Boundary Misorientation Distribution Research Articles

Nano-crystallization induced by high-energy heavy ion irradiation in UO2

Advanced microstructure investigations of the high-burnup structure (HBS) in UO2 produced by high-dose 84 MeV Xe ion irradiation are reported. Spark plasma sintered micro-grained UO2 was irradiated to 1357 dpa at 350 °C. The characteristic nano-grains and micro-pores of the HBS were formed. The grain size and grain boundary misorientation distributions of the HBS were measured using transmission electron microscopy based orientation imaging microscopy. Grain polygonization due to accumulation of radiation-induced dislocations was found to be the mechanism of nano-crystallization. The morphology of Xe bubbles was quantitatively investigated. This study provides crucial references for advanced fuel performance modeling of high-burnup UO2.

Misorientation distribution of high angle boundaries formed by grain fragmentation: EBSD-based characterization and analysis performed on heavily deformed iron

When studying crystal lattice fragmentation during plastic deformation of metals, it is of great importance to characterize high angle deformation-induced boundaries as long as their creation and evolution controls grain refinement. A problem exists however to separate a contribution of DIBs to overall misorientation distribution from a contribution of original grain boundaries, particularly when total lengths of high-angle deformation-induced and original grain boundaries are comparable. In the present study, a method making possible this separation based on electron backscattering diffraction is suggested and used to characterize evolution of deformation-induced boundary misorientations in polycrystalline iron deformed under various conditions. The method is shown to provide reasonable accuracy up to a strain of ~ 2 in the case of uniaxial compression and up to a strain of 5 in the case of multi-axial forging. It has been shown that character of the deformation-induced boundaries evolution in iron changes weakly when increasing deformation temperature from room temperature to 400C. At the same time, this evolution differs significantly in iron deformed by uniaxial compression and multi-axial forging. In all cases considered in the present study a deformation-induced boundary misorientation distribution can be represented as a superposition of three partial distributions. The first two partial distributions correlate with those obtained earlier in transmission electron microscopy studies. The third partial distribution suggested in the present study describes the highest-angle part of deformation-induced boundary misorientation distribution. Each partial distribution evolves according to its own law with proceeding deformation.

Microtexture analysis of restoration mechanisms during high pressure torsion of pure nickel

The grain refinement in severe plastically deformed pure metals is limited to few 100nm and the saturated microstructure primarily depends on deformation temperature and impurities. In present study high pressure torsion (HPT) technique is used for understanding the mechanism of microstructure restoration processes at saturation state for 99.99% pure nickel between 77 and 673K. The saturation grain size increased with deformation temperature. Electron back scatter diffraction (EBSD) measurements along with microtexture evaluation show that the grain boundary (GB) misorientation distribution remains similar for all the temperatures; however, new texture components and special GB characteristics start to develop at higher temperatures. The differences in the GB migration characteristics and the resulting microtexture changes suggest that at low temperatures GB migration is facilitated by the variations in elastic strain energy density among neighboring grains; however, with increasing temperature thermally driven migration processes induced by plastic stored energy become important.

Grain Boundary Assemblies in Dynamically-Recrystallized Austenitic Stainless Steel

The grain boundary misorientation distributions associated with the development of dynamic recrystallization were studied in a high-nitrogen austenitic stainless steel subjected to hot working. Under conditions of discontinuous dynamic recrystallization, the relationships between the grain or subgrain sizes and flow stresses can be expressed by power law functions with different grain/subgrain size exponents of about −0.76 (for grain size) or −1.0 (for subgrain size). Therefore, the mean grain size being much larger than the subgrain size under conditions of low flow stress gradually approaches the size of the subgrains with an increase in the flow stress. These dependencies lead to the fraction of high-angle boundaries being a function of the flow stress. Namely, the fraction of ordinary high-angle boundaries in dynamically-recrystallized structures decreases with a decrease in the flow stress. On the other hand, the fraction of special boundaries, which are associated with annealing twins, progressively increases with a decrease of the flow stress.

