Misorientation distributions in hot deformed NaCl using electron backscattered diffraction

EBSD orientation mapping has been used to derive subgrain boundary misorientation distributions in a series of hot deformed and etched NaCl samples. The main objective of this study has been to examine the influence of data processing, noise caused by angular resolution limits and step size on the subgrain misorientation distributions in hot deformed NaCl. Processing of non‐indexed EBSD patterns increased the average misorientations in etched NaCl. Noise contributed significantly to low angle misorientation peaks for step sizes less than the minimum subgrain size. Orientation data collected using a step size larger than the average subgrain size cumulated misorientations across individual subgrains and effectively measured an orientation gradient between steps. Orientation gradient distributions were not influenced by noise. Average misorientation values calculated from large step data correlated well with average misorientation from small step size data. Average misorientations showed a power law relationship with strain. Three types of substructures were identified using scanning electron microscopy and EBSD mapping, equiaxed subgrains, long subgrain boundaries and a core‐mantle subgrain arrangement.     

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.     

Visualization of grain subdivision by analysing the misorientations within a grain using electron backscatter diffraction

The misorientation relative to the average orientation of a grain and the point-to-point relative misorientation along a line across a moderately cold deformed grain, calculated from an electron backscatter diffraction (EBSD) dataset, are analysed in detail by visualizing both the misorientation angle and the misorientation axis. The significance of monitoring the misorientation axis is illustrated by an example of a grain subdivided into a misorientation band structure. A new technique to visualize the subdivision structure by assigning colours to misorientations in such a way that the contrast is maximized within a grain is introduced and discussed. Furthermore, some methods for grain boundary reconstruction from EBSD datasets are compared with the map of the confidence index in order to provide a validation of the accuracy of these methods.     

The interpretation of indexing of high Σ CSL grain boundaries from ceramics

Electron backscatter diffraction (EBSD) is a useful technique for measuring the orientation of individual grains and for determining grain boundary misorientations in polycrystals. However, its application to ceramics is more difficult than to metals, because the surface quality that can be achieved often makes the Kikuchi patterns blurred. As a consequence, it can be difficult, even for automated systems, to differentiate between different grain orientations, which have similar patterns. In this paper, we carry out EBSD analyses of SrTiO₃ polycrystalline material prepared with different polishing methods, and we consider the effect of different criteria in interpreting the EBSD patterns from them. In particular, we investigate the CSL statistics using both the Palumbo and Aust and the Brandon criteria in this situation.     

Analysis of local orientation gradients in deformed single crystals

Grain fragmentation and local orientation gradients in deformed single crystals are characterized using electron backscatter diffraction (EBSD) to obtain statistically reliable information. Interrogation of the dislocation substructure is accomplished by extracting information gleaned from small point-to-point misorientations as measured by EBSD. Along with an estimate of the geometrically necessary dislocation (GND) content, the point-to-point deviation from an average grain orientation is described by an orientation difference vector defined in Rodrigues space. Mapping of parameters such as GND, and divergence and gradient fields created from analysis of the difference vectors provide an alternative approach to obtain quantitative information and images from EBSD data.     

Electron backscatter diffraction characterization of inlaid cu lines for interconnect applications

Automated electron backscatter diffraction (EBSD) techniques have been used to characterize the microstructures of thin films for the past decade or so. The recent change in strategy from an aluminum‐based interconnect structure in integrated circuits to one based on copper has necessitated the development of new fabrication procedures. Along with new processes, complete characterization of the microstructures is imperative for improving manufacturability of the Cu interconnect lines and in‐service reliability. Electron backscatter diffraction has been adopted as an important characterization tool in this effort. Cu microstructures vary dramatically as a function of processing conditions, including electroplating bath chemistry, sublayer material, stacking sequence of sublayers, annealing conditions, and line widths and depths. Crystallographic textures and grain size and grain boundary character distributions, all of which may influence manufacturability and reliability of interconnect lines, are ideally characterized using EBSD. The present discussion presents some results showing structural dependence upon processing parameters. In addition, the authors identify an in‐plane orientation preference in inlaid Cu lines {111} normal to the line surface and 〈110〉 aligned with the line direction. This relationship tends to strengthen as the line width decreases.     

