Electron Channeling Contrast Research Articles

Do voids nucleate at grain boundaries during ductile rupture?

In the absence of pre-existing failure-critical defects, the fracture or tearing process in deformable metals loaded in tension begins with the nucleation of internal cavities or voids in regions of elevated triaxial stress. While ductile rupture processes initiate at inclusions or precipitates in many alloys, nucleation in pure metals is often assumed to be associated with grain boundaries or triple junctions. This study presents ex situ observations of incipient, subsurface void nucleation in pure tantalum during interrupted uniaxial tensile tests using electron channeling contrast (ECC) imaging, electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). Instead of forming at grain boundaries, voids initiated at and grew along dislocation cell and cell block boundaries created by plastic deformation. Most of the voids were associated with extended, lamellar deformation-induced boundaries that run along the traces of the {110} or {112} planes, though a few voids initiated at low-angle dislocation subgrain boundaries. In general, a high density of deformation-induced boundaries was observed near the voids. TEM and TKD demonstrate that voids initiate at and grow along cell block boundaries. Two mechanisms for void nucleation in pure metals, vacancy condensation and stored energy dissipation, are discussed in light of these results. The observations of the present investigation suggest that voids in pure materials nucleate by vacancy condensation and subsequently grow by consuming dislocations.

Applications of Electron Channeling Contrast Imaging (ECCI) in Failure Analysis of In-Situ Synchrotron X-Ray Diffraction Deformation Experiments

Journal Article Applications of Electron Channeling Contrast Imaging (ECCI) in Failure Analysis of In-Situ Synchrotron X-Ray Diffraction Deformation Experiments Get access Shirin Kaboli, Shirin Kaboli Geoscience Department and High Pressure Science and Engineering Center, University of Nevada Las Vegas, Las Vegas, United States Search for other works by this author on: Oxford Academic Google Scholar Pamela C Burnley Pamela C Burnley Geoscience Department and High Pressure Science and Engineering Center, University of Nevada Las Vegas, Las Vegas, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 568–569, https://doi.org/10.1017/S143192761700352X Published: 04 August 2017

Crystallographic Orientation Maps Obtained from Ion and Backscattered Electron Channeling Contrast

Crystallographic Orientation Maps Obtained from Ion and Backscattered Electron Channeling Contrast

Micro-damage initiation in ferrite-martensite DP microstructures: A statistical characterization of crystallographic and chemical parameters

Damage mechanisms occurring in a DP-steel microstructure at low strains, under monotonic and cyclic deformation modes have been studied by a comprehensive statistical analysis of scanning electron microscopy observations. The study aims to define local microstructural configurations of high damage susceptibility. In particular, the role of grain and interphase boundaries at triggering micro-damage features was investigated. SEM-based electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were utilized. Focused ion beam (FIB) milling was additionally applied for a 3-dimensional investigation of the crack morphology and corresponding arrangement of phases. Microstructural configurations of high damage susceptibility were deduced and confirmed through a quasi in-situ deformation experiment. Damage initiation is shown to be highly dependent on the distribution and morphology of the martensitic islands. Voids are most likely to nucleate at interphase boundaries under monotonic loading, particularly at triple junctions between ferritic grain boundaries and martensite. On the other hand, decohesion cracks are frequently observed inside of martensite where they are remarkably restricted to prior austenite grain boundaries (PAGbs). The low fracture strength of martensitic PAGbs is discussed in the light of local chemical analysis of different types of interfaces using atom probe tomography (APT).

Effects of Ausforming Procedure and Following Annealing Treatment on Microstructural Characteristics in Cobalt

The effects of ausforming procedure and subsequent annealing treatments on microstructures of cobalt (Co) samples are investigated by electron channeling contrast and electron backscatter diffraction techniques. Results show that the ausformed Co samples consist of coarsen blocky laths (single ε phase) with the irregular morphology and the ultra-fine acicular laths (dual phase: γ and ε) with the slender and rectangular morphologies. As compared to the slight reduction sample, the much denser acicular laths are observed in the heavily ausformed sample. In addition, recrystallization behavior and annealing-induced γ → ε transformation have occurred in ausformed Co samples during the annealing treatment.

