Orientation Of Individual Grains Research Articles

Influence of the Ti microstructure on anodic self-organized TiO2 nanotube layers produced in ethylene glycol electrolytes

The relationship between the microstructure of Ti substrates and the anodic growth of self-organized TiO2 nanotube layers obtained upon their anodization in the ethylene glycol based electrolytes on these substrates is reported for the first time. Polished Ti sheets with mirror-like surface as well as unpolished Ti foils were considered in this work. Grains with a wide range of crystallographic orientations and sizes were revealed by Electron Backscatter Diffraction (EBSD) and correlated with nanotube growth on both types of substrates. A preferred grain orientation with [0001] axis perpendicular to the surface was observed on all substrates. Surfaces of all substrates were anodized for 18h in ethylene glycol electrolytes containing 88mM NH4F and 1.5% water and thoroughly inspected by SEM. By a precise comparison of Ti substrates before and after anodization, the uniformity of produced self-organized TiO2 nanotube layers was evaluated in regard to the specific orientation of individual grains. Grains with [0001] axis perpendicular to the surface turned out to be the most growth-promoting orientation on polished substrates. No orientation was found to be strictly growth-retarding, but sufficient anodization time (24h) was needed to obtain uniform nanotube layers on all grains without remnant porous initial oxide. In contrast with polished Ti sheets, no specific orientation was found to significantly promote or retard the nanotube growth in the case of unpolished Ti foils. Finally, the difference between the average nanotube diameters of nanotubes grown on various grains was investigated showing non-negligible differences in the diameter for different grain orientations and substrates.

Study on surface termination of boron-doped diamond electrodes under anodic polarization in H2SO4 by means of dynamic impedance technique

Anodic oxidation is a popular way to modify termination bonds at boron doped diamond electrodes altering their electrochemical and physicochemical properties. Our studies, performed with dynamic electrochemical impedance spectroscopy technique, supported with X-ray photoelectron spectroscopy and ellipsometry analysis prove its utility in continuous on-line monitoring of impedance changes on the electrode surface under polarization conditions, which may be of great use for the optimization of working conditions of the process. Based on our results, it can be observed that oxidation of termination bonds is a multistep process, each stage initiated at different anodic polarization potential. The factors influencing oxidation of termination bonds are presented and discussed. It was also possible to draw a conclusion about the removal of sp2 carbon impurities from the electrode surface as a precondition to activate the oxidation process. The depth and time of polarization, boron uptake at the grain boundaries, as well as crystallographic orientation of individual grains influence the heterogeneity of the oxidation process. Its partial reversibility was observed as a result of cathodic polarization in the range of hydrogen evolution, however due to irreversible corrosion of sp2 impurities, hydrogenation is not complete. This form of hydrogenation additionally contaminates the electrode surface with sulphur.

Anisotropic Mechanical Properties of Pure Magnesium Analyzed by <i>In Situ</i> Nanoindentation

In this work, correlation between nanomechanical properties and crystallographic orientation of grains in 99.9 % magnesium is presented. Crystallographic orientation of individual grains was obtained by scanning electron microscope (SEM) equipped with electron backscatter diffraction (EBSD) detector. Hardness and elastic modulus of grains with known orientation were subsequently determined by in situ nanoindentation in SEM. We show that hardness decreases with increasing angle between the direction of indentation and thec-axis of grains, while elastic modulus varies non-monotonically.

