Intragranular Misorientation Research Articles

Evaluation of intragranular misorientation parameters measured by EBSD in a hot worked austenitic stainless steel

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.

Effect of Initial Texture on the Deformed Microstructure of IF Steel

The development of deformation texture and microstructure was examined for four different initial textures. IF steel sheets with a majority of α-, ε-, and γ-fiber and near random texture were prepared and cold rolled. The specimens exhibited characteristic behaviors in rolling texture evolution and deformation-induced misorientation development, according to their initial textures, especially at small strain levels. Due to the orientation dependence of intra-granular misorientation accumulation, the different texture evolutions affected the induced misorientation distribution. A larger fraction of γ-fiber orientations was related to more prominent misorientation development, while the initial texture stability simultaneously affected the misorientation development. The unstable, initial ε-fiber texture showed a stronger tendency of misorientation accumulation than the stable α-fiber during the subsequent cold rolling.

Low Temperature Superplasticity and Intragranular Misorientation in Severely Rolled Sheets of Al-4.5%Mg-0.7%Mn Based Alloys

Severely rolled sheets of Al-4.5%Mg-0.7%Mn based alloys have been prepared to achieve grain refinement leading to low temperature superplasticity. Rolling is one of straining technique which has a high possibility of industrial application. The alloys used were T1: base one adding 0.08%Zr and 0.05Ti for suppression of grain growth, F1: 0.80%Fe addition and F2: 1.92%Fe addition to increase nucleation sites of recrystallization. For comparison, M1: Al-4.2%Mg was also prepared. All of samples were 99.8% cold rolled to thin sheet 0.1mm thick. The maximum elongation of 150%, which was markedly large in consideration of specimen thickness of 0.1mm, was obtained at 533K and 1.4×10-3s-1 for T1 sample while the elongations for F1 and F2 showed less than 100%. On the other hand, M1 exhibited larger elongation of 190% at 533K and 1.4×10-3s-1. Further, difference in superplastic deformation of these alloys was investigated by using SEM/ EBSP analysis of intragranular misorientation, which reflects strain or stored energy generated during deformation with dislocation glide, to discuss the deformation mechanism.

Effect of deformation mode and grain orientation on misorientation development in a body-centered cubic steel

Strain-induced misorientation development was studied in an IF steel as a function of strain for two deformation modes, plane strain compression and simple shear. Using electron back-scattered diffraction, orientation maps of “large” areas were obtained, from which several individual grains associated with the principal texture components could be extracted so that only intragranular misorientations could be estimated for these orientations. It was observed that the increase of the misorientation angle was more prominent in simple shear than in plane strain compression and that the orientation influence was different for each mode. Considering texture evolution as a possible source of misorientation development, the lattice spin tensor was estimated with the Taylor model for the two deformation modes; both reorientation axis and angle were compared with misorientation angle and axis. The striking concordance of both quantities allows us to conclude that there is a direct contribution of texture evolution to misorientation accumulation with strain.

Three-Dimensional Characterization of Incipiently Spalled Tantalum

Three-dimensional (3-D) characterization techniques have been applied to quantitatively assess shock-induced damage in an incipiently spalled tantalum plate. A significant sample volume was imaged and volumetrically reconstructed via serial sectioning techniques. An analysis of segmented void damage revealed distributions regarding void size, morphology, spatial variations, and nearest-neighbor distances (NNDs). The latter two distributions highlight the necessity of 3-D data sets to capture the feature statistics. Very limited correlation was observed between void size and clustering tendency. Electron backscatter diffraction (EBSD) was applied to investigate spatial relationships between the damage features and the relative levels of induced plasticity. Large variations in microstructural character were found between different intervoid regions, independent of ligament width. The use of intragranular misorientation deviation (IMD) maps in two and three dimensions reveals surprisingly contiguous and complex plastic linkages not obviously predicted from the void-dominated damage field. Overall, 3-D characterization of shock-damaged material is shown to provide statistics and insight regarding the true damage field unobtainable from two-dimensional (2-D) data sets, and will provide meaningfully improved data for predictive damage models.

EBSD characterization of dynamic shear band regions in pre-shocked and as-received 304 stainless steels

The microstructure and texture variations near adiabatic shear localization regions in as-received and pre-shocked 304 stainless steels were investigated using electron backscatter diffraction (EBSD). Shear bands were developed in both steels during a forced dynamic shear deformation produced on a split-Hopkinson pressure bar. EBSD examination revealed large intragranular misorientation gradients toward the shear bands. Taylor factor and misorientation analyses were applied to explain deformation characteristics. Shear-induced deformation structure was observed to be sensitive to both initial microstructure and local orientation. Comparison of as-received and pre-shocked steels indicated that the pre-shocked condition yielded sharper shear band boundaries but moderate misorientation gradients, while the as-received condition led to substantial misorientation adjacent to the band. Shear-induced texture evolution generally resulted in the reorientation of 〈1 1 0〉 toward the shear direction for both cases. Grains with slip systems favorably oriented with respect to the applied shear path displayed less intragranular misorientation development compared to unfavorably oriented grains.

