Electron Channeling Contrast Imaging Techniques Research Articles

On fatigue crack origin with a fine granular area in matrix without defect

Fatigue crack initiation in metallic materials during very high cycle fatigue has been characterized by a subsurface crack origin with defect such as an inclusion. In this paper, fatigue damage behavior in an austenitic stainless steel has been studied using a novel progressive stepwise load increasing test method with each cycle step higher than 108 cycles. Subsurface crack origin with a fine granular area has formed in the matrix without defect. This is a new phenomenon. The mechanism has been investigated using Focused Ion Beam cross-sectioning and electron channeling contrast imaging techniques. Strain localization, grain fragmentation and local plasticity exhaustion are the main factors that cause fatigue damage and crack initiation in the matrix. This study provides a fundamental understanding how material damage and crack initiation occur in material matrix during very high cycle fatigue.

Effect of hydrogen on nanomechanical properties of Inconel 625 studied using in-situ electrochemical nanoindentation technique

In-situ electrochemical nanoindentation (ECNI) experiment is performed to probe the nanomechanical properties of Inconel 625 alloy under a hydrogen environment. The hydrogen effect on various stages of the load-displacement (LD) plot is quantified. The pop-in load, which indicates the onset of a homogenous dislocation nucleation event, is reduced with hydrogen charging, which augers well with the 'Defactant' concept. The hydrogen solute hinders dislocation glide during pop-in (reduced pop-in width), and the resultant glide resistance force is found to be linearly dependent on dissolved hydrogen concentration. We have also quantified the hydrogen-reduced activation barrier for the secondary source by examining the secondary elastic hardening stage from LD data. Hydrogen charging-induced slip line formation and nanohardness variation at different polarizations are explained using a simple diffusion-desorption model. Furthermore, the electron channeling contrast imaging technique is used to study the dislocation patterns around the indents in air and in the hydrogen environment to shed light on the difference in plastic behavior.

Hydrogen‐assisted fatigue crack propagation behavior of equiatomic Co–Cr–Fe–Mn–Ni high‐entropy alloy

AbstractThe equiatomic Co–Cr–Fe–Mn–Ni high‐entropy alloy (HEA) shows well hydrogen embrittlement resistance under monotonic tensile load. However, the fracture behavior under cyclic load is still unclear. In this study, combining with the fracture features analysis by electron back‐scattered diffraction and electron channeling contrast imaging techniques, the hydrogen‐assisted fatigue crack propagation behavior of equiatomic Co–Cr–Fe–Mn–Ni HEA under in situ electrochemical hydrogen charging was investigated. The results suggest that the hydrogen had significant accelerating effects on the fatigue crack growth rate of Co–Cr–Fe–Mn–Ni HEA. Intergranular cracking with the formation of dislocation cells was observed at low stress intensity range (ΔK) area, while transgranular cracking with deformation twins was observed at a high ΔK area. The formation of these deformation features was assisted by hydrogen‐assisted dislocation emission.

Investigation of microstructure evolution and martensite transformation developed in austenitic stainless steel subjected to a plastic strain gradient: A combination study of Mirco-XRD, EBSD, and ECCI techniques

The evolution of the microstructure and deformation mechanism at different levels of plastic strain are investigated for 304 L austenitic steel with a combination of micro X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and electron channeling contrast imaging (ECCI). A plastic strain gradient is developed along the longitudinal rolling direction in a wedge-shaped 304 L austenitic steel sample. The graded deformed microstructure includes various amounts of deformation bands, ε- and α′-martensites, and their intersections form during the plastic deformation. ECCI observations reveal several deformation mechanisms of the formation of partial dislocations, dislocations, and interactions. This study suggests geometrically necessary deformation bands (GNDBs) are introduced and stored instead of geometrically necessary dislocations (GNDs) in low stacking fault energy (SFE) materials such as austenite steels. Consequently, increasing the strain gradient leads to an increase in the geometrically necessary martensitic transformation (GNMT); this is the result of the deformation-induced martensite in these materials. In addition to the statistically stored dislocations (SSDs), GNDs are generated at the grain boundaries of the fragmented grains to preserve the continuity of the grains. Accordingly, the strain hardening of the austenite steel includes multiple interactions of the deformation bands, SSDs, GNDs, GNDBs, and GNMT. From the viewpoint of microstructure design, our study provides quantitative information about the relationship between the amount of plastic deformation and the extent of microstructure evolution, in a continues design space.