'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.     

Grain size measurement by EBSD in complex hot deformed metal alloy microstructures

The measurement of grain size by EBSD has been studied to enable representative quantification of the microstructure of hot deformed metal alloys with a wide grain size distributions. Variation in measured grain size as a function of EBSD step size and noise reduction techniques has been assessed. Increasing the EBSD step size from 5% to 20% of the approximate mean grain size results in a change in calculated arithmetic mean grain size of approximately 15% and standard noise reduction techniques can produce a further change in reported size of up to 20%. The distribution of measured grain size is found not to be log‐normal, with a long tail of very small sizes in agreement with a computer simulation of linear intercept and areal grain size measurements through randomly oriented grains. Comparison of EBSD with optical measurements of grain size on the same samples shows that, because of the ability of EBSD to distinguish twins and resolve much smaller grains a difference of up to 50% in measured grain size results.     

The role of misorientation and phosphorus content on grain growth and intergranular fracture in iron–carbon–phosphorus alloys

The relationship between the crystallography of intergranular fracture and phosphorus segregation has been investigated in a Fe?0.06wt%P?0.002wt%C alloy aged for 1 h at temperatures between 600 °C and 1000 °C. Two novel techniques were devised for the investigation: first, electron back‐scatter diffraction (EBSD) across the reconstructed fracture surface and, second, a combination of Auger electron spectroscopy, stereophotogrammetry and microscopy to measure phosphorus and carbon on fracture facets combined with EBSD measurements direct from the fracture surface. In total, 700 misorientations were measured from across the reconstructed fracture surface and in ‘control’ areas away from the fracture. It was found that Σ 3s were in general more resistant to brittle fracture than were random boundaries, and it was suggested that alloys of this type could be grain boundary engineered to improve fracture resistance by a short anneal in the austenite region to increase the final proportion of Σ 3s. Sixteen fracture facets yielded combined Auger/EBSD data. The combined Auger/EBSD methodology to acquire joint crystallographic and segregation information from facets was shown to be feasible, although laborious. There were significantly more {110} planes than any other type in the sample population of facets from which combined segregation/crystallography data had been collected. The data suggested that there was on average lower phosphorus segregation on fracture facets that were near {110} than on other intergranular fracture facets.     

Effects of microstructure on material properties of modified 9Cr-1Mo steel subject to creep-fatigue

The creep-fatigue properties of modified 9Cr-1Mo steel were studied to elucidate the effects of the microstructure on specimens deformed by fatigue and creep-fatigue via stress holding for 30 min at 550°C. First, creep-fatigue behaviors up to 7.8% nominal strains were obtained with respect to the deformation conditions. Next, the micro Vickers hardness values were determined to measure the creep-fatigue properties of the deformed materials. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to determine the grain diameters and grain average misorientations (GAMs) and distributions of dislocations and precipitates, respectively. These observations explained the creep-fatigue properties of the deformed specimens. The differences in the properties of materials subjected to fatigue and creep-fatigue are discussed both macroscopically and microscopically.     

Turning induced residual stress prediction of AISI 4130 considering dynamic recrystallization

Machining-induced residual stress distribution is strongly influenced by the machining process condition, tool geometry and workpiece material mechanical properties. The high temperature, large strain and high strain rate environment will promote the material micro-structural attribute changes. The material micro-structural attribute changes could directly affect the material mechanical properties. An analytical model is proposed for the residual stress prediction in the orthogonal turning by considering the material dynamical recrystallization induced grain growth effect. The grain size effect on the material flow stress behavior is included by adding a grain size dependent term into the traditional Johnson–Cook model. The Johnson–Mehl–Avrami–Kolmogorov model calculates the recrystallized volume fraction and grain size as a function strain, strain rate and time. The average grain size is calculated with a rule of mixture by volume. Then the modified Johnson–Cook model is embedded into a classic residual stress prediction model for the machining induced residual stress profile prediction on the machined workpiece surface. Experimental tests are conducted for the model validation. The predicted residual stress shows good approximation with the measurement in both the trend and magnitude of the residual stress. Also, the effects of cutting speed and feed rate on the residual stress profile are investigated.     

