Electron Channeling Contrast Research Articles

Dislocation motion at a fatigue crack tip in a high-nitrogen steel clarified through in situ electron channeling contrast imaging

The dislocation motion was observed at the tip of a fatigue crack in the high nitrogen steel (HNS) by in situ test through electron channeling contrast imaging. No dislocation motion was observed near the crack tip even at a stress amplitude higher than the fatigue limit. In contrast, dislocation pileup was observed a bit from the crack tip with very narrow dislocation spacing. In addition, with increasing number of cycles, the dislocations were extended to stacking fault. The stacking fault showed a reversible behavior of shrink and expansion, which may influence in enhancing the crack growth resistance of the HNS.

A 2D and 3D nanostructural study of naturally deformed pyrite: assessing the links between trace element mobility and defect structures

The links between deformation-induced micro- and nanostructures and trace element mobility in sulphide minerals have recently become a popular subject of research in the Earth sciences due to its connections with metallic ore paragenesis. It has been shown that plastic deformation in pyrite creates diffusion pathways in the form of low-angle grain boundaries that act as traps for base- and precious-metals. However, the plastic behavior of pyrite and the physiochemical processes that concentrate these trace elements in deformation-induced micro- and nanostructures remain poorly understood. In this study, we develop strategies for 2D and 3D analysis of naturally deformed sulphides by combining electron backscatter diffraction, electron channeling contrast imaging and atom probe tomography on pyrite in an attempt to better understand the underlying diffusion processes that mobilize trace elements. The combined results reveal structures associated with crystal-plastic deformation in the form of dislocations, stacking faults, and low-angle grain boundaries that are decorated by As and Co. Although our data support a dislocation-impurity pair diffusion model, we have evidence that multiple diffusion mechanisms may have acted simultaneously. In this study, we applied new data processing techniques that allow for orientation measurement of nanostructural crystal defects from atom probe tomography data. Dislocations within our studied sample occur along the (110) planes suggesting glide on {110}.

Tracking Microstructure Evolution in Complex Biaxial Strain Paths: A Bulge Test Methodology for the Scanning Electron Microscope

In this work, a novel method is presented to track site-specific microstructure evolution in metallic materials deformed biaxially along proportional and complex strain paths. A miniaturized bulge test setup featuring a removable sample holder was designed to enable incremental measurements to be performed in a scanning electron microscope, by probing the same position on the sample at different deformation levels, with electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and other imaging modes. Validation experiments were performed at room temperature on samples prepared from commercial sheet metal (dual-phase steel) and foils (stainless steel). Local strain measurements with the digital image correlation technique confirmed that proportional strain paths with a strain ratio up to 5 can be investigated using elliptical dies in the bulge test holder. It is also shown how complex strain paths can be obtained using a combination of overlapping elliptical dies. Incremental EBSD and ECCI were conducted in configurations relevant for the multi-scale investigation of localized plasticity and damage mechanisms in dual-phase steel. Quantitative information regarding microstructure evolution (phase fractions, orientation fields, dislocation structures, etc.) and regarding local strain distributions could be successfully obtained. This type of data sheds light on underlying deformation mechanisms and provides opportunities to calibrate crystal plasticity models.

Effects of β Air Cooling and Subsequent Cold Rolling on Microstructure and Hardness of Zr702 Sheet

In this work, a Zr702 sheet was subjected to β air cooling and then rolled to 15% reduction at room temperature, with their detailed microstructural characteristics characterized by electron channeling contrast imaging and electron backscatter diffraction techniques. Results show that after the β air cooling, the prior equiaxed grains in the as-received material are completely transformed into Widmanstatten structures featured by coarse plates with typical phase transformation misorientations between them. The subsequent 15% rolling allows both slip and twinning (especially the {10–12} type) to be activated readily, leading to significant grain refinement and the appearance of misorientation angle peaks around 3°–5° and 85°. Analyses on kernel average misorientations reveal that there exist very low residual strains in the β-air-cooled specimen while they are markedly increased after the 15% rolling and preferably distributed near low-angle and twin boundaries. Hardness measurements show that the specimen hardness is evidently decreased from ~ 199 to ~ 170 HV after the β air cooling, which can be attributed to grain coarsening and the scattered orientations associated with the slow β → α transformation. For the 15%-rolled specimen, however, effective grain refinement by twinning and denser low-angle boundaries jointly lead to ~ 35% hardness increment to ~ 228 HV.