Characterization of the microstructure and deformation substructure evolution in a hierarchal high-entropy alloy by correlative EBSD and ECCI

We investigate the microstructure and deformation substructure evolution in an additively manufactured 1.8 at.% N-doped FeCoNiCr high-entropy alloy (HEA) by means of correlative electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). The combinational use of EBSD and ECCI uncovers hierarchal structures of the HEA at multiple length scales, which consist of a bimodal grain structure, low angle boundaries, and dislocation networks, along with the quantitative evolution of geometrically necessary dislocations (GNDs) distribution. The ECCI technique shows comparable resolution to transmission electron microscopy (TEM) in terms of characterizing the dislocation substructures. We further clarify the strain hardening mechanisms by investigating the deformation microstructures. The excellent strain hardening rate of the HEA even at large strain is due to the jointly activation of slip bands and mechanical twining. The deformation induced twining shows strong orientation dependence.

Strain visualization of growing short fatigue cracks in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint: Intergranular cracking and crack closure

Physically short fatigue crack growth behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint was investigated in-situ in SEM to reveal effects of crack closure and tortuous crack growth path on crack-tip straining behavior with the help of high resolution digital image correlation technique. The crack growth path and associated fatigue damage at microstructural scale was characterized by electron backscatter diffraction and electron channeling contrast imaging techniques. Results showed that the short crack growth was influenced by both local microstructure and global strength gradient. The short crack could deflect intergranularly when meeting with one large grain due to misfit deformation and strain localization. The crack closure, which was accompanied during loading and unloading in terms of crack surface contact, shielded the real crack-tip and manifested itself well in both compressive strain at crack wake and strain development at crack-tip. It is indicated that strain localization at lathy boundaries and formation of sub-grains was responsible for the fatigue of short cracks.

Development of Grain Boundary Character Distribution in Medium-Strained 316L Stainless Steel During Annealing

A medium-strained (13% cold-drawing) 316L austenitic stainless steel rod was subjected to annealing between 750 and 1100 °C. Electron backscatter diffraction and electron channeling contrast imaging techniques were jointly employed to investigate the development of microstructures and grain boundary character distribution during the annealing treatments. Results show that annealing at temperatures below 900 °C does not evidently change microstructures of the as-drawn specimen. Full recrystallization is observed after the temperature reaches 1000 °C and the deformation microstructures are replaced by fine and uniform recrystallized grains. A large number of annealing twins (with Σ3 misorientation) are produced by the recrystallization, leading to markedly increased fractions of special boundaries (fSB). The maximum fSB is found to be 67.3% in the present study. In addition, for the recrystallized grains, rapid growth is noticed at relatively high temperatures due to easy migration of grain boundaries.

Direct observation of recombination-enhanced dislocation glide in heteroepitaxial GaAs on silicon

We use an electron beam to induce and directly observe dislocation glide in strained GaAs thin films on silicon substrates at room temperature using electron channeling contrast imaging (ECCI). Plastic behavior in this brittle material is kinetically facilitated by a lowering of the Peierls barrier for glide due to recombination of cathodogenerated electron-hole pairs at dislocations. The high residual strain in the GaAs film provides the overall driving force for motion. The dynamics of a variety of previously inaccessible dislocation interactions are studied by tracking individual threading dislocations with high spatial and temporal resolution. The ECCI technique has an inherently high resolution and was conducted in a convenient scanning electron microscope environment. The results and methods described in this Rapid Communication provide insight into dislocation-filtering processes and failure mechanisms in technologically important III-V devices on silicon, as well as other mismatched heteroepitaxial systems.

Microstructural characterization of Inconel 718 alloy after pulsed laser surface treatment at different powers

Abstract An annealed Inconel 718 alloy was surface-treated by pulsed laser at three different powers (100, 50 and 25 W). Microstructural changes induced by the laser treatments were characterized by use of electron backscatter diffraction and electron channeling contrast imaging techniques. Results show that both annealing twins and strengthening precipitates profusely existing in the as-received specimen are dissolved at elevated temperatures during the laser irradiation. Meanwhile, in the melting zone (MZ), densities of low angle boundaries (LABs) are greatly increased with a large number of Laves phases preferentially distributed along such LABs. For different specimens, widths and depths of their MZs are found to be gradually reduced with decreasing the laser powers. Orientation analyses reveal that the columnar grains in the MZ of the 100 W specimen could inherit orientations existing in the matrix while lower laser powers promote the formation of more nuclei with scattered orientations to grow to be granular grains in the MZ. Hardness tests reveal that the MZs of all laser-treated specimens are softer than the matrix probably due to both precipitate dissolution and grain coarsening.

