is to create a project file including the EBSD data of the sample before and after deformation,which allows the lattice rotation to be calculated and visualized using the commercial EBSD software.This method was applied to a polycrystalline Nisubjected to quasi-static compression.The lattice rotation of one grain was calculated and visualized and the activated slip systems were predicted.The comparison with the slip systems predicted by full-constraints(FC) Taylor model highlights the advantage of the present method.     

Orientation imaging microscopy of polycrystalline sodium chloride

A novel preparation technique is described that makes possible grain size analysis of polycrystalline NaCl using orientation imaging microscopy via electron backscatter diffraction (EBSD). The preparation methodology is specifically developed to overcome difficulties in preparing microporous NaCl for microscopy. The grain size and crystallographic texture of polycrystalline NaCl samples, prepared via solution pressure and sintered in the range of 650–780 °C, were able to be measured successfully with EBSD. The limitations of the preparation technique for EBSD analysis of NaCl are also discussed.     

Microtexture determination by electron back-scatter diffraction

This review describes the use of an experimental technique known as electron back-scatter diffraction (EBSD) to measure microtexture, that is, spatially specific texture measured on an individual orientation basis. Other methods of microtexture determination are briefly described and compared with EBSD. The EBSD technique itself is described in considerable detail including recent developments such as on-line automation. Those EBSD-based microtexture studies which have been reported in the literature are summarized, including those which provide a direct comparison with macrotexture measurements obtained by X-ray diffraction. The concept of microtexture as the texture of individual grains leads naturally to the idea of mesotexture as the texture of grain boundaries. Mesotexture data can be computed from the EBSD-generated microtexture measurements and this is demonstrated and examples are given. Examples of microtexture/mesotexture studies in multiphase materials are also shown. Finally, because EBSD allows the simultaneous determination of microtexture/mesotexture and microstructural information, it is pertinent to discuss ways of displaying this sort of data, and so the review is completed by a discussion of the representation of microtextures and mesotextures, including the use of Rodrigues-Frank space and orientation mapping.     

Electron microscopy analysis of structural changes within white etching areas

In the present work, crack networks with white etching areas (WEAs) in cross-sections of bearings were investigated by a complementary use of SEM and TEM with the focus on the use of orientation contrast imaging and electron backscatter diffraction (EBSD). Orientation contrast imaging was used for the first time to give detailed insight into the microstructure of WEA. A significant difference between Nital-etched and polished WEA samples was observed. It was revealed that WEAs are composed of different areas with varying grain sizes. As a result of secondary transformation, needle-shaped grains were observed within WEAs. Using EBSD analysis, evidence was obtained that WEA formation and accompanying crack growth are without relation microstructural features. In addition, an inhomogeneous chemical structure of WEA as a result of carbide dissolution is revealed by analytical investigations.     

Evaluation of macrozone dimensions by ultrasound and EBSD techniques

Titanium alloys are known to have texture heterogeneities, i.e. regions much larger than the grain dimensions, where the local orientation distribution of the grains differs from one region to the next. The electron backscattering diffraction (EBSD) technique is the method of choice to characterize these macro regions, which are called macrozones. Qualitatively, the images obtained by EBSD show that these macrozones may be larger or smaller, elongated or equiaxed. However, often no well-defined boundaries are observed between the macrozones and it is very hard to obtain objective and quantitative estimates of the macrozone dimensions from these data. In the present work, we present a novel, non-destructive ultrasonic technique that provides objective and quantitative characteristic dimensions of the macrozones. The obtained dimensions are based on the spatial autocorrelation function of fluctuations in the sound velocity. Thus, a pragmatic definition of macrozone dimensions naturally arises from the ultrasonic measurement. This paper has three objectives: 1) to disclose the novel, non-destructive ultrasonic technique to measure macrozone dimensions, 2) to propose a quantitative and objective definition of macrozone dimensions adapted to and arising from the ultrasonic measurement, and which is also applicable to the orientation data obtained by EBSD, and 3) to compare the macrozone dimensions obtained using the two techniques on two samples of the near-alpha titanium alloy IMI834. In addition, it was observed that macrozones may present a semi-periodical arrangement.     

Characterization of microstructural damage due to low-cycle fatigue by EBSD observation

Electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy was used to assess the damage due to cyclic or uniform strain. Samples of Type 316 stainless steel after fatigue and tensile tests were prepared for EBSD observation and the misorientation angle between neighboring points (local misorientation) was evaluated. It was shown that the local misorientation developed due to the cyclic and uniform strain and that its spatial distribution was not uniform. In fatigue samples, the area of large local misorientation tended to form clusters, whereas it localized to the grain boundaries in the tensile samples, and the magnitude of local misorientation and the degree of the localization increased with the strain amplitude. The degree of localization was quantified via statistical processing of the measured data. It was also shown that the source of damage (cyclic or uniform strain) and the loading direction could be deduced from the EBSD observations of the damaged sample.     

Microstructure and magnetic properties of BaTiO3-(Ni,Zn)Fe2O4 multiferroics

The microstructures of composite xBaTiO₃-(1−x)(Ni_0.5Zn_0.5)Fe₂O₄ (BT-NZF) multiferroics with various mixing ratios (x = 0.50, 0.60 and 0.70) are investigated by means of electron backscatter diffraction (EBSD) and magnetic force microscopy (MFM). The EBSD measurements reveal a change in the texture of the ferrite and the BaTiO₃ grains upon increasing the ferrite content in the sample. The sample with x = 0.70 exhibits the best ferrite texture, where only some directions are present. Furthermore, the resulting grain sizes vary from several µm (x = 0.50) to about 100 nm in the sample with x = 0.70. The MFM images reveal the presence of magnetic domains being extended over several adjacent grains, which according to the EBSD data may comprise different crystallographic orientations. In this way, we can explain the differences in the magnetic contrast obtained.