Electron Backscatter Diffraction Characterization Research Articles

Simultaneous enhancement of strength and ductility for AZ31 magnesium alloy by pre-twinning induced heterostructure

Pre-twinning was used to tailor the strength and ductility of AZ31 magnesium alloy. Simultaneous enhancements in both ductility and strength were achieved in the AZ31 after the introducing of a high fraction of twin-matrix pairs. The slip trace analysis indicated that although the area-weighted average global Schmid factors (SFs) were not significantly changed, much more non-basal slip was activated in the pre-twinned sample compared with that in the as-received sample, which was beneficial to achieving higher work hardening and obtaining higher uniform elongation. The high-resolution digital image correlation (DIC) combined with ex-situ electron backscatter diffraction (EBSD) characterization revealed that the uniformity of the grain-size scale strain distribution was improved and the local strain concentration at the boundary was reduced during the loading as result of the extra activated non-basal slip, which delayed intergranular crack initiation and increased ductility. The activation of much non-basal slip with high critical resolved shear stress, the hetero-deformation, and the fine equivalent grain size caused the strengthening of the AZ31 sample with a high fraction of twin-matrix pair interfaces. These results imply that tailoring the twin-matrix interface density is an effective strategy to achieve excellent strength-ductility combinations in HCP metals.

Finite element simulation and experimental investigation of crystallographic orientation-dependent residual stress in diamond cutting of polycrystalline aluminum

The residual stress of crystalline materials induced by deformation at grain level strongly correlates with crystal orientation. In the present work, the dependence of residual stress on crystallographic orientation in diamond cutting of polycrystalline aluminum is investigated by crystal plasticity finite element modeling and simulations. Furthermore, corresponding ultra-precision diamond cutting experiments, which have the same parameter setup with the finite element model, are also carried out. The crystallographic orientations of the specimens are depicted by electron back-scattered diffraction characterization, and the surface residual stress is measured by X-ray diffraction. Both experiment and finite element simulation results demonstrate the anisotropic characteristics of the residual stress within machined polycrystalline aluminum, which exhibits the significant correlation with crystallographic orientation. Furthermore, finite element simulations of bi-crystal cutting are also performed, which indicate that the grain boundary also has a strong influence on the generation of residual stress.

Investigations of Tungsten Inert Gas and Metal Active Gas Joints of SUS301L Stainless Steel: Electron Backscatter Diffraction Characterization and Fatigue Performance

Investigations of Tungsten Inert Gas and Metal Active Gas Joints of SUS301L Stainless Steel: Electron Backscatter Diffraction Characterization and Fatigue Performance

Large Volume 3D Electron Backscatter Diffraction Characterization of Lath Martensite in 13%Cr-4%Ni Stainless Steel by Xe Plasma FIB

Large Volume 3D Electron Backscatter Diffraction Characterization of Lath Martensite in 13%Cr-4%Ni Stainless Steel by Xe Plasma FIB

Variant selection in additively manufactured alpha-beta titanium alloys

The crystallographic arrangements of the α-phase variants in α-β titanium alloys remains less identified due to the crystallographic complexity involved while being essential to understand the α-β microstructural intricacy. To improve the current understanding, specimens of two columnar-grained α-β Ti alloys (Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo) and two equiaxed-grained α-β Ti alloys (Ti-6Al-4V and Ti-4Al-2V) were fabricated by laser metal powder deposition (LMD). Electron backscatter diffraction (EBSD) analyses were applied to more than 105 α-phase variants in each alloy. The results revealed that the Type 4 α/α variant boundary ([10¯553¯]/63.26∘) is prevalent in the two columnar-grained α-β alloys while the Type 2 α/α variant boundary ([112¯0]/60∘) is common in the two equiaxed-grained α-β alloys. Further EBSD characterisation indicates that α-variant selection tends to be more prevalent in equiaxed prior-β grains, featured by the Category I triple-α-variant clusters, which mostly terminate on dense {101¯1} planes with lower boundary energy. Conversely, columnar prior-β grains show significant Category II triple-α-variant clusters, which mostly terminate on less dense {41¯3¯0} planes with higher boundary energy. Self-accommodation to compensate for the β→α transformation strain is assumed to be the major underlying mechanism. The implications of these findings for understanding the tensile strengths are discussed in conjunction with the Schmid factor of α-variants calculated in columnar- and equiaxed-grained Ti-6Al-4V.