In Situ Neutron Diffraction Studies of Large Monotonic Deformations of Superelastic Nitinol

Superelastic Nitinol micromechanics are studied well into plastic deformation regimes using neutron diffraction. Insights are made into the nature of initial transformation, bulk transformation, plastic deformation, and unloading. Schmid factor predictions based on habit plane variants are found to best describe the very first grains that transform, prior to the transformation plateaus. However, the bulk transformation behavior that gives rise to transformation plateaus violates single crystal Schmid factor analyses, indicating that in bulk polycrystals, it is the effect of grain neighborhoods, not the orientations of individual grains, that drives transformation behaviors. Beyond the plateaus, a sudden shift in micromechanical deformation mechanisms is observed at ~8.50 %/4.75 % tension/compression engineering strain. This mechanism results in reverse-phase transformation in both cases, indicating a strong relaxation in internal stresses of the samples. It is inferred that this mechanism is most likely initial bulk plastic flow, and postulated that it is the reason for a transition from fatigue life enhancement to detriment when pre-straining superelastic Nitinol. The data presented in this work provide critical datasets for development and verification of both phenomenological internal variable-driven and micromechanical theories of transformation-plasticity coupling in shape memory alloys.

Mechanical properties and microstructure of large IN713LC nickel superalloy castings

When approving large castings for use, mechanical values are measured by room temperature tensile test on test bodies made from the centres of the castings. These test bodies often exhibit lower values than required, in particular ultimate tensile strength and elongation. The presence of fragile niobium carbides in the structure of the alloy was identified as the main cause of this situation, which originated as a result of the slow speed cooling. Because of large grain size compared to gauge length diameter, crystal orientation of individual grains also plays an important role. Interdendritic carbides are oriented parallel with dendrites thus parallel with directions in Ni superalloy crystal lattice. Grain oriented with axis parallel to tensile direction has platelet carbides oriented perpendicular to the tensile axis. In that position carbides crack easily at low grain deformation. Longitudinal metallographic cuts of selected tested specimen gauge lengths were prepared and grain orientation of individual grains was investigated by EBSD, Specimens, where grain oriented with axis parallel to tensile direction were found, performed poor elongation values 3%, compared to 7% where no such oriented grains were found.

The Significance of Local Plasticity for the Crack Initiation Process During Very High Cycle Fatigue of High Strength Steels

The present paper deals with experimental results on the fatigue damage of grade SAE 318 LN austenitic-ferritic duplex steel and a modified SAE 4150 tempering steel in the very high cycle fatigue (VHCF) regime. Particular attention was paid to the relation- ship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. Therefore, electrolytically polished bulk specimens have been fatigued under fully reversed loading in an ultrasonic fatigue testing machine while the surface has been observed in-situ by an optical microscope. The specimens were carefully analyzed by means of scanning electron microscopy (SEM) in combination with automated electron back-scatter dif- fraction (EBSD). In case of the austenitic-ferritic duplex steel under VHCF loading conditions, slip band formation is limited to the softer austenite grains. Once being formed, the bands generate high stress concentrations where they impinge the austenite- ferrite (γ−α) phase boundaries, eventually, leading to the crack initiation. In case of the tempering steel crack initiation is caused by slip band formation within the martensitic laths structure. It was found that in both materials microcracks are sensitive to changing crystallographic orientations when crossing a grain or phase boundary.

Quantification of Ridging in Ferritic Stainless Steel Sheets by Electron Backscattered Diffraction R-Value Maps

In ferritic stainless steel (FSS), undesirable surface defects of ridging appear during deep drawing. The formation of these defects is attributed to the inhomogeneous distribution of orientations of individual grains. In the present work, a new electron backscattered diffraction R(α)-value map was introduced, and the dependence of the tensile directions on the formation of ridging in an FSS sheet was discussed using this map. The results showed that large grain colonies in the R(α)-value maps lead to the formation of severe ridging in an FSS sheet.

Short fatigue crack propagation during low-cycle, high cycle and very-high-cycle fatigue of duplex steel – An unified approach

The present paper reviews experimental results on the fatigue damage of austenitic–ferritic duplex steel under various load levels ranging from LCF to VHCF, placing the focus towards the relationship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. A combination between fatigue testing of electropolished specimens and analytical electron microscopy (SEM/EBSD, TEM) revealed that under LCF loading conditions almost all the ferrite and the austenite grains showed plasticity, while under HCF and VHCF loading conditions, slip band formation was limited to the softer austenite grains and a low plastic activity is observed in the ferrite. Once being formed, the bands generate high stress concentrations, where they impinge the α–γ phase boundaries, eventually, leading to the crack initiation. This is discussed by applying a numerical simulation approach based on the finite-element (FEM) and the boundary-element (BEM) method.