Characterization of microstructure and hardness of a Zr-2.5Nb alloy surface-treated by pulsed laser

A duplex-phase Zr alloy (Zr-2.5Nb) was surface-treated by pulsed laser with modified microstructures characterized by electron channeling contrast imaging (ECC) and electron backscatter diffraction (EBSD) techniques. Hardness variation induced by the laser treatment was also examined and tried to be correlated with various microstructural characteristics. After the laser surface treatment, two modification zones are identified: the microstructure in zone I is fully composed of martensitic α plates with a few hundred nanometers in width and abundant nanotwins inside them; zone II possesses a mixed microstructure of transformed α plates, prior bulk α grains and β films. Comparative analyses with the case in single-phase Zr alloys suggest that the β films in the Zr-2.5Nb could act as stress absorber to suppress dislocation activity in α grains in zone II. Hardness measurements show that surface regions near the laser beam center can be remarkably hardened, which could be attributed to combined contribution from grain refinement, dense nanotwins and solid solution of Nb. For locations with the mixed microstructures in zone II, only compromised hardness is obtained due to less effective hardening contributor.

α→β Transformation characteristics revealed by pulsed laser-induced non-equilibrium microstructures in duplex-phase Zr alloy

Microstructures of a duplex-phase Zr-2.5Nb alloy treated by pulsed laser were characterized by electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) imaging techniques. Major attention has been paid to non-equilibrium hybrid microstructure consisting of both prior α and β phases, and α plates transformed from new β phases to probe α→β transformation characteristics in the alloy. The origin of the hybrid microstructure is attributed to the specific thermodynamic conditions induced by the pulsed laser treatment. ECC observation shows that newly formed β phases during laser heating prefer to nucleate and grow at the expense of edges of prior α grains rather than their interiors. EBSD analyses further reveal that orientations of the new β phases are not determined by the prior α grains according to the Burgers relationship but maintain those of the prior β phases in an epitaxial growth way.

SEM and ECC Imaging Study of Step-Bunched Structure on 4H-SiC Epitaxial Layers

Step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-voltage electron scanning microscopy (LVSEM) and electron channeling contrast (ECC) imaging. LVSEM observations revealed that the step bunching resulted in the formation of atomically flat wide (~250 nm) terraces on the surface, and the terraces tended to form in pairs. The two terraces in paired terraces often showed the same electron channeling contrast as each other, and the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the step-bunched structure on a vicinal 4H-SiC (0001) surface is discussed.

Electrochemical Liquid Phase Epitaxy (ec-LPE): A New Methodology for the Synthesis of Crystalline Group IV Semiconductor Epifilms.

Deposition of epitaxial germanium (Ge) thin films on silicon (Si) wafers has been achieved over large areas with aqueous feedstock solutions using electrochemical liquid phase epitaxy (ec-LPE) at low temperatures (T ≤ 90 °C). The ec-LPE method uniquely blends the simplicity and control of traditional electrodeposition with the material quality of melt growth. A new electrochemical cell design based on the compression of a liquid metal electrode into a thin cavity that enables ec-LPE is described. The epitaxial nature, low strain character, and crystallographic defect content of the resultant solid Ge films were analyzed by electron backscatter diffraction, scanning transmission electron microscopy, high resolution X-ray diffraction, and electron channeling contrast imaging. The results here show the first step toward a manufacturing infrastructure for traditional crystalline inorganic semiconductor epifilms that does not require high temperature, gaseous precursors, or complex apparatus.

Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications.

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modelling of the energy- and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.

Hydrogen effects on microstructural evolution and passive film characteristics of a duplex stainless steel

We revealed the effects of hydrogen on the microstructural evolution and passive film properties of a 2205 duplex stainless steel by the joint use of electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The microstructural analysis results show that effects of hydrogen on the two phases are different: (i) in austenite, stacking faults are induced by hydrogen, and (ii) in ferrite, hydrogen causes an increase of the dislocation density. The XPS analysis revealed that hydrogen reduced the occurrence of Cr2O3 and nitrogen in the passive film, leading to the reduction of their overall thickness. Furthermore, for the first time we demonstrated that the hydrogen release time plays an important role in the electrochemical behavior of the hydrogen charged steel.