EBSD Characterization of 7075 Aluminum Alloy and Its Corrosion Behaviors in SRB Marine Environment

Aluminum alloy 7075 is an important engineering material for ship structures. However, the corrosion of Al alloys generally exists in various environments, especially in the marine environment. Currently, the corrosion behaviors of Al alloy 7075 in sulfate-reducing bacteria (SRB) marine environment has not been well-addressed. In this paper, the corrosion effect of SRB on 7075 aluminum alloys was studied by adding SRB to real seawater. The microstructure and grain orientation of the super-hardness Al alloy 7075 were studied via the electron back-scattered diffraction (EBSD)technology, and the electrochemical impedance spectroscopy (EIS) test of the electrochemical corrosion behavior of 7075 in a variety of microorganisms, mainly SRB, in real seawater was continuously performed for 21 days. It was concluded that Al alloy 7075 has the strongest texture intensity on the (001), (111), (010), and (0–10) planes, which is 2.565. Adding SRB to real seawater accelerated the corrosion rate, and after corrosion on the 14th day, the protective film on the 7075 aluminum alloy surface was completely broken, and the impedance was significantly reduced.

A quasi-in-situ EBSD study of the thermal stability and grain growth mechanisms of CoCrNi medium entropy alloy with gradient-nanograined structure

The thermal stability and mechanical properties of a gradient-nanograined structure (GNS) CoCrNi medium entropy alloy (MEA) processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction (EBSD) characterization and Vickers micro-hardness (HV) measurements. A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes, which could effectively save specimen and time costs. The grain nucleation and growth, as well as shrink and disappearance process through Σ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed. The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nano-grained top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling. Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy, thus improving their thermal stability. The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures. Moreover, the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.

Multiple-mechanism and microstructure-based crystal plasticity modeling for cyclic shear deformation of TRIP steel

The transformation-induced plasticity (TRIP) steel has an excellent synergy between strength and ductility and is widely used in industry. Cyclic loading is often seen in its industrial application. Therefore, it is of great significance to study cyclic plastic behavior of the TRIP steel. The TRIP steel's cyclic shear behavior and microstructure evolutions were investigated by mechanical testing, electron backscatter diffraction (EBSD) characterization, and in-situ electron channeling contrast imaging (in-situ ECCI) observation. Then, a multiple-mechanism crystal plasticity constitutive model was developed, considering both dislocation slip and martensitic transformation mechanisms. Furthermore, an isotropic hardening law and a modified kinematic hardening rule were taken into account. The crystal plasticity model was implemented into DAMASK with the spectral method. A polycrystalline RVE with realistic grain morphology was constructed from the EBSD data. The simulations of the TRIP steel under cyclic shear loading showed that the samples under higher strain amplitude exhibit stronger cyclic shear hardening. The activation of the martensitic transformation mechanism promotes cyclic hardening. In grain level, the grains with Taylor factor (related to orientation) between 3.3 and 3.9 can activate more slip systems and have a better cyclic hardening ability. So, the Taylor factor is defined as an indicator describing the cyclic response of individual grains in polycrystalline materials. Considering the capability of the established modeling framework, it is of great significance to guide the TRIP steel to serve safely and help the microstructural design in grain scale.

Microstructure evolution and mechanical properties of laser additive manufactured Ti6Al4V alloy under nitrogen-argon reactive atmosphere

It is known that the reactive nitrogen atmosphere can effectively modulate the microstructure of laser powder bed fusion (L-PBF) of Ti6Al4V alloys. In the present work, high nitrogen contents of 15 and 25 vol% were applied in argon atmospheres to investigate the influence on microstructure and mechanical performances. The enhanced nitrogen solid solution was achieved with nitrogen content ranging 374 ppm and 456 ppm (parts per million) in the Ti6Al4V alloys, respectively. Besides, the EBSD (electron back-scattered diffraction) characterization shows the significant refinement of acicular martensite α′ and the EBSD phase reconstruction shows the interrupting epitaxial growth of the β phase with the increasing nitrogen content in the atmosphere. The KAM (kernel average misorientation) diagrams with an increasingly higher value indicate an increased micro-strain and dislocation density at higher nitrogen content in the atmosphere. Meanwhile, the high magnification TEM further confirms the martensite α’ refinement and also the formation of β lamella at 25 vol% nitrogen content in L-PBF atmosphere. Such microstructure evolution can be mainly attributed to the more severe thermal cycling with a higher thermal gradient and cooling rate under the atmosphere with higher nitrogen content. By applying the 25 vol% nitrogen in the atmosphere, the L-PBF Ti6Al4V alloys exhibit a significant increase in maximum compressive strength of ∼1885 MPa and maximum compressive strain of ∼10%, respectively. The underlying strengthening mechanism can be attributed to the refinement of martensite structure, solid solution, and higher dislocation density. The present work shows that the atmosphere can effectively modulate the microstructure and improve the mechanical property of L-PBF alloys.

