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.     

904L不锈钢形变奥氏体的亚结构演化及其对宏观流变应力的影响

为了揭示动态再结晶行为的本质及其对流变应力的影响,选取904L奥氏体不锈钢为研究对象,针对奥氏体在不同工艺下的热变形行为以及变形组织和再结晶组织的亚结构进行深入研究。结果表明材料的宏观应力-应变曲线在特定条件下不能准确反映微观尺寸上的动态再结晶行为。EBSD分析表明动态再结晶晶粒中亚结构特征以取向差角小于1°为主,呈周期性分布,且这种特征不会随晶粒自身长大或应变的增加而发生明显改变,即具有较好的稳定性;而变形组织中亚结构的取向差角明显增大,频繁出现大于1°的小角晶界,局部取向差角的分布范围也增大至1°～4°,即几何必需位错密度显著增加。尽管无论何种工艺下晶界均为再结晶的优先形核位置,但形核条件是不同的。对于完全再结晶组织来说,新的再结晶晶粒形核只需晶界出现局部“弓出”或弯曲;而在原始变形组织中,出现再结晶形核的晶界均存在微区高应变。     

Quantifying recrystallization by electron backscatter diffraction

The use of high‐resolution electron backscatter diffraction in the scanning electron microscope to quantify the volume fraction of recrystallization and the recrystallization kinetics is discussed. Monitoring the changes of high‐angle grain boundary (HAGB) content during annealing is shown to be a reliable method of determining the volume fraction of recrystallization during discontinuous recrystallization, where a large increase in the percentage of high‐angle boundaries occurs during annealing. The results are shown to be consistent with the standard methods of studying recrystallization, such as quantitative metallography and hardness testing. Application of the method to a highly deformed material has shown that it can be used to identify the transition from discontinuous to continuous recrystallization during which there is no significant change in the percentage of HAGB during annealing.     

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.     

Process optimization and microstructural development during superplastic-like forming of AA5083

An advanced sheet forming process was utilized by combining hot drawing and blow forming to establish a fast forming technology. As a continuation of the development in superplastic-like forming, this study dealt with the process optimization and evaluation of post-forming properties. Aluminum alloy 5083 (AA5083) parts with near-net shape were successfully fabricated at 400 °C. Thickness uniformity has been improved by optimizing the mechanical preforming (hot drawing) and adopting a strain-rate-control gas forming (blow forming). Fairly uniform microstructure can be achieved with this forming process. To investigate the microstructural information, the annealed and hot deformed samples were characterized using electron backscatter diffraction technique. Fine grains with high-angle grain boundaries occurred near the elongated grains during hot drawing stage as a result of dynamic recrystallization. Subgrain structure was also examined by characterizing the distribution of grain boundary misorientation angles. Grain growth and subgrain boundary migration were two main microstructural features observed during the gas forming stage.     

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.     

Characterization of crystallographic properties of SMC poly Si using electron backscattered diffraction

Crystallographic properties of silicide mediated crystallization (SMC) polycrystalline silicon (poly Si) and excimer laser annealing (ELA) poly Si were studied by electron backscattered diffraction. Large‐grain sized poly Si with a large fraction of low‐angle grain boundaries was acquired by SMC, and small‐grain sized poly Si with high‐angle grain boundaries especially around 60° was acquired by ELA. The thin film transistor (TFT) device characteristics were investigated in view of short‐range crystallinity (pattern quality) and long‐range crystallinity (misorientation distribution) of the specimens. Short‐range crystallinity did not significantly affect the TFT device characteristics, and long‐range crystallinity considering the low energy level of special boundaries could be better related to the TFT device characteristics of poly Si.     

Extracting twins from orientation imaging microscopy scan data

Automated electron backscatter diffraction or orientation imaging microscopy (OIM) provides spatially specific measurements of crystallographic orientation. These measurements are typically collected on regular grids. By inspecting the misorientation between neighbouring measurements on the grid, potential twin boundaries can be identified. If the misorientation is within some given tolerance of a specified twin misorientation, the boundary separating the two measurements may be identified as a potential twin boundary. In addition, for a coherent twin, the twinning planes must be coincident with the grain boundary plane. As OIM scans are inherently two‐dimensional, the scan data provide only limited information on the boundary plane. Thus, it is not possible to ascertain definitively whether the twinning planes are coincident with the boundary plane. Nonetheless, the alignment of the surface traces of the twinning planes with the trace of the boundary provides a partial indication of coincidence. An automated approach has been developed that allows data concerning both twin criterion to be extracted from OIM scans. Application of the methodology to deformed zirconium suggests that the twinning planes remain coherent during deformation. The methodology was also used to improve grain size distributions measured by OIM. These results more closely match those obtained by conventional metallography.     

采用电子背散射衍射技术研究变形温度对压缩后1235铝合金织构的影响

采用电子背散射衍射技术对经不同温度热压缩后1235铝合金的织构进行了研究,分析了在50%变形量、0.1s-1应变速率下,变形温度对该合金晶粒取向、晶界特征及织构组分的影响。结果表明:随着变形温度的升高,织构的取向聚集性弱化,织构相对体积总含量降低,晶粒内小角度晶界更稀疏,大角度晶界更加平直且连通性好,再结晶晶粒长大更充分。     

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.     

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.     

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 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.     

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.     

