Misorientation Angle Distribution Research Articles

Effect of Feed Rate in FSW on the Mechanical and Microstructural Properties of AA5754 Joints

Friction stir welding is a solid-state welding process, which is successfully applied to aluminum alloys to replace fusion welding processes. In the present work, the influence of varying the feed rate at constant rotational speed on the mechanical and microstructural properties of friction stir welded AA5754 was investigated. FSW caused dynamic recrystallization leading to microstructural changes in different zones. Electron backscattered diffraction confirmed such changes via misorientation angle and subgrain size distributions. Energy dispersive spectroscopy determined the elements present in the matrix quantitatively and their distribution through mapping. Tensile and hardness tests showed slight enhancement compared to base metal. The strain hardening exponent was used to investigate the effect of varying the feed rate on the behavior of the welded samples.

Effect of Twinning Behavior on Dynamic Recrystallization During Extrusion of AZ31 Mg Alloy

A new severe plastic deformation method, viz. integrated dual-directional extrusion and spiral deformation, was adopted to process AZ31 Mg alloy and found to be very effective for refining the grains and softening the texture. We also investigated how twinning in AZ31 alloy affected the dynamic recrystallization (DRX) during hot extrusion, revealing that multiple twins can offer increased nucleation sites and enough energy to trigger DRX. Moreover, even after dual-directional extrusion and spiral deformation, a misorientation angle of ~ 86° remained between the primary tension twin and matrix or ~ 60° between the secondary tension twin and primary twin, being induced by {10–12} tension twin variants (primary and secondary) and resulting in a dispersed misorientation angle distribution. Nucleation of DRX occurred at the tip of preexisting twins, although primary and secondary tension twins can also act as nucleation sites for DRX.

Microstructure, texture, and mechanical properties of Ti-6Al-4V joints by K-TIG welding

In this study, a Keyhole TIG welding system was successfully used to weld 12 mm thick Ti-6Al-4V alloy workpieces. This technique can be used to weld a single-sided joint using a double-sided forming process, without the preparation of a groove or the use of filler metals. Weld beads with a bright appearance and no obvious defects were formed. The experimental results show that the textures of the α phase in the base metal and weld metal are {0001} <11−20> and {01–10}<2–1–10 > , respectively. Based on the misorientation-angle distribution of the grain boundaries within the α phase, the high-angle grain boundary proportions of the heat-affected zone and weld metal are higher than that of the base metal. The impact toughness of the weld metal is superior to that of the base metal. The high-angle grain boundary proportion has an important influence on the impact toughness of the weld metal. The strength of the weld metal decreased following welding, but it meets the standard of the mechanical properties.

Misorientation angle distribution of deformation-induced boundaries provided by their EBSD-based separation from original grain boundaries: Case study of copper deformed by compression

Grain lattice fragmentation in heavily deformed metals is controlled by the formation of deformation-induced boundaries (DIBs) with misorientations evolving from low to high angles with increasing strain. A method is suggested allowing the separation of the contributions from DIBs and original grain boundaries to the overall misorientation angle distribution based on electron backscatter diffraction (EBSD) data. The peculiarities, accuracy and limitations of the method are considered by the example of copper under uniaxial compression at room temperature and 150 °C. It is shown that obtained distributions of DIB misorientation can be analyzed using their representation as a superposition of partial distributions. The partial distributions f1 and f2, which describe the low-angle interval, correlate with the distributions determined earlier by the transmission electron microscopy for two types of dislocation boundaries, while the third partial distribution f3 relates to the high-angle DIBs, whose misorientations go beyond the range of the first two distributions. The rise of deformation temperature does not affect the character of misorientation distribution development.