Martensite Reversion Duality Behavior in a Cold-Rolled High Mn Transformation-Induced Plasticity Steel

The martensite reversion behavior of a cold-rolled high Mn transformation-induced plasticity (TRIP) steel was investigated through an elaborate continuous annealing procedure. The dilatometry analysis was employed to ensure a precise heating rate (50 °C/s) control during annealing along with acquiring the critical temperature range of the martensite reversion. The magnetic measurement was used for $$ \alpha^{\prime} $$-martensite quantification through annealing, and electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) analyses were also performed to characterize the microstructure. The dilatometry results revealed the dual temperature ranges for the $$ \alpha^{\prime} $$-martensite reversion in the experimental TRIP steel; the reversion occurred in two separate temperature regions of 540 °C to 730 °C and 730 °C to 850 °C. Although the inhomogeneity in the morphology of $$ \alpha^{\prime} $$-martensite (lath type and dislocation cell type), which was detected in the ECCI image of the cold-rolled structure, might be responsible for the reversion temperature range broadening, the discontinuity of the reversion was mainly attributed to the partitioning of Mn from $$ \alpha^{\prime} $$-martensite to austenite through annealing. The microstructural characterizations indicated that the duality would also exist in the case of the martensite reversion mechanism. Accordingly, the reversion of lath-type martensite appeared to occur merely through an athermal mechanism, while the dislocation cell-type martensite might be reversed to austenite via both shear and diffusional mechanisms.

Quantitative analysis of electron channeling contrast of dislocations

Quantitative analysis was performed for electron channeling contrast (ECC) of dislocations in a [001] single-crystal 316 L stainless steel sample in order to establish the range of deviation parameter, w, for optimal dislocation imaging. Channeling contrast was evaluated from a series of ECC images taken as a function of sample tilt at ∼ 0.1° increments across 〈200〉 Kikuchi lines. The deviation parameter, w, was estimated by quantitative analysis of several parameters, namely experimental dislocation contrast intensity and dislocation feature width, as well as calculated BSE signal. Our analysis reveals that optimum dislocation imaging conditions by ECC occur for positive values of w. Under these imaging conditions, dislocations displayed strong contrast and narrow dislocation feature width, y. In particular, we found that the value of y can be greatly reduced (about two times in the present work) by using large positive values of w (w > 1.0 ± 0.5). Out of these channeling conditions, dislocations displayed weak contrast and larger y-values were measured.

Effects of pulsed laser surface treatments on microstructural characteristics and hardness of CrCoNi medium-entropy alloy

ABSTRACTA hot-swaged/annealed CrCoNi medium-entropy alloy (MEA) was surface-treated by pulsed laser at two different powers (400 and 200 W). Microstructural characteristics of the laser-modified zones were characterised and analysed by energy dispersive spectroscopy, electron backscatter diffraction and electron channelling contrast imaging techniques. Results show that melting and rapid solidification occur on the surfaces of both laser-treated specimens, and profuse annealing twins existing in the initial microstructures are essentially eliminated in the melting zone (MZ) with plentiful low-angle boundaries appearing. Meanwhile, the initial equiaxed grains are replaced by new grains in the MZ with either granular or columnar appearance (in the 2D cross-sectional views). These grains are comprised of fine cellular structures with relatively uniform sizes (∼1–2 μm in width/diameter), the formation of which is related to the segregation of Cr during solidification. With the laser power decreasing from 400 to 200 W, the volume of the MZ and grain sizes in its interior are reduced. This is due to less heat supply and faster cooling caused by reducing the laser power. Hardness tests reveal that the surfaces of both the laser-treated specimens are slightly softened (by 10–20%), and quantitative analyses suggest that this is mainly related to grain coarsening and the disappearance of annealing twins in the MZ.

