EBSD Study of Damage Mechanisms in a High-Strength Ferrite-Martensite Dual-Phase Steel
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Dual-phase steel
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AbstractAbstractThe applicability of electron backscattered diffraction (EBSD) to studies of thermo mechanical processing is outlined and its relationship to other microtexture methods is discussed. Applications of the technique are grouped into three types. First, the measurement of orientation/misorientation of large numbers of crystallites enables the orientation gradients and misorientation distributions to be measured. Second, EBSD at specific microstructural heterogeneities enables detailed information to be obtained about annealing mechanisms. Third, EBSD in conjunction with in situ scanning electron microscope annealing is shown to be a powerful techniquefor studying boundary mobilities and the development of microstructure.
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An extensive characterization of hot deformed austenitic stainless steel was carried out using the electron backscatter diffraction technique. Special emphasis was given to the misorientation parameters related to different length scales. These parameters show a behaviour that is sensitive to the amount of applied strain and also lead to increasing values for both the strain and the scale length. At the same time, the use of different thresholds and scan steps in the evaluation of the parameters were analyzed in order to assess the validity of the results.
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Work has been carried out recently, which demonstrates misorientation measurements recorded by using electron backscatter diffraction (EBSD) enables one to undertake local post mortem plastic strain quantification once the degree of misorientation is calibrated against plastic strain. The present paper builds on this work and investigates the possibility of determining strain in individual grains. Due to the anisotropy of crystalline grains, polycrystalline material deform inhomogeneously on a microstructural level. In this study, the local strain induced in a pure copper specimen during tensile loading measured using EBSD was compared to in-situ strain measurements using optical microscopy imaging in conjunction with image correlation technique. By applying an averaging procedure for improving the accuracy of the measured EBSD data, the distribution of the misorientation within grains was quantified, and, as one would expect, it tended to be highest near the grain boundaries.
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Abstract Electron backscatter diffraction (EBSD) has become a common technique for measuring crystallographic orientations at spatial resolutions on the order of tens of nanometers and at angular resolutions <0.1°. In a recent search of EBSD papers using Google Scholar™, 60% were found to address some aspect of deformation. Generally, deformation manifests itself in EBSD measurements by small local misorientations. An increase in the local misorientation is often observed near grain boundaries in deformed microstructures. This may be indicative of dislocation pile-up at the boundaries but could also be due to a loss of orientation precision in the EBSD measurements. When the electron beam is positioned at or near a grain boundary, the diffraction volume contains the crystal lattices from the two grains separated by the boundary. Thus, the resulting pattern will contain contributions from both lattices. Such mixed patterns can pose some challenge to the EBSD pattern band detection and indexing algorithms. Through analysis of experimental local misorientation data and simulated pattern mixing, this work shows that some of the rise in local misorientation is an artifact due to the mixed patterns at the boundary but that the rise due to physical phenomena is also observed.
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The applicability of electron backscattered diffraction (EBSD) to studies of thermo mechanical processing is outlined and its relationship to other microtexture methods is discussed. Applications of the technique are grouped into three types. First, the measurement of orientation/misorientation of large numbers of crystallites enables the orientation gradients and misorientation distributions to be measured. Second, EBSD at specific microstructural heterogeneities enables detailed information to be obtained about annealing mechanisms. Third, EBSD in conjunction with in situ scanning electron microscope annealing is shown to be a powerful techniquefor studying boundary mobilities and the development of microstructure.
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Crystal orientations of creep damaged Type 316 stainless steel were measured by 10 organizations using the same specimens, passed in a round robin, in order to investigate the scatter in material damage assessment using the electron backscatter diffraction (EBSD) technique. The measurements were performed according to the EBSD measurement guideline issued by the Society of Material Science, Japan. Two misorientation parameters, the local and intra-grain misorientations, were calculated using mapping data of measured crystal orientations. It was shown that the area averaged local and intra-grain misorientations correlated well with the degree of the inelastic strain caused by the creep damage. Although the area averaged local misorientation showed eminent scatter, the scatter in the area averaged intra-grain misorientation was relatively small. The scatter in the area averaged local misorientation was deduced to be brought about by the error in the crystal orientation measurements. Since the accuracy of the crystal orientation measurement depends on various factors and is difficult to control, the correlation between the degree of the creep damage and the local misorientations obtained by one SEM/EBSD system is difficult to apply to other SEM/EBSD systems. On the other hand, the area averaged intra-grain misorientation is not affected much by the error in the crystal orientation measurements and the values obtained by various organizations using different SEM/EBSD systems were almost the same. It was concluded that the area averaged intra-grain misorientation can be used for measurement of the creep damage (inelastic strain). The empirical relationship between the area averaged intra-grain misorientation and the degree of the creep damage can be shared regardless of the SEM/EBSD system used.
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Abstract : Equal-channel angular (ECA) pressing is a promising method to achieve refinement of grain size to 1.0 micrometer or less for aluminum and its alloys. Computer-aided electron backscatter diffraction (EBSD) analysis of high purity aluminum (99.99%) which had been subjected to one pass, four passes, and twelve passes through an ECA die was performed. Grain and subgrain size and boundary misorientation distributions during such large-strain deformation processing were of particular interest. A texture was present after one pressing and the boundary misorientation distribution had a peak at 5 deg - 10 deg although boundaries were present in all misorientation ranges. Fine equaixed grains were achieved after twelve passes through the ECA die, accompanied by random orientation and misorientation distributions.
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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.
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