Microfabrics of omphacite and garnet in eclogite from the Lanterman Range, northern Victoria Land, Antarctica

We examined the microfabrics of omphacite and garnet in foliated eclogite to determine the influence of the layered structure on seismic observations in subduction zone. The analyzed eclogite, from the Lanterman Range, northern Victoria Land, Antarctica, is characterized by layering in which the modal abundances of garnet and omphacite vary. For garnet, the low aspect ratios, similar angular distribution of long axes relative to the foliation in both layers, uniform grain size distribution, near-random crystallographic preferred orientations (CPOs), and misorientation angle distributions are indicative of passive behavior during deformation. In contrast, omphacite shows relatively high aspect ratios, a low angle between the long axes of crystals and the foliation, a wide grain-size distribution, and distinctive CPOs, suggesting dislocation creep as the main deformation mechanism. The results of fabric analyses are consistent with strain localization into omphacite or omphacite-rich layers rather than garnet or garnet- rich layers. The single-crystal seismic anisotropy of garnet is very weak (AVP = 0.2%, AVS = 0.5–0.6%), whereas that of omphacite is much stronger (AVP = 3.7–5.9% and AVS = 2.9–3.8%). Seismic anisotropy of the omphacite-rich layers shows an increase of 329% for AVP and 146% for AVS relative to the garnet-rich layers. Our results demonstrate the importance of the layered structure in strain localization and in the development of the seismic anisotropies of subducting oceanic crust.

Free surface effects on the recrystallization of compressed, stable, Al-Mn single crystals

The free surface effect on the grain crystallography and morphology during recrystallization has been studied in stable Goss {110}〈001〉 and brass {110}〈112〉 - oriented single crystals of Al-1%wt.Mn alloy. The samples were plane strain compressed up to a strain of about 0.5 to develop a homogeneous structure comprising two sets of symmetrical microbands and then lightly annealed. The variation of recrystallization after annealing was then analysed by optical microscopy and SEM/EBSD on different cross-sections. The results are discussed in terms of the misorientation angle and axis distributions. The free surface provokes high frequency nucleation leading to a small grain size compared to internal sections of the bulk sample. However, the free surface and surface ‘quality’ do not modify the orientations of the recrystallized grains, i.e. similar groups of recrystallized grain orientations were observed for all sections. This leads to a strong misorientation relation between deformed/recovered area orientations and the limited number of groups of recrystallized grain orientations, basically the same as those observed during bulk recrystallization of similar crystals.

Using electron backscatter diffraction to measure full crystallographic orientation in Antarctic land-fast sea ice

ABSTRACTWe have mapped the full crystallographic orientation of sea ice using electron backscatter diffraction (EBSD). This is the first time EBSD has been used to study sea ice. Platelet ice is a feature of sea ice near ice shelves. Ice crystals accumulate as an unconsolidated sub-ice platelet layer beneath the columnar ice (CI), where they are subsumed by the advancing sea–ice interface to form incorporated platelet ice (PI). As is well known, in CI the crystal preferred orientation comprises dominantly horizontalc-axes, while PI hasc-axes varying between horizontal and vertical. For the first time, this study shows thea-axes of CI and PI are not random. Misorientation analysis has been used to illuminate the possible drivers of these alignments. In CI the misorientation angle distribution from random pairs and neighbour pairs of grains are indistinguishable, indicating the distributions are a consequence of crystal preferred orientation. Geometric selection during growth will develop thea-axis alignment in CI if ice growth in water is fastest parallel to thea-axis, as has previously been hypothesised. In contrast, in PI random-pair and neighbour-pair misorientation distributions are significantly different, suggesting mechanical rotation of crystals at grain boundaries as the most likely explanation.

Improving the precision of orientation measurements from technical materials via EBSD pattern matching

We use pattern matching of experimental and dynamically simulated backscattered Kikuchi diffraction (BKD) patterns to increase the orientation precision of electron backscatter diffraction (EBSD) measurements. In order to quantify the improvement in orientation precision, we analyze the experimental distribution of the kernel average misorientation (KAM) angles. We find that for the same raw data, i.e. the same EBSD data acquisition time budget, the pattern matching approach improves the KAM resolution by an order of magnitude compared to orientation data delivered from the conventional Hough-transform based data analysis. This quantitative improvement enables us to interpret small orientation changes in plastically deformed materials which are hidden in noisy orientation data delivered from the reference EBSD system.As an application example, we analyze a ferritic-martensitic steel (P92) sample before and after low-cycle fatigue (LCF) loading (±0.3% strain) at 620 °C. Whereas the low precision of the EBSD orientation data from the manufacturer software does not allow a reliable discrimination of the gradually changing microstructure, we find very clear systematic differences of the local microstructure after the pattern matching orientation refinement of the initial, raw pattern data. For the investigated P92 sample, the KAM-angle histograms are well described by two log-normal distributions indicating the already tempered and the remaining and mostly untempered martensite.