Effect of the Grain-boundary Misorientation Distribution on the Intergranular Voltage Relaxation of Bi1.65Pb0.35Sr2Ca2Cu3 O 10+δ Ceramic Samples

The impact of the grain boundary misorientation distribution (GBMD) on the intergranular voltage relaxation (V − t) curves at zero applied magnetic field in Bi1.65Pb0.35Sr2Ca2Cu3O10+δ (Bi-2223) ceramic samples has been investigated. Changes in the GMBD were realized by subjecting powders of Bi-2223 to two different uniaxial compacting pressures (UCP) before the last heat treatment of the samples. The GBMD was then determined from X-ray rocking curves and revealed significant differences between intergranular media of the specimens. It was found that the UCP results in a two-time reduction in the population of high-angle grain boundaries (𝜃 > 12∘) while the orientation homogeneity of the grain boundaries rises ∼ 30 %, indicating an improvement of the degree of texture of the materials. Such changes are mirrored in the behavior of the V − t curves which are explained by invoking differences in the rearrangement of the transport current driven by the angular dependence of the critical current density along grain boundaries. Numerical simulations of the V − t curves support the experimental V − t results and further suggest the occurrence of current localization in conductive paths within the materials.

Relations between grain boundary character distribution, misorientation distribution and two-point orientation distribution

Relations are found between grain boundary character distribution, misorientation distribution and two-point orientation distribution. Some of these relations are analogies for polycrystals of the Debye–Anderson–Brumberger formula and Berryman formula established for two-phase composites.

Localized microstructural characterization of a dissimilar metal electron beam weld joint from an aerospace component

Hydrogen induced cold cracking (HICC) and hydrogen embrittlement (HE) are influenced by the microstructural evolution, residual plastic strain (i.e. local misorientation), recrystallization of grains and the resultant grain boundary characteristic distribution (GBCD) brought about by welding processes. HICC and HE are known to cause failures in aerospace components and it is vitally important to quantify the microstructural evolution, degree of residual plastic strain and determine the GBCD across dissimilar weld joints in order to assess the susceptibility of the weld joint to these phenomena. In this investigation a full a microstructural characterization study was carried out at various locations within and around a dissimilar weld joint of pulse-plated nickel (PP-Ni) and Inconel 718 (IN718), taken from an aerospace component. Areas examined included the base metals, weld fusion zone and heat affected zones on both sides of the weld joint, formed via electron beam welding. Scanning electron microscopy (SEM) in combination with electron backscatter diffraction (EBSD) was employed to measure the residual plastic strain, grain structure, grain size distribution, crystal orientation distribution, grain boundary misorientation distribution and GBCD of the dissimilar metal weld joint. Finally a metallurgical examination was carried out using SEM on the IN718 HAZ in order to investigate the secondary phase precipitation arising from the welding process. The results shows large variety of GBCD, crystallographic orientation distribution, local plastic strain distribution and grain size, shape and structure distribution across dissimilar weld joint. And these localized microstructural characterized data sets need to be carefully transferred using data-driven approach in order to develop predictive multiscale material modelling for hydrogen induced cracking and hydrogen embrittlement.

Prominent role of basal slip during high-temperature deformation of pure Mg polycrystals

Several controversies regarding the mechanical behavior of Mg at moderate temperatures and quasi-static strain rates remain unsolved. In particular, the fundamental reasons behind the anomalous grain size dependence of the flow stress during power law slip-dominated creep as well as of the observed decrease in the strain anisotropy with increasing temperature are still unknown. This work aims to shed light on these two issues by bringing new experimental evidence obtained by electron backscattered diffraction-aided slip trace analysis. Two pure Mg polycrystals, with average grain sizes (d) of 36 and 19μm, and similar textures and grain boundary misorientation distributions, were prepared by rolling and annealing, then subsequently tested in tension along the rolling direction at 150 and 250°C and at 10−3s−1. Their macroscopic mechanical response is related to the corresponding microstructure and texture evolution after straining, as well as to the frequency of slip traces corresponding to basal, prismatic and pyramidal 〈c+a〉 systems. This work highlights the prominent role of basal slip during high-temperature deformation of microcrystalline pure Mg and suggests that this enhanced basal activity might be key to rationalizing the two controversies under scrutiny. Furthermore, the current study attests to the impact on slip activity of grain size–texture correlations and of percolation between grains that are favorably oriented for basal slip during high-temperature deformation of pure Mg.