Relationship between indentation plastic zone size and residual stresses in plastically deformed Fe

Indentation induced plastic zone size was studied using electron channelling contrast imaging in polycrystalline Fe. Nanoindentation tests were carried out in samples that were compressed to different degrees of thickness reduction. Both strain hardening and residual stresses induced by the compression contributed to significant hardness elevations at all levels of thickness reduction. By comparing between the un-deformed and pre-deformed samples, texture and strain hardening effect were found to contribute to the differences between stresses at micro and macro scales. A correction for texture effects applied to average micro-stresses brought their values close to the macro-stresses. For each sample, a linear relationship was found between the cube of the plastic zone size measured and the inverse of the hardness. The slope of the linear trend line was shown to increase with increased compressive macro-stress level as determined by X-ray diffraction (XRD). This newly discovered empirical relationship demonstrated the potential of predicting macro-scale residual stress levels from nanoindentation experiments.

{332}<113> detwinning in a multilayered bcc-Ti–10Mo–Fe alloy

We have studied {332} detwinning mechanism in a bcc-Ti–10Mo–xFe (x = 1–3 wt%) multilayered alloy upon annealing treatment at 900 °C by electron backscatter diffraction and electron channeling contrast imaging. Our analysis reveals that in the present material the detwinning process consists of two independent detwinning events that occur at two different microstructural regions, namely twin tips located at grain interiors and grain boundaries. Boundary dissociation reactions and mobilities were analyzed by tracking the evolution of the twin structure upon thermal annealing. We find that the first detwinning process is characterized by the evolution of incoherent twin boundaries into a Σ3 boundary and its subsequent migration. The second detwinning event is characterized by the detachment of the twin crystal from a grain boundary by the formation and migration of Σ11 {113} incoherent twin boundaries. Both detwinning modes can be explained from a thermodynamic standpoint where the boundary dissociation processes minimize the boundary free energy.

ECCI/EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels

Fatigue test data of duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue (VHCF) regime is presented. Fatigue testing was conducted using ultrasonic fatigue test equipment operating at 20kHz under fully reversed tension-compression load condition. Scanning Electron Microscope (SEM) analysis of the fracture surfaces and external surfaces of failed specimens was conducted. Electron Channelling Contrast Imaging (ECCI) and Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens was done to analyse the accumulation of plastic fatigue damage and fatigue crack growth in the grains adjacent to the external surface and crack initiation site. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 was carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the Crystallographic Growth Region (CGR) showed features like grain boundaries and fatigue striations on the fracture surfaces. SEM analysis of the external surfaces of fatigue loaded specimens showed inhomogeneous accumulation of plastic fatigue damage. ECCI/EBSD analysis showed Persistent Slip Bands (PSBs) in ferrite grains in LDX 2101 grade but no PSBs were observed in any grains of 2304 SRG specimens. The barrier effect of grain and phase boundaries on short fatigue crack propagation was observed. TEM analysis of thin foils cut from the failed specimens of LDX 2101 showed stacking faults in austenite grains and they were seen to stop at the grain and phase boundaries.

Microstructural characteristics of cobalt treated by high-speed laser surface melting under high power

The microstructural evolution of cobalt (Co) treated by high-speed laser surface melting (LSM) under high power is symmetrical investigated by using electron backscatter diffraction and electron channeling contrast imaging techniques. Four distinctly different microstructural characteristics from surface to substrate are revealed in Co samples treated by LSM under high power: ε martensite plates in the elongated columnar grains for melted zone (MZ); a dual-phase microstructure (γ phase and ε phase) for heated affect zone (HAZ); deformation twins for stress affected zone (SAZ); and recrystallization microstructure for basal metal (BM). The high-speed laser treatment on Co sample shows some remarkable effects: (i) deepening the modified depth in MZ; (ii) obtaining the ultra-fine grains in HAZ; (iii) emerging one new region (SAZ). The micro-hardness reaches the highest value in HAZ due to fine grain strengthening. The metallurgical processes for the different laser-modified regions are symmetrically discussed in this paper.