The impact toughness of novel keyhole TIG welded duplex stainless steel joints

Abstract Keyhole tungsten inert gas (K-TIG) welding is a novel, high-efficiency welding method. In this study, 10.8-mm-thick duplex stainless steel (DSS) plates were welded via K-TIG welding, without groove preparation or filler metals. The microstructure, grain size and grain boundary misorientation angle distribution (GBMAD) of the weld metal (WM) were analyzed to characterize its impact toughness. Charpy impact tests were carried out, and the results proved that the impact absorbed energy (IAE) of the WM increases with the increase in heat input. This finding indicates that the austenite volume fraction and the GBMAD have an influence on the impact property of the WM.

Grain refinement in the equal channel angular pressing process: simulations using the crystal plasticity finite element method

Grain refinement due to severe plastic deformation is simulated using the crystal plasticity finite element method in the total Lagrangian setting. A rate-independent model with the regularized Schmid law is applied. As an example, a single pass of the equal channel angular pressing process is considered. Texture evolution, misorientation angle distributions and maps of new grains are presented. A special algorithm for tracking the creation of new grains in finite element simulations is developed. The results are analysed with respect to experimental data available in the literature. The possible mechanisms leading to the fragmentation of grains in a face centred cubic material are discussed. The influence of the quality of the microstructure representation on the simulation results is assessed.

Microstructure Evolution in Cold-Rolled Pure Titanium: Modeling by the Three-Scale Crystal Plasticity Approach Accounting for Twinning

A three-scale crystal plasticity model is applied to simulate microstructure evolution in hcp titanium subjected to cold rolling. Crystallographic texture and misorientation angle development, as an indicator of grain refinement, are studied. The impact of twinning activity on both phenomena is accounted for by combining the original three-scale formulation with the probabilistic twin-volume consistent (PTVC) reorientation scheme. The modeling results are compared with available experimental data. It is shown that the simulated textures are in accordance with the experimental measurements. The basic components of misorientation angle distribution, especially in the range of high angle boundaries, are also well reproduced.

Five-parameter intervariant boundary characterization of martensite in commercially pure titanium

The intervariant boundary characteristics of a commercially pure Ti microstructure formed by the β→α martensitic phase transformation were described according to the crystallography of the displacive transformation and the boundary plane orientation. The martensitic transformation created a microstructure whose grain boundary misorientation angle distribution had four distinct peaks that were consistent with the misorientations between the variants produced by the Burgers orientation relationship. Interestingly, about 60% of population corresponded to 60°/[112¯0] intervariant boundaries. Three-variant clusters with a triangular morphology were observed frequently. This configuration is consistent with the phenomenological theory of martensite, which predicts that these clusters, separated by 60°/[112¯0] boundaries, have a lower transformation strain than other possible variant cluster arrangements. Other intervariant boundaries resulted from the impingement of different combinations of distinct three-variant clusters. The five-parameter boundary analysis revealed a strong anisotropy in the plane orientation distribution, showing that boundaries have a tendency to terminate on prismatic {hki0} and pyramidal {101¯1} planes, when misorientation was ignored. The dominant 60°/[112¯0] intervariant boundaries had symmetric tilt (1¯101) boundary planes, which are a low energy configuration.

Achieving room temperature superplasticity in the Zn-0.5Cu alloy processed via equal channel angular pressing

The Zn-0.5Cu alloy was investigated in as-cast, homogenized state and after carrying out equal channel angular pressing four times at room temperature via route BC. Microstructure analysis using light microscopy and SEM/EBSD was performed, as well as tensile tests, misorientation angle distribution and texture analysis. The initial microstructure with the average grain size of 560 µm was refined to a grain size of approx. 1 µm. The obtained microstructure consists of uniaxial grains separated mostly by high angle grain boundaries. Mechanical characterization was performed at strain rates from 2 × 10−6 s−1 to 1 s−1 at room temperature. Ultrafine-grained zinc-copper alloy exhibited 510% elongation with the strain rate sensitivity equal 0.31, which was the first observation of room temperature superplasticity in Zn-Cu alloys. Based on the microstructure, misorientation angle and texture analysis, the main operating deformation mechanisms were distinguished for samples deformed at strain rates from 2 × 10−5 s−1 to 10−1 s−1.