Effect of grain size on slip activity in pure magnesium polycrystals

The aim of this work is to investigate the effect of grain size on slip activity, a critical issue for materials design that remains unsolved. Three pure magnesium polycrystals with average grain sizes (d) of 36, 19 and 5μm and with very similar textures and grain boundary (GB) misorientation distributions were designed by selecting appropriate rolling and annealing conditions. The three microstructures were tested in tension along the rolling direction (RD) at 50°C and at an strain rate of 10−3s−1 and the incidence of the different deformation mechanisms was evaluated by electron backscattered diffraction (EBSD)-assisted slip trace analysis. A clear transition from non-basal to basal slip-dominated flow takes place when d decreases below 36μm, i.e. at grain size ranges that are very relevant for most structural applications. This change in slip activity is shown to be consistent with an increase in the CRSSnon-basal/CRSSbasal ratio with decreasing d, i.e. with a more potent GB strengthening of non-basal than basal systems. Accordingly, a transition in the mechanisms responsible for the accommodation of local stress concentrations at GBs takes place with decreasing grain size. Intergranular compatibility becomes increasingly difficult with grain refinement due to the progressively lower availability of active deformation mechanisms. Macroscopically, these phenomena translate to a reduction of work hardening and ductility with decreasing grain size.

Microstructural Characterization of a Magnesium Alloy Processed by Equal Channel Angular Pressing

<p>Samples of as-extruded ZK60 magnesium alloy were subjected to 6 passes of equal-channel angular pressing (ECAP) following route A. The processing temperature was decreased with the number of passes. The structural evolution of the deformed samples was analyzed using Electron Backscattered Diffraction (EBSD) and X-Ray Diffraction. The grain boundary misorientation distribution showed a reduction in the grain size accompanied by<br />a large proportion of high angle grain boundaries and the presence of recrystallization processes. XRD results showed that with the increment of the applied strain the peaks presented a slight variation of the angles.</p>

Microstructure-Mechanical Properties Relationships for Complex Microstructures in High Strength Steels

Due to the increased complexity of steel microstructures, when considering the application of available Hall-Petch type equations for yield strength prediction, a number of difficulties raises. For example, the correlation between grain size measurements by EBSD technique and optical microscopy (OP) in complex microstructures is required in order to integrate data to the traditional equations developed for OP results and ferrite-pearlite microstructures. Besides, the introduction of some additional terms to the equations to account for precipitation, C in solution and forest dislocation contributions presents some difficulties that need to be overcome to improve prediction accuracy. Different microstructures (ferrite-pearlite, bainite, quenched and Q&T) have been produced by thermal and thermomechanical treatments, followed by microstructural characterisation and mechanical testing. A Hall-Petch coefficient dependent on the boundary misorientation distribution is proposed. This approach allows dealing in a similar way ferritic, bainitic and martensitic microstructures. The Hall-Petch coefficient, thus defined, corresponds to the previously proposed by Pickering for ferrite, while bainitic microstructures give a smaller value. Additionally, the equation used to express the fracture appearance transition temperature of ferritic-pearlitic microstructure has been generalized from the developments made in the calculation of the yield stress.