In-Situ SEM/EBSP Analysis during Annealing in a Pure Aluminum Foil for Capacitor

In-situ SEM/EBSP analysis has been performed during the evolution of the cube texture in a pure aluminum foil. In general, foils for capacitor are manufactured in an industrial process of casting, homogenizing, hot rolling, cold rolling (CR), partial annealing (PA), additional rolling (AR) and final annealing (FA). The foil samples after CR or AR in the process were analyzed by the SEM/EBSP technique at a constant temperature which was step-heated repeatedly by 10-20K from a room temperature to 623K or 598K. In a CRed sample, cube ({001}<100>) grains begin to grow preferentially at 503K to cover the sample. On the other hand, in a sample subjected to PA at 503K and AR, cube grains coarsened rapidly and preferentially at more than 533K in contrast to other oriented small grains remaining their sizes. Further, intragranular misorientation analysis revealed that the misorientation, which corresponds to dislocation density or strain, was much smaller in cube grains than in S ({123}<634>) and Cu ({112}<111>) ones.

Microstructure and texture of a lightly deformed TRIP-assisted steel characterized by means of the EBSD technique

The microstructural and textural changes after a tensile strain of 10% were observed by orientation contrast measurements in a TRIP-assisted steel. On the undeformed samples it was shown that the electron back scatter diffraction (EBSD) technique could be used successfully for determining the volume fraction of the microstructural constituents bainite, ferrite and austenite, whereas after deformation only the BCC and FCC phases could be separated. The results show that the tensile strain of 10% gave rise to a drop in residual austenite content from 10 to 4%, which was also confirmed by magnetic measurements. The texture data showed only minor orientation rotations after 10% tensile strain for the BCC ferrite and bainite grains, whereas the residual austenite did show a significant texture change. By meticulously monitoring the local intra-granular misorientations it was concluded that the BCC phases (ferrite and bainite) took up the larger part of the nominal strain whereas the residual austenite primarily responded to the mechanical load by a partial (stress-induced) martensite transformation. Hence, the texture change observed in the residual austenite could be attributed to the orientation selective character of the phase transformation.

Change in crystallographic orientation distribution during superplastic deformation in an Al–Zn–Mg–Cu alloy

The change in crystallographic orientation distribution during superplastic deformation in an Al–Zn–Mg–Cu alloy has been investigated in order to reveal the deformation mechanism. The well-known strain rate dependence of the deformation behavior was examined by SEM/EBSP (scanning electron microscopy/electron back scatter diffraction pattern) analysis. Fraction of low angle grain boundaries increased after deformation at the high strain rate, presumably due to dislocation activity, while fraction of random boundaries was high in the specimen deformed at the low strain rate. Progressive randomization of the initial texture was also found during deformation at the low strain rate. Further, the intragranular misorientation, grain boundary misorientation and local orientation are analyzed in detail to discuss the accommodation process.

Analysis of Intragranular Misorientation Related to Deformation in an Al-Mg-Mn Alloy

Intragranular misorientation reflects strain generated during deformation with dislocation glide. The SEM/EBSP (scanning electron microscope/ electron back scatter diffraction pattern) technique provides is “kernel average misorientation (KAM)” as the most appropriate quantity to evaluate the strain or the stored energy for a given point. The KAM is defined for a given point as the average misorientation of that point with all of its neighbors. In the present paper two analyses of the intragranular misorientation using the SEM/EBSP technique for a cyclic deformation at room temperature and a high temperature deformation in an Al-Mg-Mn alloy are reviewed.

Discrete dislocation density modelling of single phase FCC polycrystal aggregates

A new dislocation-mechanics based crystallographic theory has been developed to model the mechanical behaviour of single-phase FCC polycrystal aggregates. In the theory, dislocations are discretised into edge and screw components with intrinsically different relative mobilities and are subject to different dynamic recovery processes. The theory has been implemented within a finite-strain and rate-dependent constitutive framework, and applied to a thin polycrystal Cu specimen to investigate the effect of intragranular lattice misorientations on deformation behaviour. These misorientations are representative of low angle grain boundaries, which are known to play an important role in the microstructural evolution of polycrystals under monotonic and cyclic deformation. This study reveals that the presence of these misorientations strengthen the material response by suppressing and re-distributing the localisation of slip within the grains, as well as inhibiting the formation of sub-grains. Through the discretisation of dislocations, the model also predicts a higher proportion of edge dislocations in the vicinities of localised slip regions.

Change in Crystallographic Orientation Distribution during High Temperature Deformation in an Al-Mg-Mn Alloy Sheet Consisting of Coarse- and Fine-Grained Layers

The change in crystallographic orientation distribution during high temperature deformation for an Al-Mg-Mn alloy sheet consisting of the coarse-grained surface and the fine-grained center layers has been investigated in order to reveal the deformation mechanism. The grain size dependence of the deformation behavior is discussed in the identical deformation condition by using the specially-prepared sheet. The grain structures in the coarse-grained surface layer of the sample deformed at 713 K are elongated in the tensile direction corresponding to the macroscopic elongation to failure. The structures related to the maximum elongation in both of the surface and center layers have preferred orientations of the tensile deformation. Further, the intragranular misorientation, grain boundary misorientation and high strain rate deformation are analyzed in detail.

