Electron Backscatter Diffraction Method Research Articles

A comparative study of plastic deformation mechanisms in room-temperature and cryogenically deformed magnesium alloy AZ31

Abstract In this work, a commercial cast magnesium alloy AZ31 was compressed at room and cryogenic temperatures (RT and CT) to study how the twins and dislocations affect the flow stress, plastic strain and strain hardening. Cryogenic deformation suppresses the dislocation activities to enable the effects of dislocation slip and twinning on plasticity to be separately interrogated. A quasi-in-situ Electron Backscatter Diffraction (EBSD) method was used to trace the evolution and distribution of twins and geometrically necessary dislocation (GND) density manifested as kernel average misorientation (KAM) in the same group of grains before and after plastic deformation, at RT and CT respectively. Compared to the RT deformation, higher flow stress, higher strain hardening rate and moderately reduced strain to failure were observed in the CT deformation. These interesting findings were rationalised by the subsequent EBSD analysis. It is found that the increased fraction of finer twins and twin-twin interactions rather than dislocation density leads to higher stress and higher strain hardening at CT. This suggests that the hardening effect induced by twin boundary and twin-twin interactions is considerably more effective than dislocation density hardening.

Microtexture and Rolling Deformation Behavior Analysis of the Formation Mechanism Fe3O4 at the Interface Formed on Hot-Rolled High-Strength Steel

In order to better understand the formation mechanism of tertiary oxide scale in high-strength steel during hot rolling, the microtexture of the oxide layer has been characterized and analyzed by the electron backscatter diffraction (EBSD) method. The results show that the Fe3O4 phase in the oxide layer has a two-phase heterogeneous morphology, Fe3O4 in the oxide layer comprises columnar grains, and Fe3O4 near the substrate comprises spherical grains. As the reduction rate increases, the Fe2O3 layer is gradually wedged into the surface of the Fe3O4 layer. Fe3O4 forms a <110> fiber texture at a reduction rate of 10%. The inner layer of the oxide scale comprises spherical grains, and Fe3O4 is preferentially nucleated and precipitated in the direction of Fe surface grains <110> texture. With the increase in the reduction rate, the {112}<−1−21> directional slip system shows the lowest Schmidt factor value, so the grains with a low Schmidt factor exhibit higher stored strain energy. The formation of the spherical Fe3O4 seam layer close to the steel matrix is the result of the combined effect of the stress state at the matrix and ion diffusion.

The effects of prior creep–fatigue on the strain rate sensitivity of a P92 welded joint

The high-temperature creep–fatigue (CF) interaction on the service damage of P92 welded joint was uncovered based on the evolution of local strain rate sensitivity of strength by nanoindentation. The individual creep, fatigue and creep–fatigue loadings were performed to reveal the CF interaction on both the structure and mechanical properties of the welded joint. And CF loadings interrupted at lifetime fractions of 10%, 20%, 30%, 60%, 100% were adopted to acquire the high-temperature service damage evolution. The structure changes of the heterogeneous welded joint during service damage were characterized by using electron backscatter diffraction method. According to structure evolution including the changes of grain size and dislocation density during high-temperature deformation, we systematically discussed the local variation mechanism of hardness and SRS at different microstructural zones.

New Approach to Distinguish Copper Molten Marks Based on Quantitative Microstructure Analysis Using Electron Backscatter Diffraction

Molten marks identified at a fire site can aid in determining the cause of a fire. Quantitative analysis of such molten marks has not yet been reported, despite several identification methods for molten marks being proposed based on optical micrographs. Herein, we propose a new methodology to establish the quantitative microstructure parameters of the molten marks using electron backscatter diffraction (EBSD). The globules were generated by heating the copper conductor to 1100°C in a non-energized state. Then, the arc beads were prepared by shorting the copper wire at 25°C and 900°C in an energized state. The globules did not show evident demarcation lines and indicated that the microstructure consisted of globular or dendritic grains; this demonstrated that their characteristics were distinctly different from those of arc beads. However, the primary arc beads (PABs) shorted at 25°C exhibited a strong (001) texture perpendicular to the demarcation line and comprised large fractions of columnar grains with a small grain aspect ratio (GAR). The microstructure of secondary arc beads (SABs) shorted at 900°C presented a mixture of elongated and equiaxed grains with a large GAR and no specific development of texture. The GAR and (001) fraction perpendicular to the demarcation line could be discriminant parameters of PABs and SABs. Consequently, EBSD methods can be newly applied for the classification of globules, PABs, and SABs based on the quantitative microstructure information and orientation distribution.

