EBSP study of the annealing behavior of aluminum deformed by equal channel angular processing

Commercial purity aluminum (99.5%) was fabricated by equal channel angular pressing (ECAP) up to total accumulated strains of approx. 10. The annealing behavior of material deformed to total strains of approx. 1 and 10 was investigated, using heat treatments of 2 h at various temperatures from 100 to 500 °C. The microstructure of the annealed materials was characterized using the electron back-scatter pattern technique. A number of parameters were determined including the distribution and average values of both the boundary spacings and misorientations. For samples deformed to a total strain of 1, annealing resulted in discontinuous recrystallization. For samples deformed to a total strain of 10, annealing resulted in microstructures exhibiting characteristics of both uniform coarsening and, in a number of places, of discontinuous recrystallization. An attempt was made, based on the boundary spacing distributions, to separate these two components. The grain size after annealing was still however small, being just 6.4 μm after 2 h at 300 °C.     

Microstructure evolution in a large—grained TiAl alloy

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Towards high accuracy calibration of electron backscatter diffraction systems

For precise orientation and strain measurements, advanced Electron Backscatter Diffraction (EBSD) techniques require both accurate calibration and reproducible measurement of the system geometry. In many cases the pattern centre (PC) needs to be determined to sub-pixel accuracy. The mechanical insertion/retraction, through the Scanning Electron Microscope (SEM) chamber wall, of the electron sensitive part of modern EBSD detectors also causes alignment and positioning problems and requires frequent monitoring of the PC. Optical alignment and lens distortion issues within the scintillator, lens and charge-coupled device (CCD) camera combination of an EBSD detector need accurate measurement for each individual EBSD system.This paper highlights and quantifies these issues and demonstrates the determination of the pattern centre using a novel shadow-casting technique with a precision of ∼10 μm or ∼1/3 CCD pixel.     

形变量对冷轧高纯镍高温退火行为的影响

应用电子背散射衍射分析(EBSP)对形变量分别为90%、96%和98%的冷轧高纯镍在高温下完全再结晶退火后的微观组织和织构进行分析.结果表明,立方织构随着退火温度和形变量的增大而升高,98%形变量的镍试样在高温退火后可以得到集中的立方织构和较小的晶粒尺寸,是超导覆膜基带的较好选择.     

The effect of SiO2 addition on the development of low-Σ grain boundaries in PTC thermistors

Electron backscatter pattern analysis has been used to characterise, using the coincidence site lattice model, the distribution of grain boundary structures in a series of BaTiO₃ based positive temperature coefficient of resistance (PTC) thermistors, prepared with 0, 1.0, 2.0 and 3.0 at.% SiO₂ additions. As the SiO₂ content was raised, the proportion of random, high-angle grain boundaries in the microstructure increased steadily from 85.7% to 89.6%, while the proportion of grain boundaries indexable in the range Σ3–Σ29 decreased from 14.3% to 10.4%, and the Σ3 grain boundary population fell from 5.9% to 3.6%. At the same time the proportion of Σ3 twin boundaries remained approximately constant at 3.0 ± 0.3%. Significantly more Σ3 grain boundaries than would be expected in a randomly oriented, untextured material were observed in all samples. The variation in grain boundary types with SiO₂ addition is discussed in terms of grain boundary energy and its effect on PTC performance.     

Determining dislocation cell sizes for high-strain deformation microstructures using the EBSP technique

The effect of several data collection and processing choices has been examined for high‐resolution electron back‐scatter pattern (EBSP) investigation of a highly deformed sample. The results were compared with a transmission electron microscope (TEM) investigation of the same sample. The estimated dislocation cell size was examined as a function of data cleaning strategy, line intercept vs. reconstruction method, critical misorientation angle definition and step‐size. The best agreement with the TEM results was obtained using a modified relative reconstruction algorithm on fine step‐size maps allowing some of the noise in the data to be overcome. Step sizes of up to one‐quarter the average cell size yielded similar values for the estimated average cell size. As a result of the mixture of both high‐ and low‐angle boundaries, single diffraction condition TEM images may give larger cell size estimates than the EBSP data. Orientation noise in the EBSP data, however, still limits the extent to which quantitative information can be extracted.     