ZK60 alloy processed by ECAP: Microstructural, physical and mechanical characterization

Lately, magnesium alloys have been extensively investigated through severe plastic deformation (SPD) methods in order to extend their application to structural components. In the present work, as-extruded ZK60 magnesium alloy samples were subjected to 4 passes of equal-channel angular pressing (ECAP) at a processing temperature of 250°C, following route Bc. The microstructural evolution of the deformed grains was analyzed using electron back-scattered diffraction (EBSD). The Orientation Imaging Microscopy (OIM) maps, together with the grain boundary misorientation distribution showed a reduction in the grain size accompanied by a large proportion of high angle grain boundaries. Calorimetric analysis showed a slight increase in the recrystallization temperature of the ECAPed magnesium samples. Mechanical tests showed an improvement in the elongation-to-failure after 4 ECAP passes, which were about 2 times higher than in the as-extruded sample. Also a brittle to ductile transition was observed.

Microstructure and mechanical properties of multi-pass friction stir processed aluminum alloy 6063

The effect of overlapping between consecutive passes of Friction Stir Processing (FSP) was examined on the microstructure, microtexture, and mechanical properties of Al 6063. FSP was carried out at three overlapping (OL) percentages, i.e. 25, 50, and 75, and at constant rotational and traverse speeds of the FSP tool. Microstructural analysis based on grain boundary misorientation distribution, grain size and microtexture were made by Electron Back Scattered Diffraction (EBSD), while mechanical properties were evaluated by microhardness and tensile tests. For the three OL percentages, similar grain refinement (in the range of 3–6μm) occurred in the processed zone. The microtexture measurements in the OL area show the formation of shear texture, having a stronger intensity of B/B¯ component by increasing the OL percentage. On the other hand, the tensile and microhardness tests indicate a reduction in the strength and microhardness of FSP samples with increasing the OL percentage due to the dissolution of the hardening precipitates. The overlapping effect was further evaluated on the thermal stability of microstructure, microtexture, and mechanical properties of FSP samples by applying solution heat-treatment and aging (SHTA). The results show that SHTA led to grain growth with an average grain size of 35μm. However, it caused limited weakening of the shear texture in the OL area with no sign of recrystallization. In addition, SHTA led to precipitate formation and thus restoring the strength and microhardness to the levels of the base metal.

Development of Boundary Misorientations During Grain Growth in Silicon Steels

Changes in grain size, texture and misorientation distributions have been monitored during extensive normal grain growth in 3%Si steels. The boundary misorientation distributions deviate significantly from the Mackenzie relationship. Comparisons of correlated and uncorrelated distributions show large excesses of low angle boundaries. However, these are not a result of low energy boundaries being favoured during grain growth since the deviation diminishes as growth proceeds. The effect originates in the nucleation of grains in colonies of similar orientation during primary recrystallisation. A slight tendency for promotion of 60º boundaries may indicate some preference for the retention of lower energy twin boundaries during grain growth in silicon steel.

Kinetic Monte Carlo simulation of the electrodeposition of polycrystalline copper: Effects of substrates and deposition parameters on the microstructure of deposits

A two-dimensional cross-sectional poly-lattice kinetic Monte Carlo (2DCSP-KMC) model has been developed for simulation of the electrodeposition of polycrystalline copper on either a copper or gold substrate. The model has proved capable of capturing the effects of the deposition parameters, including the applied electrode potential, the concentration of Cu2+ ions and the temperature on the resultant deposit microstructure, the evolution of the grain density, the grain size, the variance of grain size and the grain boundary misorientation distribution. Three unit shapes namely the funnellike, columnar and pyramidal shapes, resulting from the competition between the growth of a grain and its neighbouring grains, are abstracted from a variety of morphology of individual grains to describe the effects of the deposition parameters on the deposit microstructure. The fundamentals of the dependence of these effects on substrate have been discussed and ascribed to the competition amongst the possible KMC events at each KMC step.