Temperature-Dependent Mechanical Stability of Retained Austenite in Thermomechanically Processed 3Mn Steel

The temperature-dependent mechanical stability of retained austenite in medium-Mn transformation induced plasticity 0.17C-3.3Mn-1.6Al-1.7Al-0.22Si-0.23Mo thermomechanically processed steel was investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) methods. Specimens were deformed up to rupture in static tensile tests in a temperature range 20–200°C. It was found that deformation temperature affects significantly the intensity of TRIP effect. In case of specimens deformed at temperatures higher than 60°C, a gradual temperature-related decrease in the stability of γ phase was noted. It indicates a progressive decrease of the significance of the TRIP effect and at the same time the growing importance of the thermally activated processes affecting a thermal stability of retained austenite.

Effect of uniaxial loading direction on mechanical responses and texture evolution in cold pilgered Zircaloy-4 tube: Experiments and modeling

Mechanical responses of cold pilgered Zircaloy-4 (Zr-4) tubes are measured under different uniaxial (tension/compression) loading directions. Tensile tests were carried out along three directions: rolling direction (RD), transverse direction (TD) and 45° to the rolling direction (45RD) at quasi static strain rate. Compression test was also performed in the RD. Texture was characterized at intermediate strains with electron backscattered diffraction method. For different uniaxial loading conditions, anisotropic yield stress and strain hardening behavior can be observed. Such complex mechanical behavior is well explained with the variations in activation of deformation mechanisms predicted by the Schmid factor distribution and visco-plastic self-consistent (VPSC) model. In order to calibrate the VPSC model’s parameters more accurately, both mechanical responses and texture changes for four different directional tests are compared with VPSC simulations. The calculated Lankford coefficients are also calibrated with anisotropic tensile results. Furthermore, a deepening understanding of the texture evolution under different loading directions were disclosed based on VPSC method. The results indicate that the activity of deformation mechanisms changes significantly with different uniaxial loading directions. Non-prismatic slips with higher critical resolved shear stress, e.g. basal and pyramidal slip, make larger contributions to the yield and strengthening stress resulting in the highest stress in TD compared with RD and 45RD. Compression test along RD prefers the activation for tensile twinning (TTW). Meanwhile, TTW can be detected in 45RD and TD tension owing to the spread distribution of initial texture towards TD.

Texture Formation in a Polycrystalline Fe–Ni–Co–Al–Ti–B Shape Memory Alloy

In polycrystalline Fe-Ni-Co-Al-based shape memory alloys, control of the recrystallization texture is significantly important to improve the ductility by suppressing the brittle precipitates of the β-B2 phase at grain boundaries during aging treatment. In this paper, the texture evolution in the recrystallization process was systematically investigated by means of the electron backscatter diffraction (EBSD) method in an Fe–Ni–Co–Al–Ti–B polycrystalline alloy. The development of a {110}<112> texture was confirmed in the 98.5% cold-rolled sheet specimen. After primary recrystallization annealing, the γ matrix containing the β phase showed the continuous recrystallization remaining in the same orientation with a deformation texture after annealing at 1000°C. Grain growth of the γ phase was interrupted by the β phase. Then, abnormal grain growth of {210}<001> grains began occurring in concurrence with the dissolution of the β phase and the main recrystallization texture changed from {110}<112> to {210}<001> at temperatures higher than 1100°C.

Microstructure of zirconium fuel claddings: TEM and EBSD studies of as-received and ne-utron-irradiated materials

Zirconium fuel claddings act as a first barrier against release of fission products during nuclear power plant operation and interim storage of the spent fuel. During the reactor operation, cladding tubes are exposed to different stress level at elevated temperatures and neutron irradiation in corrosive environment. It causes a material degradation by corrosion, cladding embrittlement by hydrides and radiation-induced damage or radi-ation growth and creep of the fuel rods. The irradiation damage effects mainly contribute to the loss of material ductility. In our study, microstructure of as-received (non-irradiated) Zr-alloys used in LWR (Zr1Nb, Zr-1Nb-1.2Sn-0.1Fe, Zr-1.5Sn-0.2Fe-0.1Cr) were examined by electron microscopy methods. Transmission electron microscope (TEM) was used to describe the microstructure of claddings used in different reactor conditions and identify the radiation-induced damage, which is presented on Zr1Nb irradiated to one standard campaign in the VVER-1000 active zone. Following Electron Backscatter Diffraction (EBSD) method on transparent foils complements the TEM results in larger area, i. e. by grain size and orientation or analysis of local misorienta-tion after irradiation. Radiation-induced damage was observed in Zr1Nb metallic matrix as type disloca-tion loops, presence of radiation-induced precipitates or partial amorphization of the secondary phase particles. EBSD method showed no changes in crystallographic orientation, but a local increase of dislocation density can be affected by neutron irradiation.