In Situ Macroscopic Tensile Testing in SEM and Electron Channeling Contrast Imaging: Pencil Glide Evidenced in a Bulk β-Ti21S Polycrystal

In this paper, we report the successful combination of macroscopic uniaxial tensile testing of bulk specimen combined with In situ dislocation-scale observations of the evolution of deformation microstructures during loading at several stress states. The dislocation-scale observations were performed by Accurate Electron Channeling Contrast Imaging in order to follow the defects evolution and their interactions with grain boundaries for several regions of interest during macroscopic loading. With this novel in situ procedure, the slip systems governing the deformation in polycrystalline bulk β-Ti21S are tracked during the macroscopic uniaxial tensile test. For instance, curved slip lines that are associated with “pencil glide” phenomenon and tangled dislocation networks are evidenced.

Recovery kinetics in Al-0.3%Cu after heavy rolling

The recovery kinetics of a nanoscale lamellar structure Al-0.3%Cu alloy, which was produced by 98% cold rolling, during annealing at temperatures from 100 to 200 °C have been analyzed in this study. Microstructural characterization using electron channelling contrast in the scanning electron microscope shows a uniform coarsening of the lamellar structure and gradual precipitation of second phase particles (Al2Cu) during annealing at these temperatures. Fitting of the microhardness values for samples annealed for between 4 and 4096 minutes at different temperatures with a recovery model reveals a change, in the apparent activation energy from 79 kJ/mol at the beginning to 105 kJ/mol at the end of recovery annealing, which leads to a discussion of the recovery mechanisms.

Effect of heterogeneous laser surface treatment on mechanical properties of interstitial free steel

Laser surface treatment was applied to a Ti-added interstitial-free steel by linear-scanning. The distance between each linear scan was 1.5 mm and the width of the laser-modified zone for each scan was 1 mm. These process parameters result in a heterogeneous surface, containing hard laser-modified lamella separated by non-treated material. It is found that after the treatment, higher strength and good ductility can be obtained. Electron channelling contrast imaging was used to investigate the structure of the sample, which was then compared with the untreated one to reveal the origin of the improved mechanical behaviour of the laser surface-treated steel.

Two beam toy model for dislocation contrast in ECCI

Dislocation contrast in the SEM, as observed though electron channelling contrast imaging (ECCI), is commonly treated analogously to the contrast in the TEM. This perception is based on early studies done for dislocations parallel with the surface where the surface relaxation is negligible. However, for threading dislocations (TD) that interact with the surface (normal or inclined), as is the case for nitrides materials, g b type invisibility criteria are no longer fully applicable to ECCI, especially in forward geometry [1]. Dislocations change locally the lattice curvature and Bragg diffraction conditions in the crystal, affecting the form and diffracting behaviour of the electron wavefunction in that region. More explicitly, Howie and Whelan [2] had shown that dislocation contrast is the result of interband transitions between Bloch waves states which, in turn, are caused by the change in the displacement field, u(r), around the dislocation or local “strain”. Dynamical models have been used successfully to both predict and characterise dislocations in ECCI [3]. Nevertheless, the behaviour of dislocation contrast in ECCI in particular and diffraction contrast in the SEM in general remains somewhat opaque. In the work we investigate the behaviour of contrast causing strain as a means of insight into this problem.

Observation of the Stacking Faults in In0.53Ga0.47As by Electron Channeling Contrast Imaging

The observation and interpretation of Frank stacking faults, Shockley stacking faults, Lomer dislocations, and 60° misfit dislocations, which have similar line shapes in the (001) In0.53Ga0.47As crystalline surface, are performed with the electron channeling contrast imaging (ECCI) technique. To minimize the backscattered electron (BSE) contrast that resulted from the surface morphology, a relatively flat region is first selected and compared with an atomic force microscopy (AFM) image and then, subsequently, examining ECCI with transmission electron microscopy (TEM)‐like invisibility criteria. By orthogonally choosing the diffraction vector g between (220) and (2‐20), misfit dislocations seem to be always visible but partially faint in the g parallel to the line direction on the surface. With respect to the image contrast, Frank stacking faults and Lomer dislocations are likely to be completely invisible for parallel g. The criteria are further confirmed by cross‐sectional TEM analysis, which shows a preferred homogeneous surface nucleation.