Crystallographic analysis of facets using electron backscatter diffraction

Applications of electron backscatter diffraction (EBSD), also known as backscatter Kikuchi diffraction in the scanning electron microscope (SEM) are first and foremost microtexture and grain boundary misorientation analysis on a single polished section in the specimen. A more subtle and revealing approach to analysis of these data is to use EBSD to probe the orientations of planar surfaces, i.e. facets, which bound crystals. These surfaces include: • grain or phase boundaries • fractures • cracks It is of great interest to know the crystallography of such facets since it provides a key to understanding the physical properties of them.
As far as investigation methodology is concerned, surfaces or facets associated with polycrystals are of two types: exposed or unexposed. Exposed facets, such as a fracture surface, can be viewed directly in the SEM, whereas unexposed facets, such as a grain boundary, are usually revealed as an etched trace on a polished surface. Photogrammetric methods can be used to obtain the positional orientation of an exposed facet, and the crystallographic orientation is obtained either directly from the surface or by indirect sectioning. Calibrated sectioning is required to obtain the equivalent parameters for an internal surface. The present paper compares the methods for obtaining and interpreting the crystallography of facets, with illustrations from several materials.     

Electron backscattered diffraction as a tool to quantify subgrains in deformed calcite

In this work, we investigated processing methods to obtain subgrain sizes from electron backscattered diffraction data using samples of experimentally deformed calcite (CaCO₃) polycrystals. The domain boundary hierarchy method, based on area measurements of domains enclosed by boundaries larger than a given misorientation angle, was applied to these calcite samples and was found to be limited by: (i) topological problems; (ii) undersampling of large grains; and (iii) artefacts caused by nonindexing. We tested two alternative methods that may reduce the problems: (i) the measured linear intercept hierarchy method, based on measurements of linear intercept between boundaries having larger misorientations than a given minimum angle; and (ii) the calculated linear intercept hierarchy method, based on the total length of boundaries having misorientations larger than a given minimum angle. The measured linear intercept hierarchy method was found to produce results more representative for the microstructure than the calculated linear intercept hierarchy method, because the calculated linear intercept hierarchy method has a significant uncertainty related to the grid‐based nature of the measurements. Preliminary results on calcite suggest that the measured linear intercept hierarchy method is related, in a complex way, to deformation conditions such as stress, strain and temperature as well as to the characteristics of subgrain rotation and grain boundary migration processes.     

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.     

Evidence of multimicrometric coherent γ′ precipitates in a hot‐forged γ–γ′ nickel‐based superalloy

This paper demonstrates the existence of large γ’ precipitates (several micrometres in diameter) that are coherent with their surrounding matrix grain in a commercial γ–γ’ nickel‐based superalloy. The use of combined energy dispersive X‐ray spectrometry and electron backscattered diffraction (EBSD) analyses allowed for revealing that surprising feature, which was then confirmed by transmission electron microscopy (TEM). Coherency for such large second‐phase particles is supported by a very low crystal lattice misfit between the two phases, which was confirmed thanks to X‐ray diffractograms and TEM selected area electron diffraction patterns. Dynamic recrystallization of polycrystalline γ–γ’ nickel‐based superalloys has been extensively studied in terms of mechanisms and kinetics. As in many materials with low stacking fault energy, under forging conditions, the main softening mechanism is discontinuous dynamic recrystallization. This mechanism occurs with preferential nucleation on the grain boundaries of the deformed matrix. The latter is then being consumed by the growth of the newly formed grains of low energy and by nucleation that keeps generating new grains. In the case of sub‐solvus forging, large γ’ particles usually pin the migrating boundaries and thus limit grain growth to a size which is determined by the distribution of second‐phase particles, in good agreement with the Smith–Zener model. Under particular circumstances, the driving force associated with the difference in stored energy between the growing grains and the matrix can be large enough that the pinning forces can be overcome, and some grains can then reach much larger grain sizes. In the latter exceptional case, some intragranular primary γ’ particles can be observed, although they are almost exclusively located on grain boundaries and triple junctions otherwise. In both cases, primary precipitates have no special orientation relationship with the surrounding matrix grain(s). This paper demonstrates the existence of high fractions of large γ’ precipitate (several micrometres in diameter) that are coherent with their surrounding matrix grain, in a commercial γ–γ’ nickel‐based superalloy. Such a configuration is very surprising, because there is apparently no reason for the coherency of such particles.     

Effect of annealing temperatures on the secondary re-crystallization of extruded PM2000 steel bar

The ferritic oxide dispersion-strengthened alloy PM2000 is an ideal candidate for high-temperature applications as it contains uniform nano-oxide dispersoids, which act as pinning points to obstruct dislocation and grain boundary motion and therefore impart excellent creep resistance. The development of the microstructure during re-crystallization of oxide dispersion-strengthened alloys has been discussed by a number of authors, but the precise mechanism of secondary re-crystallization still remains uncertain. Hence, this work is aimed at investigating the re-crystallization behaviour of extruded PM2000 bar for different annealing temperatures, using electron backscatter diffraction, in particular, to determine grain orientations, grain boundary misorientation angles, etc. The results show that the as-extruded bar microstructure comprises both low-angle grain boundaries pinned by oxide particles and high-angle boundaries that will have inherent boundary mobility to allow boundary migration. In addition, dynamical re-crystallization was found in the outer region of the non–heat-treated PM2000 bar, which suggested that deformation heterogeneities can be introduced during thermo-mechanical processing that enhance the nucleation of re-crystallization. Subsequent heat treatments promote and stimulate secondary re-crystallization, giving rise to large grains with few sub-grain boundaries.     

Diffraction effects and inelastic electron transport in angle‐resolved microscopic imaging applications

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle‐resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in‐depth theoretical modelling of the energy‐ and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.