Low threading dislocation density GaAs growth on on-axis GaP/Si (001)

We report a systematic study of high quality GaAs growths on on-axis (001) GaP/Si substrates using molecular beam epitaxy. Various types of dislocation filter layers and growth temperatures of initial GaAs layer were investigated to reduce the threading dislocation densities in GaAs on GaP/Si. Electron channeling contrast imaging techniques revealed that an optimized GaAs buffer layer with thermal cycle annealing and InGaAs/GaAs dislocation filter layers has a threading dislocation density of 7.2 × 106 cm−2, which is a factor of 40 lower than an unoptimized GaAs buffer. The root-mean-square surface roughness was greatly decreased from 7.8 nm to 2.9 nm after the optimization process. A strong enhancement in photoluminescence intensity indicates that the optimized GaAs template grown on on-axis (001) GaP/Si substrates is a promising virtual substrate for Si-based optoelectronic devices.

Multiple slip dislocation patterning in a dislocation-based crystal plasticity finite element method

Dislocation structures forming during cyclic loading of fcc metals are fatigue damage precursors. Their specific structures are caused by the motion and interactions of dislocations. Depending on the load conditions, the grain orientation, the stacking fault energy, a variety of different dislocation structures appear in the material such as labyrinths, cells, veins and persistent slip bands. We present a continuum dislocation-based model for cyclic fatigue and incorporate it into a crystal plasticity finite element solver. A method for the simulation of dislocation junction formation is introduced, which reproduces the behaviour of discrete objects, such as dislocations, in a continuum framework. The formation of dislocation walls after 50 and 100 deformation cycles at 0.95% and 0.65% strain amplitude starting from an initial random dislocation distribution is predicted for 〈001〉 and 〈11¯0〉 oriented crystals. Simulations and cyclic tension-compression experiments of polycrystalline 316L stainless steel are performed to compare our model with another model based on edge and screw dislocation densities. The simulated dislocation structures and experimental results, obtained with the electron channeling contrast imaging technique, are compared using a 2D orientation distribution function of the dislocation structures. The dominant orientation of dislocation walls is predicted by the new model; it turns out to be perpendicular to the intersection line between the two slip planes involved in their formation and at an angle of around 45o from the loading axis. This agrees well with the experimental observations and represents a step forward for understanding the formation mechanism of these dislocation structures.

Three-dimensional visualization of dislocations in a ferromagnetic material by magnetic-field-free electron tomography

In conventional transmission electron microscopy, specimens to be observed are placed in between the objective lens pole piece and therefore exposed to a strong magnetic field about 2 T. For a ferromagnetic specimen, magnetization of the specimen causes isotropic and anisotropic defocusing, deflection of the electron beam as well as deformation of the specimen, which all become more severe when the specimen tilted. Therefore electron tomography on a ferromagnetic crystalline specimen is highly challenging because tilt-series data sets must be acquired without changing the excitation condition of a specific diffraction spot. In this study, a scanning transmission electron microscopy (STEM) tomography method without magnetizing a ferromagnetic specimen has been developed for three-dimensional (3D) visualization of dislocations in α-Fe, which is a typical ferromagnetic material. Magnetic-field-free environment down to 0.38 ± 0.07 mT at the specimen position is realized by demagnetizing the objective lens pole piece of a commercial STEM instrument. By using a spherical aberration corrector with the magnetic-field-free environment, an “aberration corrected Low-Mag STEM mode” with no objective lens field reaches a convergence semi angle ∼1 mrad and a spatial resolution ∼5 nm, and shows an adequate performance of imaging dislocations under a two-beam excitation condition for a low-index diffracted beam. The illumination condition for the aberration corrected Low-Mag STEM mode gives no overlap between the direct beam disk (spot) and neighboring diffraction disks. An electron channeling contrast imaging technique, in which an annular detector was located at a doughnut area between the direct beam and the neighboring diffracted beams, was effectively employed with the aberration corrected Low-Mag STEM mode to keep image intensity high enough even at large specimen-tilt angles. The resultant tomographic observation visualized 3D dislocation arrangements and active slip planes in a deformed α-Fe specimen.

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.