Inhomogeneous Gradient Microstructure and Mechanical Properties of Thick Copper Plate via Friction Stir Welding

Inhomogeneous microstructural gradient of the friction stir welding (FSW) joint in T2 thick copper plates was studied in the present work. Microstructures along the thickness direction of the nugget zone (NZ) were systematically characterized via using electron back-scattering diffraction (EBSD) method. The corresponding mechanical properties of different layers in the NZ, involving tensile performance and microhardness, were examined as well. Through our characterization results of each layer in the NZ, it is found that a significant gradient microstructure is generated due to the temperature and strain gradient produced by FSW process. Refined equiaxed grains dominate the NZ’s lower areas, while coarse grains with a mass of twins occupy the upper regions. Along the thickness direction from the top to bottom, the mean grain size of the NZ gradually declines from 15.56 ± 8.79 to 5.74 ± 3.06 μm. Such refined microstructure in the lower layer of the NZ leads to the enhancement of mechanical properties of the specimens due to the Hall–Petch relationship. Particularly, the strength and hardness of the lowest layer are found to be comparable with the cold-rolled base metals. The result further uncovers the underlying mechanism about how the microstructural gradient forms on FSW and how it affects the mechanical performance of the weld joint. The work also inspires that, through controlling the heat input of the present FSW method, the gradient microstructure of NZ can be rationally adjusted to improve the welding quality.

Nature of the Clinoenstatite Rim in Refractory Forsterite-Rich Objects from Carbonaceous Chondrites: First Results of Study by the Method of Electron Backscatter Diffraction (EBSD)

A clinoenstatite rim occurs in many refractory forsterite-rich objects; however, specialized studies of it have not been carried out. The first data from the study of refractory forsterite-rich objects by electron backscatter diffraction (EBSD) are reported. It is shown that the clinoenstatite rim in refractory forsterite-rich inclusions has a reactionary nature and was formed as a result of nebular processes. The rim is almost entirely composed of clinoenstatite. Forsterite grains have a block internal structure with a maximum misorientation angle of 10° within a single crystal.

Quasi-in-situ observations of low-angle grain boundaries, twins and texture evolution during continuous annealing in a cold-rolled Mg-Zn-Gd alloy

The evolution of texture, low-angle grain boundaries and twins in a cold-rolled Mg-Zn-Gd alloy during continuous annealing at 250 °C were tracked by quasi-in-situ X-ray diffraction and quasi-in-situ electron backscatter diffraction method, and the influence of extension twins on texture was discussed. The results show that during the whole annealing process, the main texture component is always split toward transverse direction (TD) and the tilting angle hardly changes. However, the peak position changes between the two TD texture components. During continuous annealing, most pre-existing low-angle grain boundaries retain their original misorientation (including angles and axes) and hardly migrate. In addition, a new low-angle grain boundary occurs when two grains with similar orientation grow to connect with each other. Boundaries of pre-existing extension twins tend to migrate with different velocity. Moreover, with the migration, extension twins grow and gradually swallow their matrix grains completely, and thus re-orientate the TD orientations of twinned-grains from one side to another. Considering that nuclei display no preferred orientation between +TD and –TD, the changing peak position is considered to be caused by the migration of ETBs.