Intra-granular strains and grain boundary morphologies of dynamically recrystallized quartz aggregates in a mylonite

Dynamic recrystallization process has been investigated by analyzing intra-granular strains and grain boundary morphologies of sheared quartz aggregates deformed under high greenschist facies condition collected from the Hatagawa mylonite zone in northeast Japan. Microstructural features indicate pervasive operation of both subgrain rotation and grain boundary migration. The grains favorably oriented for operation of {10 1 ̄ 1}〈11 2 ̄ 0〉 and {10 1 ̄ 0}〈11 2 ̄ 0〉 slip systems tend to have small mean intra-granular misorientations (less than 3°) and occupy a large volume in the aggregate. Grains favorably oriented for (0001)〈11 2 ̄ 0〉 slip show large intra-granular misorientations, often exceeding 10°, and are often isolated as thin layers and equant small grains between those favorably oriented for prism〈 a〉 and rhomb〈 a〉 slip. The energies of subgrain boundary developed in the latter grains are estimated to be about 0.28 J m −2, and are 1.5–2 times larger than the former. The morphological features of grain boundaries were well described by means of the curvature distribution along the boundary. The serration of grain boundaries is controlled by the misorientation and spacing of subgrain boundaries developed in the grain. We have demonstrated that serrated grain boundaries have a common asymmetric feature in the curvature distribution and this morphological feature can be used for the determination of direction of the grain boundary migration.

N-site modeling of a 3D viscoplastic polycrystal using Fast Fourier Transform

We present a formulation to compute the local response of elastic and viscoplastic anisotropic 3D polycrystals based on the Fast Fourier Transform (FFT) algorithm. This formulation is conceived for periodic heterogeneous microstructures and also for materials with random spatial distribution of heterogeneities. The approach is of the n-site kind, provides an exact solution of the equilibrium equation and has better numerical performance than small-scale FEM. The viscoplastic FFT formulation combined with an ad-hoc microstructure updating scheme is used to predict local states, morphology and texture evolution of ideal f.c.c. polycrystals. The model predicts strain localization, intragranular misorientation and subgrain formation and overall textures which are smoother than those obtained with classical 1-site schemes, in better agreement with experiments.

Dislocation density analysis in single grains of steel by X-ray scanning microdiffraction

A new set-up for X-ray micro-diffraction has been developed on the ESRF beamline ID22. It allows microscopic characterization of materials with micrometer resolution. This facilitates the measurement of material quantities as average size of the coherently diffracting volume, local dislocation density, residual stress, local fluctuation of the residual stress, and intragranular misorientation from single grains of a polycrystalline material. The first application on an IF-Ti steel after different thermo-mechanical treatments is presented.

Single Grain Analysis of Dislocation Density in Cold Rolled IF-Ti Steel with a New High-Resolution Microdiffraction Technique

A new set-up for X-ray microdiffraction has been developed on the ESRF beamline ID22. This set-up allows very local characterisations of materials, the size of the focussed beam being only of a few microns and the position of the beam on the specimen being determined with an accuracy of about 30 microns. Data such as average size of the coherently diffracting volume, local dislocation density, residual stresses, local fluctuation of the residual stresses, intragranular misorientation, can be obtained from the X-ray intensity distribution on a 2-D detector. The first application on an IF-Ti steel after different thermomechanical treatments (recrystallization, cold-rolling, annealing) is presented.

塑性変形に伴う結晶粒内の方位差の発達

With the aim of measuring local strain distribution using electron back-scattered diffraction (EBSD), the evolution of intragranular misorientation during tensile and creep deformations was studied. Test materials include a low-alloyed Cr-Mo steel, Type 304 stainless steel, Ni-based superalloy and pure iron, which were deformed up to 6 pct strain. Although all of the test materials were annealed, misorientations of 0.4 to 0.5 degrees were measured prior to deformation. Misorientation angles subtracted by these initial values linearly increase with plastic strain. The slope was shown to be independent of the kinds of test materials, deformation temperatures and strain rates. The ratios of misorientation angle to strain for the crept Cr-Mo steel with a ferrite-lath structure and aluminum deformed at elevated temperature, calculated from the published works, were shown to lie on the extrapolated line of the present study. TEM observations showed various deformation structures. Slip lines with 0.5 to 1 μm intervals were observed in INCO718, while a tangled dislocation structure was observed in SUS304. Dislocation tangling has developed to a preform of cellular structures in the pure iron strained to 3pct. Dislocations pinned by carbides sparsely distributed in the crept Cr-Mo steel. It was concluded from these observations that intragranular misorientation develops by local rotation of crystal orientations without forming cellular dislocation or subgrain structures.

X-ray diffraction measurement of intragranular misorientation in α-brass subjected to reverse plastic strain.

X-ray diffraction measurement of intragranular misorientation in α-brass subjected to reverse plastic strain.