A study of recrystallization in single-phase aluminium using in-situ annealing in the scanning electron microscope

In‐situ annealing experiments were performed in the scanning electron microscope on a single‐phase Al?0.13Mg alloy cold rolled to different strain levels. Once the validity of the technique had been verified by comparison of the recrystallization kinetics and final grain size with bulk annealed samples, the method was used in combination with electron back‐scattered diffraction (EBSD) to study the potential mechanisms for recrystallization in this alloy. During annealing of material rolled to moderate strains (?_t < 0.7), the primary mechanism was strain‐induced boundary migration (SIBM). In material rolled to higher true strains (?_t > 1.4), recrystallization occurred extensively along pre‐existing cube bands and EBSD measurements showed that the mean size of cells within the cube bands was larger than for all other orientations measured, suggesting a size advantage was responsible for the strengthening of cube texture during recrystallization. SIBM was shown to occur concurrently with the nucleation along cube bands but this contributed a lower proportion of nucleation sites during recrystallization.     

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.     

A new concept for producing ultrafine-grained metallic structures via an intermediate strain rate: Experiments and modeling

A novel experimental methodology to produce ultrafine-grained metallic microstructures, which is applied on aluminum is proposed in this work. In fact, the ultrafine-grained aluminum polycrystal is made from commercial purity powder by a combination of hot isostatic pressing (HIP) and dynamic severe plastic deformation (DSPD). After the first step, the bulk consolidated material showed a random texture and homogeneous microstructure of equiaxed grains with an average size of 2 μm. The material is then subsequently impacted, using a falling weight at a maximum impact velocity of 9.2 m/sec. The resulting material shows a microstructure having an average grain size of about 500 nm with a strong gradient of fiber-like crystallographic texture perpendicular to the impact direction. The mechanical properties of the impacted material are then characterized under compression tests at room temperature under a strain rate of 10^?4 s^?1. The effect of the change of the deformation path on the mechanical response parallel and perpendicular to the impact direction is also investigated. These results are discussed in relation with microstructure. Further, a new extension of a micromechanical approach developed by Abdul-Latif et al., [2] is proposed to predict the grain size effect on the enhancement of the mechanical strength of polycrystals. Within the framework of small strain hypothesis, the elastic anisotropy of the grain and grain rotation are neglected for the sake of simplicity. The local inelastic deformation heterogeneity is determined through the slip theory. It is assumed that the yield strength increases linearly with decreasing grain size as in Hall–Petch relationship. It is obviously recognized that the model with its new extension describes fairly well the effect of the grain size on the strain–stress behavior of the sub-micrometer aluminum.     

Recrystallization behaviour of the nickel-based alloy 80 A during hot forming

The dynamic recrystallization as well as meta‐dynamic and static recrystallization of the nickel‐based alloy 80A was investigated by means of electron backscatter diffraction (EBSD). Specimens were hot compressed at a temperature of 1120°C and a strain rate of 0.1/s at varying strain and soak times to describe the recrystallization behaviour. Various approaches were tested in order to differentiate between recrystallized and deformed grains based on EBSD data. The grain orientation spread was clearly found to be the most reliable procedure. A high twinning of the recrystallized grains was observed, and as a consequence the measured grain size was strongly dependent on whether the coherent and incoherent twin boundaries were regarded as genuine boundaries or removed.     