A correlative investigation of grain boundary crystallography and electronic properties in CdTe thin film solar cells

Evaluating the impact of grain boundaries on the functional properties of CdTe thin films, consistent with processes used in photovoltaic solar cells, requires a direct correlation between their crystallography and electronic behavior. In the present work, we propose a novel comprehensive approach, combining focused ion beam/electron backscatter diffraction tomography (3D-EBSD) and quantitative cathodoluminescence (CL). While the former enables a full five parameter characterization of the interfaces, the latter is used to probe the spatial distribution of recombination centers and their characteristics. In addition, critical issues associated with sample preparation are also discussed. Monte Carlo simulations, together with electron channeling contrast imaging (ECCI), are employed to evaluate the effects of ion-sputtering damage on the CL response of CdTe thin films, as well as to overcome the resolution limit of EBSD characterization. The results obtained show that, at the exception of coherent twin boundaries, all interfaces behave as non-radiative recombination centers, exhibiting significant recombination velocities. Furthermore, there is no direct correlation between the misorientation parameters of the interfaces and their recombination properties. In contrast, trends can be observed when considering the crystallography of the boundary planes.

Anisotropy effects during dwell-fatigue caused by δ-phase orientation in forged Inconel 718

Inconel 718 is a commonly used superalloy for turbine discs in the gas turbine industry. Turbine discs are often subjected to dwell-fatigue as a result of long constant load cycles. The effect of anisotropy on dwell-fatigue cracking in forged turbine discs have not yet been thoroughly investigated. Crack propagation behaviour was characterised using compact tension (CT) samples cut in different orientations from a real turbine disc forging. Samples were also cut in two different thicknesses in order to investigate the influence of plane strain and plane stress condition on the crack propagation rates. The samples were subjected to dwell-fatigue tests at 550°C with 90s or 2160s dwell-times at maximum load. Microstructure characterisation was done using scanning electron microscopy (SEM) techniques such as electron channelling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and light optical microscopy (LOM). The forged alloy exhibits strong anisotropic behaviour caused by the non-random δ-phase orientation. When δ-phases were oriented perpendicular compared to parallel to the loading direction, the crack growth rates were approximately ten times faster. Crack growth occurred preferably in the interface between the γ-matrix and the δ-phase.

Observation of individual stacking faults in GaN microcrystals by x-ray nanodiffraction

X-ray nanodiffraction was used for the investigation of basal stacking faults in a-GaN microcrystallites. The method made it possible to find the positions of individual stacking faults in a chosen crystallite, and the resulting positions were compared with the observation of individual faults by electron channeling contrast in scanning electron microscopy. The x-ray diffraction data revealed that the faults occur in closely positioned pairs; the stacking faults in a pair have opposite displacement vectors.

Design and fabrication of enhanced lateral growth for dislocation reduction in GaN using nanodashes

The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing {1−101} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing {11−22} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109cm−2 to 3.0×107cm−2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.

Threading dislocations in MBE grown AlInSb metamorphic buffers: Revealed and counted

The authors compare four methods to investigate the threading dislocations (TDs) observed in metamorphic buffers used in the growth of InSb quantum well on GaAs (001) substrates. Three types of buffers with varying number of Al0.24In0.76Sb interlayers (N = 0, 1, and 3) were studied. Cross-sectional scanning transmission electron microscopy (STEM) revealed an effective dislocation filtering by the interlayers. Individual TDs were identified with atomic-force microscopy (AFM) as distinct morphological features of dislocation outcrops on the surface. Threading dislocation density (TDD) is reduced by 1 order of magnitude with three interlayers, consistent with the STEM observation. TDD measured with a scanning electron microscope in electron channeling contrast imaging (ECCI) mode agrees closely with the AFM analysis, except for the N = 0 buffer where the ECCI gives TDD lower by more than a factor of two. The etch pit density of N = 3 buffer, measured with a Nomarski differential interference contrast microscope after defect selective etching (DSE), is almost 1 order of magnitude lower than the TDD measured by AFM and ECCI. Due to the large pit size, the used etching recipe only works well for samples with TDD lower than 107 cm−2. AFM, ECCI, and DSE are excellent alternatives to transmission electron microscopy in the process of metamorphic buffer optimization. The AFM technique offers the additional advantage of high vertical resolution morphology mapping. Such capability is of great importance for the optimization of metamorphic buffers from the perspective of surface smoothness improvement.