Effect of Grain Misorientation Angle on Twinning Propagation in Ti–15Mo Alloy

This study was carried out to evaluate the effect of grain misorientation angle distribution on the deformation behavior and twinning of Ti–15Mo alloy. Cold rolling exhibited a significant texture with grains oriented along the {111}//normal direction, which correlate with a higher fraction of low-angle boundaries. This material showed a lower yield strength and higher elongation than those of the hot rolled material. The twinning propagation mainly occurred between neighboring grains with a low-angle relation. Consequently, the texture development was correlated with low-angle boundaries and affected by the increase in the twinning density, which increased the strain hardening rate.

Reconstruction of deformed microstructure using cellular automata method

The kinetics of microstructural evolution phenomena like recrystallization, grain growth, and phase transformation of deformed materials is affected by the characteristics of deformed microstructure. In fact, average grain size, grain morphology, texture and grain boundary properties of the deformed material determine the microstructure characteristics. In this paper, the reconstruction of deformed microstructure and changes in the microstructure in mesoscale are studied. Accordingly, the normal growth, topology deformation, and reconstruction of texture and grain boundary misorientation techniques are used to reconstruct the deformed microstructure. Therefore, probabilistic cellular automata method with hexagonal cells is used to create a microstructure with equiaxed grain morphology followed by a new modified topology deformation technique. In this technique, the quality and the quantity of the plastic deformation are considered by applying the deformation gradient tensor to the undeformed microstructure. Finally, a set of crystal orientations is created using a probabilistic algorithm relevant to the real texture of the material and then the crystal orientations are assigned to the deformed grains in such a way that satisfy the misorientation angle distribution. The accuracy of the numerical approaches is verified by comparing the experimental and the simulated results.

Microstructural characterization of a V2C and V8C7 ceramic-reinforced Fe substrate surface compound layer by EBSD and TEM

Microstructural characterization of a vanadium carbide (V2C and V8C7) surface compound layer was performed by electron backscatter diffraction (EBSD), scanning electron microscopy and transmission electron microscopy. The phase composition and phase distribution of the compound layer and the morphologies, grain sizes and crystallographic orientations of the vanadium carbides were investigated through elemental distribution and crystallographic analyses. The results indicated that the compound layer was composed of four distinct zones consisting of a compact and uniform V2C layer (thickness of 3.2 μm) over the topmost surface followed by a nonuniform V + V2C (2.4 μm) zone, a V8C7 layer (10.7 μm) and a relatively deep V2C + V8C7+α-Fe + Fe3C zone (23.2 μm). The V2C layer was found in the V-rich region, and the EBSD results revealed that this phase grew mainly along the {0001} and {11–20} orientations. However, the V8C7 phase appeared to be nearly random, which indicates that the growth of this phase had no specific orientation. The V2C and V8C7 phases exhibited low-angle grain boundaries, and their misorientation angle distributions were mainly concentrated within the range of 0–5°. Furthermore, nanoindentation tests demonstrated that the hardnesses of the V2C and V8C7 layers were 22.16 ± 0.52 and 32.42 ± 0.61 GPa, respectively. Examination of the vanadium carbide powders obtained by chemical extraction indicated that the V2C grains with a hexagonal structure exhibited equiaxial and short rod-like morphologies, and the V8C7 particles with a cubic structure possessed short rod-like and spherical morphologies.

Ultrafine gradient microstructure induced by severe plastic deformation under sliding contact conditions in copper

Sliding contact induces severe plastic deformation (SPD) at the surface of ductile materials and induces a microstructural gradient associated to a significant increase of hardness toward the surface. This gradient allows observing all stages of grain refinement in SPD, as illustrated here by the analysis of polycrystalline electrolytic copper tested in a coaxial tribometer. Materials tested in the cold-rolled state and after annealing were characterised by high-resolution electron backscattering diffraction and nano-indentation. The incremental plastic strain produces an ultrafine microstructure in the top layers, which gradually changes to the original size in unaffected material. In cold-rolled material, an intermediate recrystallised layer is observed. The separation of the Misorientation Angle Distribution (MAD) in a low-angle portion and a high angle portion allows characterising the accumulation of strain induced misorientation, while the Kernel Average Misorientation (KAM) provides information on the evolution of substructure at the finest levels. The results point toward a process where strain-induced effects compete continuously with recrystallisation, except for the surface layer in the cold-rolled material, where dynamic recrystallisation is dominant. Combining the information from KAM and subgrain size distribution, the measured hardness can be explained as a combination of grain size and dislocation hardening.