Dynamic growth model of Fe2Al5 during dissimilar joining of Al to steel using the variable polarity cold metal transfer (VP-CMT)

Further development of the car lightweight in automotive industry significantly depends on the reliability of the Al/steel dissimilar joint, since more and more Al alloy parts are used in the car manufacturing. It has been proved that the Al/steel dissimilar joint strength can be effectively increased when the intermetallic compound (IMC) growth, especially the Fe2Al5 layer thickness, is controlled at certain level. Because the Fe2Al5 is the dominant phase of the entire IMC layer in the Al/steel dissimilar joint, and cracks usually initiate at this location when damage is conducted. Therefore, for the automotive industry assembly line, the prediction model monitoring the Fe2Al5 layer thickness of each Al/steel weld seam is desired to ensure the dissimilar joint quality and reliability. In the present research, a numerical dynamic growth model based on element diffusion laws is established to calculate the IMC layer thickness of Fe2Al5 phase during the variable polarity cold metal transfer (VP-CMT) welding of aluminum to steel in the function of welding heat input and location. To build up effective connection between the welding heat input and the actual temperature history of Al/steel interface, a finite element analysis (FEA) model is customized for the present VP-CMT process. The Fe2Al5 layer thickness calculation results are further validated using electron back-scattered diffraction (EBSD) characterization under scanning electron microscopy (SEM), confirming a 2.3 % deviation between the model prediction and actual layer thickness.

Atomic-scale study of dislocation-grain boundary interactions in Cu bicrystal by Berkovich nanoindentation

The plastic behavior of metal materials is mainly dominated by dislocation movement, whereas for polycrystalline metals, dislocation-grain boundary (GB) interactions play a significant role in the mechanical response. In the present work, deformation mechanisms of single crystal and bicrystal Cu workpieces subjected to Berkovich nanoindentation are studied by experiments and molecular dynamics (MD) simulations. The numerical model considers crystallographic orientations of the individual grains determined by electron backscatter diffraction (EBSD) characterization of the designated location on the polycrystalline surface. Experimental results indicate as the distance between the indentation and the GB decreases, the hardness of the tested specimen decreases and Young's modulus increases. Meanwhile, the corresponding simulation results reveal the anisotropic nature of dislocation evolution under the indenter for different crystallographic orientations of the single crystal Cu. Furthermore, GB as a complicated factor to the dislocation motion leads to the unusual deform behavior of materials in the vicinity of the GB, and dislocation-GB interactions introduce different deformation mechanisms. For the GB with a high angle of [1-11] 59.8°, dislocation slip along the GB is believed to play an important role in the decrease of the hardness.

Strain localization and crack initiation behavior of a PM Ni‐based superalloy: SEM‐DIC characterization and crystal plasticity simulation

AbstractThe strain localization and crack initiation behavior of a powder metallurgy (PM) nickel‐based superalloy FGH4098 under low cycle fatigue (LCF) loads at room temperature is investigated by combining scanning electron microscope (SEM)‐digital image correlation (DIC) characterization with crystal plasticity simulation. The elastic anisotropy near annealing twin boundaries contributes to strain localization and results in high propensity of LCF failure. The effect of local microstructure on the fatigue crack initiation at {111} slip bands and tortuous crack propagation is revealed. Meanwhile, local crystal plasticity finite element cyclic deformation simulation is carried out based on explicit microstructure reconstructed from electron backscatter diffraction (EBSD) characterization. Four metrics of accumulated shear strain, effective plastic strain, shear strain energy dissipation density, and Fatemi–Socie parameter are selected as fatigue indicator parameters to predict crack initiation site. Fatemi–Socie parameter is proved to be a promising metric for crack initiation prediction.