EBSD: a powerful microstructure analysis technique in the field of solidification

This paper presents a few examples of the application of electron back-scatter diffraction (EBSD) to solidification problems. For directionally solidified Al–Zn samples, this technique could reveal the change in dendrite growth directions from <100> to <110> as the composition of zinc increases from 5 to 90 wt%. The corresponding texture evolution and grain selection mechanisms were also examined. Twinned dendrites that form under certain solidification conditions in Al–X specimens (with X = Zn, Mg, Ni, Cu) were clearly identified as <110> dendrite trunks split in their centre by a (111) twin plane. In Zn–0.2 wt% Al hot-dip galvanized coatings on steel sheets, EBSD clearly revealed the preferential basal orientation distribution of the nuclei as well as the reinforcement of this distribution by the faster growth of <10 1 0> dendrites. Moreover, in Al–Zn–Si coatings, misorientations as large as 10° mm⁻¹ have been measured within individual grains. Finally, the complex band and lamellae microstructures that form in the Cu–Sn peritectic system at low growth rate could be shown to constitute a continuous network initiated from a single nucleus. EBSD also showed that the α and β phases had a Kurdjumov–Sachs crystallographic relationship.     

基于应变梯度塑性理论的微塑性成形力学性能

不同厚度的T2紫铜试样的单向拉伸、微硬度和微弯曲试验表明,材料的力学行为与内禀的材料特征参数相关：厚度为30μm的板材,其拉伸强度比厚度为150μm的板材提高了28%,平均晶粒尺寸D为50μm的细晶,其拉伸强度比平均晶粒尺寸D为120μm的粗晶拉伸强度提高了33%,拉伸时呈现出“越小越强”的特征;当压痕深度与板材厚度的比值大于0.2时,压入深度越大,压痕硬度越大,呈现出“越大越硬”的现象;回弹角随板料厚度的减小而增大,当材料厚度小于一定值（0.06mm）时,材料的应变梯度硬化效应使得回弹角随板料厚度的变化更为剧烈,这种变化与采用应变梯度塑性理论预测的结果基本一致。     

High-throughput determination of grain size distributions by EBSD with low-discrepancy sampling

Because microstructure plays an important role in the mechanical properties of structural materials, developing the capability to quantify microstructures rapidly is important to enabling high-throughput screening of structural materials. Electron backscatter diffraction (EBSD) is a common method for studying microstructures and extracting information such as grain size distributions (GSDs), but is not particularly fast and thus could be a bottleneck in high-throughput systems. One approach to accelerating EBSD is to reduce the number of points that must be scanned. In this work, we describe an iterative method for reducing the number of scan points needed to measure GSDs using incremental low-discrepancy sampling, including on-the-fly grain size calculations and a convergence test for the resulting GSD based on the Kolmogorov–Smirnov test. We demonstrate this method on five real EBSD maps collected from magnesium AZ31B specimens and compare the effectiveness of sampling according to two different low discrepancy sequences, the Sobol and R₂ sequences, and random sampling. We find that R₂ sampling is able to produce GSDs that are statistically very similar to the GSDs of the full density grids using, on average, only 52% of the total scan points. For EBSD maps that contained monodisperse GSDs and over 1000 grains, R₂ sampling only required an average of 39% of the total EBSD points.     

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.     

The characterization of low-angle boundaries by EBSD

A method of accurately measuring misorientations by electron backscatter diffraction (EBSD), which is an extension of that proposed by Wilkinson and based on the comparison of diffraction patterns, is described. The method has been applied to linescans, and found to improve the angular resolution by a factor of more than 30. The consequent improvement in determining misorientation axes is also analysed. Small changes of orientation very close to some low-angle boundaries were investigated and found to be artefacts of the analysis. Measurements of the area from which diffraction patterns are generated show this to be much larger than the effective spatial resolution of EBSD, and it is concluded that this may be a limiting factor in the use of EBSD for microstructural characterization.