The preferential growth and related textural evolution during static recrystallization in a cold-rolled Mg–Zn–Gd alloy

The grain growth process plays an important role in the texture formation in magnesium alloys. The microstructural and micro-textural evolution of a cold-rolled Mg–Zn–Gd alloy during annealing at 350 °C for 60–190 min were tracked by quasi-in-situ electron backscatter diffraction method. The results show that grain growth takes place gradually with the annealing time increasing. Moreover, the TD-split texture maintains the texture type but alters in three aspects - the increased tilting angle, the decreased pole intensity and the widened distribution of high-intensity area. Grains with their c-axis tilting 45–70° from normal direction show preferential growth which is closely associated with the texture changes. The original grain size advantage is one of the important factors leading to the growth advantage, some grain boundaries, such as 50–60° [ 1 ¯ 2 1 ¯ 0 ], 50–60° [ 2 ¯ 7 5 ¯ 0 ], 60–70° [ 1 ¯ 2 1 ¯ 0 ] (∑18b), and 70–80° [ 10 1 ¯ 0 ] (∑10) are also considered to be related to this preferential growth.

Quasi-in-situ observing the rare earth texture evolution in an extruded Mg-Zn-Gd alloy with bimodal microstructure

The static recrystallization and associated texture evolution were investigated in an extruded Mg-Zn-Gd alloy with bimodal microstructure based on a quasi-in-situ electron back-scatter diffraction (EBSD) method. The typical rare earth (RE) texture formed during annealing, evolving from the bimodal microstructure with [10 1 ¯ 0] basal fiber texture that consisted of fine recrystallized (RXed) grains and coarse unrecrystallized (unRXed) grains elongated along the extrusion direction. In both RXed and unRXed regions, the RXed nucleation produced randomized orientations without preferred selection and the RXed grains with RE texture orientation had more intensive growth ability than those with basal fiber orientation, thereby leading to the preferred selection of RE texture orientation during grain growth. The relationships between stored strain energy, solute drag, grain growth and texture evolution are discussed in detail. This study provided direct evidence of the RE texture evolution in an extruded Mg-RE alloy, which assists in understanding the formation mechanisms for RE texture during extrusion and better developing wrought Mg alloys with improved formability.

Strain rate and mechanical stability in determining deformation behavior of beta Ti alloys

The effect of strain rate on flow behaviors of three beta Ti alloys with different beta phase stability was studied. The alloys were subjected to compression deformation at four strain rates ranging from 7 × 10−5 to 7 × 10−2 s−1 at room temperature. Microstructure evolution was characterized using laser scanning microscopy and electron backscattered diffraction methods. The effect of strain rate on the yield strength was found to vary between the alloys increasing with increased stability. The microstructural observations showed that the alloys exhibited different deformation mechanisms: deformation induced martensite formation, twinning and slip band formation. The strain hardening exponent (n) depended on the deformation mechanism, i.e., the stability. In the low and intermediate stability alloys showing martensite formation and combination of the deformation mechanisms respectively, n was about 0.07 independent of strain rate. In the alloy with the highest stability it was only 0.01 when the deformation mechanism was dislocation slip at low strain rates but increased significantly to ≈0.13 at high strain rates when also twinning occurred. The strain rate sensitivity factor (m) was about 0.014 and 0.017 for the low and intermediate stability alloy respectively independent of strain, but very low ≈ 0.002 for the highest stability alloy though increasing to ≈ 0.011 with increasing strain. Based on the experiments, the stability-strain rate-mechanism diagram was introduced to predict the effect of phase stability and strain rate on the deformation mechanism of beta Ti alloys.

Influence of microstructure and texture evolution on magnetic properties attained by annealing of a cold-rolled Fe-Co-10V semi-hard magnetic alloy

Typically, in processing of Fe-Co-V type magnetic alloys, hot rolled sheets are being severely cold rolled and then annealed at a specific temperature for an appropriate soaking time. In this work, in order to achieve the optimal magnetic properties, effect of various annealing cycles after 86% cold rolling on a 40Fe-50Co-10V (wt%) alloy were examined and the correlation between magnetic properties and microstructure and texture evolution was investigated. In order to supplement extensive microstructure evolution data available from previous publications, texture analysis was carried out using X-ray and electron backscatter diffraction methods and magnetic properties were determined via a vibrating sample magnetometer. It was observed that by increasing the annealing temperature the saturation magnetization decreased due to the formation of paramagnetic (Co,Fe)3V precipitates and austenite, while the coercivity increased in a reverse manner. The impact of austenite on decreasing the saturation magnetization was more pronounced than that of precipitates. Phase boundaries were more effective than grain boundaries in promoting the coercivity. However, nano-size (Co,Fe)3V precipitates inside the ferromagnetic matrix were also important in enhancing the coercivity. Besides formation of the precipitates, variations of the intensity of 〈111〉 direction as hard magnetic direction and intensity of {111}〈112〉 texture component, led to the alteration of magnetic remanence attained at 500–750 °C. Based on application of these alloys as semi hard and hard magnets, the optimal magnetic properties are achieved at a temperature range of 550–650 °C.