Static recovery of A5083 aluminum alloy after a small deformation through various measuring approaches

Static recovery was confirmed to be the dominant softening mechanism during annealing for the studied A5083 aluminum alloy. The kinetics of static recovery, described based on the activation parameters (activation energy and volume of static recovery) and the static softening fraction, were mainly studied through two thermomechanical tests, namely, double-pass compression and stress relaxation tests. A new approach was proposed to measure the static softening fraction in the stress relaxation test. In general, a higher temperature or strain rate accelerates static recovery, while interestingly, the effect of pre-strain on static recovery is opposite in cases with and without external stress, owing to the enhanced static recovery by external stress during annealing. In addition, microhardness tests and electron backscatter diffraction (EBSD) characterization were also conducted to verify the accuracy of the results. The grain average misorientation approach based on EBSD characterization was confirmed to be effective in distinguishing and quantifying static recovery. It is noteworthy that the special stress hardening phenomenon occurring at 400 °C and 0.1/s is caused by strain aging, showing the complexity of the material behaviors after deformation of the studied aluminum alloy.

Elimination of the charge effect on zirconia coatings using a focused ion beam for characterization of the three-dimensional microstructure

How to improve the acquisition efficiency of electron back scatter diffraction (EBSD) and meanwhile avoid the drifting of images caused by the charge effect is still an issue that needs to be solved urgently for three-dimensional (3D) EBSD characterization of non-conductive ceramic materials. The purpose of this study is to solve the problem based on an experimental and theoretical basis using the yttria-stabilized zirconia (YSZ) coating as the research subject. To eliminate the charge effect, the charge equilibrium point of the YSZ coating was determined using the Duane–Hunt limit method. When the incident energy is equal to the E2 value (3.06 keV), the charge effect on the scanning electron microscopy images of the YSZ coating disappears. However, the incident energy of 3.06 keV exhibits lower EBSD collection efficiency for a 3D EBSD result because of the long exposure time; therefore, increasing the incident energy to 7 keV under the large beam current is a practical way to improve EBSD collection efficiency, which inevitably results in the charge problem again. Furthermore, Ga ion implantation caused the charge effect on the YSZ coating at 7 keV under the large beam current to disappear, which can be attributed to the decrease in bandgap widths and the improvement of electrical conductivity of the t-YSZ phase after Ga ion implantation based on first principles. Calculations of the cohesive energies reveal that Ga ions are more likely to occupy the lattice positions of the YSZ coating after Ga ion implantation. Finally, a high-resolution 3D EBSD image of the YSZ coating was obtained.

On the strain effect in dislocation loops produced by self-ion irradiation to emulate in-service reactor condition

A series of experiments combining deformation, irradiation damage, and high temperature have been carried out. First, small tensile samples (less than 500 μm thick) will be subjected to constant deformation (supported by a suitably instrumented mini tensile machine) during irradiation of Fe ions at different temperatures (room temperature, RT, and 450 °C). Subsequently, FIB (Focused Ion Beam) samples will be extracted from the irradiated specimen, and an EBSD (Electron Backscatter Diffraction) and TEM (Transmission Electron Microscopy) characterisation will be carried out. The material studied is a high purity Fe- BCC model alloy. The objective is to determine the possible synergy between the effect of deformation and temperature during irradiation on the evolution of radiation-induced defects, whereby the chosen dose allows us to determine a potential bias for the generation of such defects.

Characterization of hot workability of Ti-6Cr-5Mo-5V-4Al alloy based on hot processing map and microstructure evolution