Influence of strain amplitude on the development of dislocation structure during cyclic plastic deformation of 304 LN austenitic stainless steel

In the present study, electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) were used to document the evolution of dislocations in a 304 LN austenitic stainless steel subjected to cyclic plastic deformation at room temperature using different strain amplitudes. In particular EBSD was used to quantify the development of geometrically necessary dislocation density (GND) with varying strain amplitudes. Solution annealed multiple specimens were cyclically deformed at varying strain amplitudes ranging from ±0.25 to ±1.2% under completely reversed straining conditions until complete failure. It is found that GND density, which is independent of the character of the dislocations, increases with the increase of strain amplitude. This is consistent with the cyclic hardening characteristic of the investigated steel. It is found that the evolution of GND structure during the course of deformation is heterogeneous in nature with small grains having higher GND density than the coarse grains. The clustering of GNDs is visible near grain boundaries leaving the grain interior free. EBSD can capture the variation in GND storage patterns with strain amplitudes. The heterogeneity of the cyclic deformation induced GND structure decreased with the increase of the strain amplitude. The evolution of GND structure is also found to be sensitive to the size and orientation of the austenite grains. It has been found that the GND density is maximum in {111}<0-11> grains, those with the highest Taylor factor. The fidelity of dislocation storage pattern with strain amplitudes is assessed using ECCI. This is consistent with the GND storage pattern shown by EBSD and explains the cyclic hardening behavior of the steel.

Insight into hydrogen effect on a duplex medium-Mn steel revealed by in-situ nanoindentation test

Medium-Mn steel is the newly developed steel acting as a promising candidate of the 3rd-generation advanced high strength steels. In the present study, the effect of hydrogen on the mechanical behavior and martensite transformation process in a duplex medium-Mn steel is investigated by in-situ nanoindentation test. The mechanical response of individual phase: ferrite, and retained austenite by introducing hydrogen is studied. With the presence of hydrogen, the reduction of activation energy for dislocation nucleation was verified by using a stress-biased, statistical thermal activation model. The stacking fault energy (SFE) was reduced, which was revealed by electron channeling contrast (ECC) technique. Magnetic force microscopy (MFM) revealed a suppression of retained austenite to α′-martensite transformation with the presence of hydrogen, which is related to hydrogen induced SFE reduction and enhanced slip planarity.

Formation mechanism of dislocation patterns under low cycle fatigue of a high-manganese austenitic TRIP steel with dominating planar slip mode

We investigated the evolution of dislocation patterns under low cycle fatigue in a Fe–17Mn-1.5Al-0.3C (wt. %) steel with transformation-induced plasticity (TRIP). To this end, quasi in-situ electron channelling contrast imaging under controlled diffraction conditions (cECCI) was employed. Cross-correlation based electron backscattering diffraction (CC-EBSD) with high angular resolution was applied to measure the distributions of residual strain/stress and geometrically necessary dislocations (GNDs). Planar slip mode was found to be dominating in this low stacking fault energy (SFE) steel under the small strain amplitudes that were applied here (<0.8%). However, un-expected complex dislocation patterns (typical for materials with high SFE and cross slip), e.g. dislocation tangles and walls, were often observed. These structures were found always to be accompanied with a dense and homogeneous arrangement of slip traces, which can be ascribed to the highly planar slip in conjunction with high passing stresses. The interaction of different slip systems forms junctions, which assist the formation of dislocation dipoles and later on trigger the formation of dislocation walls. These walls consist of primary faulted dipoles in their interior and pile-ups of dissociated dislocations of opposite signs on their two sides. It was found that the dislocations in the walls belong to two major, collinear slip systems. The distribution of stresses and strains over channels and walls corresponds well to a refined composite model of Mughrabi.

Dislocation analysis of a complex sub-grain boundary in UO2 ceramic using accurate electron channelling contrast imaging in a scanning electron microscope

Abstract In this work, accurate electron channelling contrast imaging (A-ECCI) assisted by high resolution selected area channelling patterns (HR-SACP) was used to characterize the structure of a complex low sub-grain boundary in a creep deformed uranium dioxide (UO2) ceramic. The dislocations were characterized using TEM-style g·b = 0 and g·b × u = 0 contrast criteria. Misorientations across the boundary were measured using HR-SACPs with 0.04° precision and high accuracy EBSD. The boundary was determined to be asymmetric and mixed in nature, composed of two distinct regions with different dislocation morphologies and a misorientation below 0.5°. The A-ECCI, HR-SACP, and HR-EBSD results are consistent, confirming A-ECCI as a powerful tool for characterizing even complex dislocations structures using scanning electron microscopy. This is particularly true for UO2, since this material is very difficult to thin, which makes TEM examination of sub-boundaries over the scale of several micrometers difficult. Furthermore, in this study, the change in dislocations arrangement along the breath of the complex low angle sub-grain boundary is related to the misorientation across the boundary.