Problems in determining the misorientation axes, for small angular misorientations, using electron backscatter diffraction in the SEM

The errors associated with calculating misorientation axes from electron backscatter diffraction (EBSD) data have been assessed experimentally. EBSD measurements were made on the same grains after imposed rotations of 2°, 5°, 7°, 10°, 12°, 17°, 27° and 180° around the normal to the specimen surface. The misorientation magnitudes and the misorientation axes associated with the imposed rotations have been calculated from the EBSD data. Individual measurements of misorientation axes are precise for misorientation magnitudes greater than ≈ 20°. The errors must be appreciated when assessing misorientation data at lower misorientation magnitudes and particularly at magnitudes less than 5°. Where misorientation axes can be characterized by the distribution of axes from a number of individual measurements, current EBSD techniques are satisfactory, for data sets of 30 measurements, as long as misorientation magnitudes are 10° or more. With larger data sets it may be possible to extend this approach to smaller misorientation magnitudes. For characterization of individual misorientations less than 5°, new EBSD techniques need to be developed.     

Annealing Texture Evolution of Ti-IF Steel during Batch Annealing

Texture development and recrystallization behavior of Ti-IF steel during Batch Annealing (BA) were investigared with X-ray diffraction and EBSP in lab, and the results were compared with research work on continu ous annealing (CA) with rapid heating rate. The basic tendencies are similar that eary nucleation takes place in <111>ND fibers, <110>RD fibers are consumed at the end of recrystallization, and <111>ND texture dominates over annealing texture. However, the detailed texture transformation during batch annealing is different and somewhat more compicated than tha in rapid-heating process. Moreover, misorientation plays an important role in texture transformation of BA In addition, the results of EBSP are consistent with that of ODF well.     

Large strain deformation of Ni3Al+B: Part II. Microstructure and texture evolution during recrystallization

The microstructure evolution and texture development of the intermetallic compound Ni₇₆Al₂₄, doped with minor additions of boron, was investigated during annealing subsequent to cold-rolling. Upon annealing of cold-rolled specimens, a hardness increase prior to recrystallization was observed. This is attributed to a relaxation process of the dislocation cores during annealing. Recrystallization proceeded with very different kinetics locally and a very small recrystallized grain size was obtained. From grain growth kinetics, the activation energy for grain boundary motion was found to be close to the activation energy for self-diffusion of Ni in Ni₃Al. The recrystallization texture was almost random. However, three texture components could be invariably recognized ({013}100, {012}021 and {112}294). They are related by special orientation relationships to the brass rolling orientation. A high frequency of annealing twins occurred in the recrystallized microstructure. The misorientation distribution function (MODF) shows a preference of certain low Σ boundaries, namely small-angle boundaries (Σ1) and twins (Σ3).     

A comparison of grain imaging and measurement using horizontal orientation and colour orientation contrast imaging, electron backscatter pattern and optical methods

The problems associated with the definition of a grain, grain size measurement, and the issues associated with making one- and two-dimensional measurements on a three-dimensional structure are discussed. The relatively new scanning electron microscope (SEM)-based techniques of colour orientation contrast imaging (COCI) and automated electron backscatter pattern (EBSP) are explained and examples given. Comparisons with conventional (horizontal) orientation contrast imaging (HOCI) in the SEM are made. A direct comparison is made between conventional metallographic methods and these new techniques on precisely the same region of an interstitial free iron specimen. Both optical imaging and HOCI were found to miss a large number of grain boundaries (7 and 12%, respectively), and to create boundaries (≈ 2%). COCI was found to be reliable, with only 3% of boundaries missed. EBSP was taken to be the standard against which the others were compared, as it unambiguously measured changes in crystallographic orientation. Errors in the number of grain boundaries that are detected have a considerable effect on grain size measurements, e.g. mean linear intercept, and a follow-on effect on the modelling of mechanical properties. New methods for increasing the acquisition speed of orientation maps are discussed, along with examples. The combination of COCI (for grain location) and EBSP (for orientation measurement) is promising, but requires improvements in either imaging or image analysis to be totally reliable.