Microstructure and intergranular corrosion behavior of HAZ in DP-TIG welded DSS joints

In this study, 10.8-mm-thick workpieces of S32101 duplex stainless steel were welded using a keyhole deep penetration tungsten inert gas welding system. The misorientation angle distribution of grain boundaries in the heat affected zone was analyzed using the electron backscatter diffraction technique. Double-loop electrochemical potentiokinetic reactivation tests were performed to assess the intergranular corrosion resistance, based on the degree of sensitization. The volume fractions of ferrite and austenite and the grain boundary misorientation angle affect the intergranular corrosion of the welded joints. The relationship between the grain boundary misorientation angle of the austenite and the degree of sensitization was established. The volume fraction of austenite and the frequency of Σ3 coincidence site lattice boundary, increases with the increasing heat input. The misorientation angle distribution in the austenite, the frequency of Σ3 coincidence site lattice boundary, and the volume fraction of CrN influence the intergranular corrosion resistance of the heat affected zone.

Deconvoluting error in measurement of low angle misorientation distribution

Misorientation angle distribution gives information about the type and fraction of grain boundaries present in a material. Since grain boundaries affect various mechanical and functional properties of the material, the distribution of grain boundary misorientation is important in order to evaluate these properties. This becomes particularly important when we want to study the microstructure in finer detail, such as understanding the average misorientation within a grain. One of the techniques increasingly used in past two decades for characterization of grain boundary misorientation is electron back scatter diffraction (EBSD). Reliable detection of very small misorientation angles using conventional EBSD system is quite challenging due to the presence of measurement error. This makes the comprehensive characterization of microstructures difficult and prone to error. In order to prevent such problems, it is important to understand the nature of measurement error and find ways to minimize it. The present work aims to elucidate the effect of measurement error on the observed misorientation angle and its statistical distribution in low misorientation angle regime. A true strain of 0.3 was imposed during cold-rolling of Cu-5%Zn alloy sample. The rolled sample was then subjected to in-situ heating from room temperature to 500 °C (∼0.58 Tm). It was found that the overall measurement error in misorientation distribution consists of random error caused by limited angular precision and systematic error which manifests primarily in the statistical distribution of low angle misorientation. In this work, we show a way to deconvolute this overall error based on the measurement technique. We further show that this systematic error is not limited to any particular measurement technique, rather related to the presence of a lower bound in the measurement.

A comparative study of microstructure and tensile properties of Ti2AlNb joints prepared by laser welding and laser-additive welding with the addition of filler powder

The influence of the addition of filler powder on the microstructure and properties of laser-welded Ti2AlNb joints was comparatively investigated using scanning electron microscopy, transmission electron microscopy, electron back scattered diffraction, and tensile tests. The heat affected zone (HAZ) of laser-additive-welded joints was divided into B2, B2 + α2, and B2 + α2 + O — three regions with increasing distance from the fusion line. The HAZ of laser-welded joints could only be divided into two regions, viz., B2 + α2 and B2 + α2 + O. The microstructure of the fusion zone was composed of a single B2 phase for both laser welding and laser-additive welding. Columnar grains were observed in the fusion zone of laser-welded joints, while the B2 grains in the fusion zone of laser-additive-welded joints were basically equiaxed. A misorientation angle distribution analysis showed that the fraction of high-angle grain boundaries of laser-additive-welded joints was higher than that of laser-welded joints. The addition of filler powder promoted heterogeneous nucleation during solidification in laser-additive welding. Following tensile tests at room temperature, failure tended to occur in the fusion zone of the laser-welded joints and in the HAZ of the laser-additive-welded joints. The laser-additive-welded joints exhibited better tensile properties because of the higher Mo content as well as the equiaxed microstructure of the fusion zone.