Influences of Laser Surface Alloying with Cr on Microstructural Characteristics and Hardness of Pure Ti

A high-purity Ti sheet was treated by laser surface alloying (LSA) with Cr at two different powers (100 and 200 W), with microstructural features in various laser-modified zones characterized by energy-dispersive spectrometry, electron channeling contrast imaging, and electron backscatter diffraction techniques. Hardness variation induced by the LSA was also examined and correlated with these microstructural features. Results show that at both laser powers there are two modification zones with distinct microstructural characteristics: (i) melted zone (MZ) near the surface, composed of martensitic α plates supersaturated with Cr and dense nanotwins inside them; (ii) heat-affected zone (HAZ) beneath the MZ, featured by irregular-shaped grains and substructures with varied sizes and little Cr in their interiors. Hardness measurements show that remarkable hardness increase (~ 2.5 times that of the matrix) could occur in the MZ after the LSA treatments. This can be ascribed to combined contribution from grain refinement, the presence of abundant nanotwins, and solid solution of Cr. The subgrains in the HAZ, produced by dislocation recovery, have only marginal contribution to hardness increase. The hardness of the Ti(Cr)-200W specimen is slightly lower than that of the Ti(Cr)-100W specimen, which is related to the dilution of Cr and the reduced cooling rate associated with enlarged modification zone at higher power. After comparing with laser surface treatments without alloying, it is confirmed that the LSA with Cr in the present work is much more effective in hardening the surface of pure Ti.

Twinkling graphene on polycrystalline Cu substrate: A scanning electron microscopy study

An unexpected “twinkling” appearance of partially covered chemical vapor deposition-grown graphene on the Cu substrate, showing strong grain orientation-dependent contrast of graphene—brighter, darker, or even invisible against the oxidized substrate—was observed with a scanning electron microscope. The intricate interplay between the electron channeling contrast, oxidation layer, and the imaging parameters was discussed. Imaging conditions affording stronger mass-thickness contrast was proposed to lessen the twinkling effect and thus enable unambiguous discrimination of graphene from the substrate. The findings reported here will have important implications for the reliable characterization of graphene and other 2D materials as well as the growth mechanism study of graphene.

Graphene to graphene, and substrate to substrate: how to reliably differentiate supported graphene from polycrystalline substrates using SEM?

Electron channeling contrast (ECC) for the polycrystalline substrate is commenly observed in a SEM image for graphene supported on polycrystalline substrates, making it challenge to differentiate graphene from substrate and determine the layer-numbers of graphene using SEM. Two feasible methods that allow accurate determination of layer-numbers of graphene using SEM were proposed and clearly demonstrated. The first method is to adjust the brightness of local grain in an SEM image with image-processing software, making the gray-scale values of different grains equal to each other. The second method is to tilt the sample stage to a certain angle during imaging process, giving rise to the identical orientation between neighboring grains relative to the incident electron beam. The findings reported here will have important implications for unambiguous differentiation graphene and other 2D materials from the supported polycrystalline substrate.

Defect Characterization of InAs/InGaAs Quantum Dot p-i-n Photodetector Grown on GaAs-on-V-Grooved-Si Substrate

The performance of semiconductor devices on silicon can be severely degraded by the presence of dislocations incurred during heteroepitaxial growth. Here, the physics of the defect mechanisms, characterization of epitaxial structures, and device properties of waveguide photodetectors (PDs) epitaxially grown on (001) Si are presented. A special GaAs-on-V-grooved-Si template was prepared by combining the aspect ratio trapping effects, superlattice cyclic, and strain-balancing layer stacks. A high quality of buffer structure was characterized by atomic force microscopy (AFM) and electron channeling contrast imaging (ECCI) results. An ultralow dark current density of 3.5 × 10–7A/cm2 at 300 K was measured under −1 V. That is 40× smaller than the best reported value of epitaxially grown InAs/GaAs quantum dot photodetector structure on GaP/Si substrate. Low frequency noise spectroscopy was used to characterize the generation and recombination related deep levels. A trap with an activation energy of 0.4 eV was id...