Mechanisms of anisotropic friction in nanotwinned Cu revealed by atomistic simulations

The nature of nanocrystalline materials determines that their deformation at the grain level relies on the orientation of individual grains. In this work, we investigate the anisotropic response of nanotwinned Cu to frictional contacts during nanoscratching by means of molecular dynamics simulations. Nanotwinned Cu samples containing embedded twin boundaries parallel, inclined and perpendicular to scratching surfaces are adopted to address the effects of crystallographic orientation and inclination angle of aligned twin boundaries cutting the scratching surface. The transition in deformation mechanisms, the evolution of friction coefficients and the friction-induced microstructural changes are analyzed in detail and are related to the loading conditions and the twinned microstructures of the materials. Furthermore, the effect of twin spacing on the frictional behavior of Cu samples is studied. Our simulation results show that the crystallographic orientation strongly influences the frictional response in different ways for samples with different twin spacing, because the dominant deformation mode varies upon scratching regions of different orientations. A critical inclination angle of 26.6° gives the lowest yield strength and the highest friction coefficient, at which the plasticity is dominated by twin boundary migration and detwinning. It is demonstrated that the anisotropic frictional response of nanotwinned Cu originates from the heterogeneous localized deformation, which is strongly influenced by crystallographic orientation, twin boundary orientation and loading condition.

The influence of the grain boundary strength on the macroscopic properties of a polycrystalline aggregate

In this work a model, based on a X-ray diffraction contrast tomography data of a stainless steel wire with a diameter of 0.4mm is presented. As measured 3D grain geometry and crystallographic orientation of individual grains are directly transferred into a finite element model. Anisotropic elasticity and crystal plasticity constitutive laws are used for the bulk grain material while the grain boundaries are explicitly modeled using the cohesive zone approach. A parametric study on the effects of the grain boundary strength and other cohesive zone parameters on the macroscopic response and damaged grain boundary area of a polycrystalline aggregate is presented. Recommendations for the cohesive zone parameters values aimed at achieving low damaged grain boundary area during numerical tensile tests are given while at the same time taking into account the numerical stability of the simulations.

Deformation banding in a Nb polycrystal deformed by successive compression tests

A large-grained Nb polycrystal was successively uniaxially compressed to different cumulative strains to study the evolution of both the microstructure and the orientation of individual grains. After each deformation step, one grain which shows a banded structure was characterized in detail using electron backscatter diffraction (EBSD) to better understand the formation process of the deformation band (DB). The generation of the DB, which is observed after 11.5% reduction, involves the splitting of a transition band (TB) into two rotation fronts moving in opposite directions. These rotation fronts subseuqently evolve into the boundary of a DB whose alignment is associated with the observed primary and secondary slip systems. Post-analysis consists not only of slip system analysis based on image quality maps but also of lattice rotation analysis based on EBSD data. The latter shows that the orientation evolutions in the matrix and the DB are around the same rotation axis, but with opposite sense of rotation. In addition, grains with similar orientation but different deformation behaviors are compared to investigate the underlying difference in the mechanism of grain splitting. It is found that the formation of a DB occurs where a long-range stress field was created at a lower strain. Since the location of this long-range stress field is orientation dependent, whether or not a grain shows a banded structure strongly depends on its initial orientation. At the end of this paper, a mechanism for the formation of a band structure is suggested.