Effect of Stoichiometry and Ordering on the Microstructure of Titanium Monoxide TiO y.

The structure of titanium monoxide TiOy with different stoichiometries and long-range order degrees was studied by using X-ray diffraction, electron backscatter diffraction, Raman spectroscopy, and electron microscopy methods. It was established that the composition of the phases formed in annealed TiOy depends on the titanium monoxide stoichiometry. A new phase precipitation mechanism is proposed. The migration of vacancies in dislocations and their accumulation on grain boundaries play an important role in the formation of new phases. The stoichiometry of quenched titanium monoxide (TiOy) was found to affect the intensity of the peak associated with the vibrational mode of the Ti–O bond in Raman spectra.

Investigation of Size Effects Due to Different Cooling Rates of As-Quenched Martensite Microstructures in a Low-Alloy Steel

Martensite transformation is a complex mechanism in materials that is classically initiated by a suitable heat treatment. This heat treatment process can be optimized based on a better understanding of the physical mechanisms on the length scale of several prior austenite grains. It is therefore appropriate to consider individual process steps of heat treatment in isolation. The aim of this study is to characterize the microstructural size changes caused by a variation of the cooling rate applied during the quenching process. For this purpose, individual martensitic microstructures from different heat treatments are analyzed using the electron backscatter diffraction (EBSD) method. With special orientation relationships between the parent austenite and martensite, the structure of the prior austenite grains and the close packet plane packets can then be reconstructed. The influence of the heat treatments on these characteristics as well as on the martensite blocks is thus quantified. No significant influence of the quenching rate on the sizes of martensite blocks and packets could be found.

Oriented nucleation causing unusual texture transition during static recrystallization annealing in cold-rolled Mg–Zn–Gd alloys

The static recrystallization, nuclei orientations, and texture evolution of cold-rolled Mg–Zn–Gd alloys during annealing were tracked by quasi-in-situ electron backscatter diffraction method. An unusual texture transition from “rolling direction (RD)-split” to “transverse direction (TD)-split” occurred and oriented nucleation was observed. Nuclei originating from boundaries of the parent grain with “RD” orientation mainly exhibited “TD” orientations, and showed preferential misorientation relationship around 62.5° <uvt0> (including basal axes <11–20>, <10–10>, <41–50> and <52–70>) with respect to the parent grain, which may be related to basal <a> dislocations and low boundary energy.

Crystal Plasticity Analysis of the Relation between Micro‐Texture and Surface Ridging for a 21%Cr Ferritic Stainless Steel

The micro‐texture of ferritic stainless steel generated by rolling and annealing processes is supposed to behave plastic anisotropy and heterogeneous strain response during forming, especially for the high Cr content ferritic stainless steel with single ferrite phase. The relation between crystallographic orientations and their strain responses during loading are analyzed for a 21% Cr ferritic stainless steel sheet. The crystallographic orientation distributions in surface and center layers as well as in cross section are characterized by an electron backscatter diffraction (EBSD) method. It indicates that texture gradients along thickness with center layer having a higher density of &lt;111&gt;//ND texture are formed. Some grain clusters with similar oriented grains gathering together to be parallel to the rolling direction are observed. Then, the experimental orientation distributions are condensed and incorporated into a developed crystal plasticity finite‐element model (CPFEM), in which the multiple slip systems and the interaction between neighbor grains are considered. The simulation results confirm that the heterogeneous distributions of normal strain and shear strain preserved for the different crystal orientation grains result in the surface ridging. The texture gradients can help to decrease the tendency of surface ridging. The simulation results show a reasonable agreement with the tensile test results.

Orientations of nuclei during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei. The results show that orientation distribution of nuclei is associated with nucleation mechanism. The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation. Differently, discontinuous static recrystallization nuclei formed within the parent grains (TD-45° orientation) show random orientations and a variety of misorientation angles but preferred axes <52¯73> or <52¯70>. Interestingly, a special oriented nucleation is found. Discontinuous static recrystallization nuclei originated from boundaries of the parent grain (TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70° <112¯0> (∑18b). It is speculated that these two special misorientation relationships are related to the dislocation type.