Crystallographic Analysis of Isothermally Transformed Bainite in 0.2C–2.0Mn–1.5Si–0.6Cr Steel Using EBSD

The crystallography of bainite, transformed isothermally at 450 °C in 0.2C–2.0Mn–1.5Si–0.6Cr steel, was investigated by electron backscatter diffraction (EBSD) analysis. The orientation relationship (OR) was found to be closer to Nishiyama–Wassermann (N–W) than Kurdjumov–Sachs orientation relationship. Bainite microstructure consisted of parallel laths forming a morphological packet structure. Typically, there were three different lath orientations in a morphological packet. These orientations were dictated by a three specific N–W OR variants sharing the same {111} austenite plane. A packet of bainite laths with common {111} austenite plane was termed as crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size. Locally, crystallographic packets with only two dominant orientations were observed. This indicates strong local variant selection during isothermal bainite transformation. The relative orientation between the variants in crystallographic packets was found to be near 60°/<110>. This appears to explain the strong peak observed in the grain boundary misorientation distribution near 60°. Bainite also contained pronounced fraction of boundaries with their misorientation in the range of 2.5°–8° with quite widely dispersed rotation angles. Spatially these boundaries were found to locate inside the bainite laths, forming lath-like sub-grains.

Visco-plastic self-consistent modelling of a grain boundary misorientation distribution after equal-channel angular pressing in an AZ31 magnesium alloy

This study applies a visco-plastic self-consistent (VPSC) model to an AZ31 alloy processed by equal-channel angular pressing. The study focuses on the possibility of reproducing a grain misorientation distribution and the distribution of coincident site lattice boundaries in the model framework. Co-rotation and a magnesium misorientation scheme are employed together with the conventional VPSC model to improve its predictions. The results of the model are then compared with experimental data.

High Strain Rate Superplasticity in a Micro-grained Al–Mg–Sc Alloy with Predominant High Angle Grain Boundaries

Friction stir processing (FSP) was applied to extruded Al–Mg–Sc alloy to produce fine-grained microstructure with a grain size of 2.2 μm. Electron backscatter diffraction (EBSD) result showed that the grain boundary misorientation distribution was very close to a random grain assembly for randomly oriented cubes. Super-plastic investigations in the temperature range of 425–500 °C and strain rate range of 1×10−2−1×100 s−1 showed that a maximum elongation of 1500% was achieved at 475 °C and a high strain rate of 1×10−1 s−1. The FSP Al–Mg–Sc exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. The origin for enhanced superplastic deformation kinetics in the FSP alloy can be attributed to its high fraction of high angle grain boundaries (HAGBs). The analyses of the superplastic data and scanning electron microscopy (SEM) examinations on the surfaces of deformed specimens indicated that grain boundary sliding is the main superplastic deformation mechanism for the FSP Al–Mg–Sc alloy.

Achieving homogeneity in a Cu–Zr alloy processed by high-pressure torsion

A copper alloy, Cu–0.1 %Zr, was subjected to severe plastic deformation at room temperature using quasi-constrained high-pressure torsion. Disks were strained through different numbers of revolutions up to a maximum of ten turns under an applied pressure of 6.0 GPa and then examined to evaluate the evolution in the Vickers microhardness, Hv, and the microstructure. The results show lower values of Hv in the center regions of the disks in the early stages of processing but a gradual evolution to a high degree of hardness homogeneity after five and ten turns. Under conditions of hardness homogeneity, the distributions of the grain boundary misorientations are essentially identical at the center and the periphery of the sample. Homogeneity was further confirmed by conducting tensile testing at elevated temperatures where similar stress–strain curves and similar elongations to failure were recorded after processing through five and ten turns of HPT.

Grain boundary assembles developed in an austenitic stainless steel during large strain warm working

Abstract The evolution of strain-induced grain boundaries in a 304-type austenitic stainless steel during multiple warm forging was studied. The developed grain boundary distributions significantly depended on the processing temperature. The misorientation distribution of deformation boundaries that evolved after total strain of 4 at 500 °C is characterised by a sharp maximum against small angles below 10° and a flat‐type distribution with almost the same fractions for different misorientations for high-angle boundaries. On the other hand, two sharp peaks on the boundary misorientation distribution are developed after processing at 800 °C. One of them corresponds to low-angle subboundaries and the other results from twin boundaries with misorientations of 60°. The difference in the characters of strain-induced grain boundaries is associated with different mechanisms of dynamic recrystallization operating at 500 and 800 °C.