Angular Precision of Automated Electron Backscatter Diffraction Measurements

Electron backscatter diffraction (EBSD) has become the preferred technique for characterizing the crystallographic orientation of individual grains in polycrystalline microstructures due to its ability to rapidly measure orientations at specific points in the microstructure at resolutions of approximately 20-50nm depending on the capabilities of the scanning electron microscope (SEM) and on the material being characterized. Various authors have studied the angular resolution of the orientations measured using automated EBSD. These studies have stated values ranging from approximately 0.1° to 2° [1-6]. Various factors influence the angular resolution achievable. The two primary factors are the accuracy of the detection of the bands in the EBSD patterns and the accuracy of the pattern center (PC) calibration. The band detection is commonly done using the Hough transform. The effect of varying the Hough transform parameters in order to optimize speed has been explored in a previous work [6]. The present work builds upon the earlier work but with the focus towards achieving the best angular resolution possible regardless of speed. This work first details the methodology used to characterize the angular precision then reports on various approaches to optimizing parameters to improve precision.

Mechanical response of multicrystalline thin films in mesoscale field dislocation mechanics

A continuum model of plasticity, Phenomenological Mesoscopic Field Dislocation Mechanics ( PMFDM), is used to study the effect of surface passivation, grain orientation, grain boundary constraints, and film thickness on the mechanical response of multicrystalline thin films. The numerical experiments presented in this paper show that a surface passivation layer on thin films introduces thickness dependence of the mechanical response. However, the effect of passivation decreases in films with impenetrable grain boundaries. The orientation of individual grains of the multicrystal also has a significant effect on the mechanical response. Our results are in qualitative agreement with experimental observations. A primary contribution of this work is the implementation of a jump condition that enables the modeling of important limits of grain boundary constraints to plastic flow , independent of ad-hoc constitutive assumptions and interface conditions.

Stress Measurement Using EBSD Analysis of Grains in Copper Foil

A cyclic biaxial stress measurement method using electrodeposited copper foil was examined. The crystallographic orientations of individual grains that undergo grain growth in copper foil subjected to cyclic loading were analyzed by electron backscatter diffraction (EBSD). One of the slip directions in most of the grains corresponded to the direction of maximum shear stress when the biaxial stress ratio was negative. However, the number of grains with other orientations gradually increased as the biaxial stress ratio approached zero. On the basis of these features, we propose biaxial stress measurement using EBSD analysis of grown grains in copper foil. Our new method has excellent resolution compared with other stress-strain measurement methods since it can measure the average biaxial stress ratios in an area of 500 μm × 500 μm.

Twinning, grain orientation and texture variation of AZ31 Mg alloy during compression by EBSD tracing

In order to investigate the micro-mechanism of warm forming of Mg alloys, three specimens cut from a rolled AZ31 sheet were chosen to be compressed along the Rolling Direction (RD) at 100 °C, 170 °C and 230 °C, separately. During compression, an in situ measurement of grain orientation in the plane of RD × TD (Transverse Direction) was carried out with EBSD method. Experimental and analytical results show that temperature has remarkable impact on activation of twinning and variation of texture. As the temperature was raised from 100 °C to 230 °C, the number of grains with twins activated decreased substantially during deformation, and rolling texture varied from quick vanishing at 100 °C to always existing at 230 °C. Tracing for orientation of individual grains during deformation shows that there are obvious different orientation changes between grains with twins activated and those without twins activated. Twinning plays a significant effect on texture variation during compression. The extension twin variant really activated during deformation is the one with maximal Schmid factor.

Cyclic Stress Measurement in a Microscopic Region Using EBSD Analysis of Grown Grains Occurred in the Copper Foil

A cyclic biaxial stress measurement method using electrodeposited copper foil was examined. The crystallographic orientations of individual grains that undergo grain growth in copper foil subjected to cyclic loading were analyzed by electron backscatter diffraction (EBSD). An individual grain had the preferred orientation that depended on the biaxial stress condition. The grain slip direction with a high Schmid factor corresponded closely to the direction of first principal shearing stress (maximum shearing stress). When the biaxial stress ratio approaches zero, the difference between the first principal shearing stress and the second principal shearing stress gradually becomes small. Therefore, grain orientation under such a condition was influenced by both principal shearing stresses. On the basis of these features, biaxial stress measurement using a pole figure and an orientation imaging map were proposed in this paper. There is a specific restriction in this method. However, the method has excellent resolution compared with other stress-strain measurement methods since it can measure the principal stresses in an area of 500 μm × 500 μm.