Ti-6Cr-5Mo-5V-4Al (Ti-6554) alloys with excellent comprehensive properties are expected to become the preferred material for large-scale parts in the aviation field. However, the processing parameters of large-scale parts have a significant impact on the microstructure, so it is necessary to optimize the hot working process to improve the comprehensive mechanical properties of the alloy. In this paper, hot compression experiments of Ti-6554 alloy were carried out in the temperature range from 680 to 830 ℃ and strain rate range from 0.001 to 10 s−1. The hot workability of the Ti-6554 alloy was studied based on the hot processing map and microstructure evolution. The Arrhenius constitutive model of the two phase region and single phase region was established. It was found that the thermal activation energy was higher at high value in the adopted range of strain rate, and the average thermal activation energy gradually decreased with raising the strain. The peak efficiency in the hot processing map occurred at 680 ℃/0.001 s−1 and 770 ℃/0.001 s−1 with efficiency values of 0.47 and 0.48, respectively. The instability regions were mainly concentrated at a high value in the adopted range of strain rate, and the typical instability phenomenon was flow localization. With raising the strain rate and temperature, the volume fraction and average size of the α phase decreased due to the dynamic phase transformation. Dynamic recovery (DRV) was the major deformation mechanism at low temperatures and low strain rates. The deformation mechanism gradually changed to DRX with the increase of temperature and strain rate. However, the increase of temperature at a higher strain rate could not improve the level of dynamic recrystallization (DRX). Based on electron backscatter diffraction (EBSD) characterization, the DRX types of the β phase were determined to be discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX). DDRX mainly occurred at high temperatures and low strain rates. Furthermore, the spheroidizing mechanism of the equiaxed α phase was also analyzed. First, under the action of compressive stress, the aspect ratio of the equiaxed α phase gradually increased and became the lamellar α phase. Subsequently, the low angle grain boundaries (LAGBs) gradually changed to high angle grain boundaries (HAGBs), accompanied by wedging of the β phase. Finally, the spheroidization process was completed.

Fatigue crack initiation behaviors around defects induced by welding thermal cycle in superalloy IN617B

In this study, fatigue crack initiation behaviors around defects induced by welding thermal cycle in IN617B were investigated by in-situ fatigue tests combined with DIC (Digital Image Correlation) and EBSD (Electron Back Scattering Diffraction) characterization. Two mechanisms of fatigue crack initiation around defects were clarified and the distance of crack initiation position from defects was proposed to determine which mechanism was dominant. Intergranular fatigue crack initiation caused by pile-up of dislocations was dominant in the far-field region from defects, while intragranular fatigue crack initiation caused by cracking at the interface between dislocation bands and matrix was dominant in the near-field region from defects. Welding defects altered the fatigue crack initiation mechanism by changing the deformation mode of the near-field region, thereby accelerating the fatigue failure process of IN617B alloy.

Mechanical properties and crystallographic texture of non-oriented electrical steel processed by repetitive bending under tension

Improving the magnetic properties of non-oriented electrical steel (NOES) through the optimization of crystallographic texture has been an on-going research activity for decades. However, using traditional rolling and annealing procedures, the obtained final textures were usually very similar, i.e., exhibiting the {111} (γ) and <110> (α) fibres, which were not the desired {001} texture (θ-fibre) for optimal magnetic quality. In the current work, a 1.8 wt% Si NOES was processed using a new sheet metal deformation method, i.e., repetitive bending under tension (R-BUT), also known as continuous bending under tension (C-BUT), to modify the texture of the electrical steel. The hot-rolled and annealed NOES plates were repeatedly bent and unbent when they were pulled under tension. The deformed plates were then heat treated at different temperatures for various times. Neutron diffraction and electron backscatter diffraction (EBSD) characterisation of the macro- and micro-textures proved that the R-BUT process significantly reduced the undesired {111} texture while promoting the {001} texture. The cube texture, which rarely formed after conventional rolling and annealing, was also seen in the R-BUT samples after annealing. It was shown that, the shear plastic deformation (induced by R-BUT) played a significant role in promoting the desired textures. In addition, the results indicated that the NOES processed by R-BUT could be deformed beyond its common formability limit, which may provide a method to address the poor workability challenge of high silicon electrical steels.

Electron Backscattered Diffraction Characterization of S900 HSLA Steel Welded Joints and Evolution of Mechanical Properties

Electron Backscattered Diffraction Characterization of S900 HSLA Steel Welded Joints and Evolution of Mechanical Properties

Study on the High-Temperature Deformation and Dynamic Recrystallization Behavior near the Interface of Stainless Steel Cladding

To ensure the long service life of concrete buildings in the marine environment, it is urgent to develop building materials with good corrosion resistance and weatherability. Stainless steel cladding is suitable for a highly corrosive environment and provides cost advantages. This paper investigated the deformation coordination and the microstructure evolution near the cladding interface of stainless steel/carbon steel. The stress-strain curves at different temperatures and strain rates were analyzed on the basis of high-temperature compression experiments. In addition, the sin-hyperbolic constitutive model was constructed, and the optimized parameters were obtained using electron backscatter diffraction characterization technology. The results show that at the deformation temperature of 1100 °C and the strain rate of 1 s−1, the deformation coordination increases significantly near the interface, accompanied by a large number of recrystallized grains, which has a positive impact on the comprehensive performance of the materials.