Potential-Dependent Surface Morphology and Microtexture Evolution of Electrodeposited Copper Films

Two distinct morphologies were observed for copper films electrodeposited from an organic additive-free acid sulphate electrolyte near the limiting current density, namely facetted columns and spheroidal nodules. Facetted columns form for overpotentials η = 200–280 mV while nodules form for η > 240 mV. Electron Back Scattering Diffraction (EBSD) was applied to cross-sections of the columnar films to determine the crystallographic orientation of individual grains. Over the first few microns, the substrate has a strong influence, but for greater thicknesses, the columnar film develops a strong <110> and weak <112> fibre texture.

Distribution Characteristics of In-Grain Misorientation Axes in Cold-Rolled Commercially Pure Titanium and Their Correlation with Active Slip Modes

The distribution characteristics of in-grain misorientation axes (IGMA) in cold-rolled pure titanium were investigated using electron backscatter diffraction (EBSD). Depending on the orientation of individual grains, two distinct IGMA distribution patterns were observed: one with strong intensities of IGMA around ⟨0001⟩ and the other with those around ⟨uvt0⟩. Analyses based on the Taylor axes and Schmid factors of possible slip modes suggested that the former pattern arises from predominant activation of prism ⟨a⟩ slip, while activation of $$ \{ 11\bar{2}2\} \langle \bar{1}\bar{1}23 \rangle $$ slip under the suppression of prism ⟨a⟩ slip results in the latter pattern. It was also found that prism ⟨a⟩ slip becomes more active with increasing strain, playing a critical role in the plasticity of pure titanium. The present work demonstrates that IGMA analysis of EBSD data may be used to explore the active slip mode in polycrystalline hexagonal-close-packed (hcp) metals deformed to moderate to large strains.

Investigation of the accuracy of modeling the orientation distribution function of polycrystalline materials from individual orientations

The accuracy of reconstructing the orientation distribution function of grains in polycrystalline materials by the kernel method from a set of individual orientations has been studied. In particular, the influence of choosing the smoothing kernel in the kernel method and the influence of the dependence on the resulting distribution are taken into account. The rotations obeying the normal distribution on the group SO(3) and obtained by the statistical test method are used as an orientation set. Some aspects of electron backscattering diffraction measurements are considered. This problem is urgent in view of the expanding potential of the experimental study of the texture of polycrystalline materials, especially for electron microscopy, which makes it possible to measure the orientations of individual grains.

EBSD studies of the stress-induced B2–B19′ martensitic transformation in NiTi tubes under uniaxial tension and compression

In situ electron backscattering diffraction (EBSD) investigations were conducted on polycrystalline NiTi tube specimens during tensile and compressive deformation. The long-range cooperative and catalytic martensitic transformation under tension induces the transformation to proceed in the form of helical Lüders band. Propagation of the band is closely related to the spatial distribution of the orientations of individual grains. In uniaxial compression, the larger variation in Schmid factors, and consequently the larger variation in the critical transformation stresses among grains, leads to a homogeneous martensitic transformation, and therefore the absence of the Lüders band. To interpret the observed tension–compression asymmetry, a crystallographic model of the critical transformation stress and transformation strain for polycrystalline NiTi under tension and compression is proposed. The model defines three crystallographic regions: tension-favorable, compression-favorable and neutral zones. The orientation population in which tensile strains are larger than compressive strains is much higher than that of orientations with higher compressive strains. For resolved shear stress, orientation populations favoring tension and compression do not show any great difference.