Analysis of dislocation configurations in a [0 0 1] fcc single crystal by electron channeling contrast imaging in the SEM.

We have investigated the dislocation configurations in a [0 0 1] single crystal of a face-centered alloy (316L stainless steel) by the electron channeling contrast imaging (ECCI) technique in the scanning electron microscope under controlled diffraction conditions. Specific dislocations such as piled-up dislocations and Lomer-Cottrell dislocations were characterized by the analysis of dislocation contrast and dislocation line trace analysis. The ECCI technique also allows the sound estimation of the local resolved stress acting on gliding dislocations by the analysis of the radius of curvature following a transmission electron microscopy-based geometrical approach.

Analysis of FIB-induced damage by electron channelling contrast imaging in the SEM.

We have investigated the Ga+ ion-damage effect induced by focused ion beam (FIB) milling in a [001] single crystal of a 316 L stainless steel by the electron channelling contrast imaging (ECCI) technique. The influence of FIB milling on the characteristic electron channelling contrast of surface dislocations was analysed. The ECCI approach provides sound estimation of the damage depth produced by FIB milling. For comparison purposes, we have also studied the same milled surface by a conventional electron backscatter diffraction (EBSD) approach. We observe that the ECCI approach provides further insight into the Ga+ ion-damage phenomenon than the EBSD technique by direct imaging of FIB artefacts in the scanning electron microscope. We envisage that the ECCI technique may be a convenient tool to optimize the FIB milling settings in applications where the surface crystal defect content is relevant.

Tribological Behavior of a Cold-Sprayed Cu–MoS2 Composite Coating During Dry Sliding Wear

Two cold spray coatings, one pure Cu and the other a Cu–MoS2 composite coating, were studied for their tribology performance in dry air. It was demonstrated that a small amount of MoS2 (1.8 ± 0.99 wt%) could significantly decrease coefficient of friction (CoF) from around 0.7 (Cu coating) to 0.14–0.15. MoS2 patches on the wear track exhibited a lower local CoF, and the main velocity accommodation mechanism was shearing MoS2-containing debris. Even though the coating wear rates were high in the early sliding (8.61–12.8 nm/cycle in penetration depth during the first 100 cycles), slow wear (0.12–0.22 nm/cycle) over the subsequent sliding was observed. It was also found that the presence of MoS2 helped to achieve high endurance of the first steady-state CoF. The dynamics of the process, material transfer, and phase transformation were examined using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The MoS2 patches developed on the wear track and the counterface served as reservoirs to replenish MoS2 in the contact and became depleted with sliding. Cross-sectional microstructure revealed by electron channeling contrast imaging technique showed a layer of sliding-induced microstructure, 3–5 µm thick for the Cu–MoS2 coating, and 10–30 µm thick for the Cu coating.

Two-Dimensional Observation of the Core–Mantle Model for Superplastic Flow in an ODS Ferritic Steel

Two-dimensional grain movements were microscopically observed in high-temperature shear deformation of an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure that was sheared in a direction perpendicular to the grain long axis. The microstructure was analyzed using electron back-scattered diffraction and electron channeling contrast imaging techniques before and after the shear deformation. Clear grain switching events, which are assumed to occur via grain-boundary sliding (GBS), were observed and the switching mechanism was characteristic of the core–mantle superplasticity model proposed by Gifkins; dislocation densities got much higher in narrow areas near the grain boundaries (mantles) than the grain interiors (cores). The mantle regions typically appeared in protruding portions of grains that was likely resistant to GBS, and low-angle boundaries were found to emerge at the core–mantle boundaries via slipping of dislocations within the mantle regions.

Microstructural changes of Zr702 induced by pulsed laser surface treatment

In this work, the surface of a fully recrystallized Zr702 is treated by pulsed laser following which microstructural changes are investigated by use of electron backscatter diffraction and electron channeling contrast imaging techniques. The pulsed laser treatment results in three distinctly different microstructural features from the surface to the substrate: fine α plates with a few hundred nanometers in width (zone I), irregular-shaped grains with varied sizes (zone II), and essentially unchanged equiaxed grains (zone III). The α plates result from rapid phase transformation due to easy heat extraction of the pulsed laser with dense nanoscale twins inside those plates closer to the surface. The origin of the irregular-shaped grains is found to be related to insufficient recrystallization of antecedently formed α plates near the substrate. Hardness tests reveal highest value (∼356.7 HV) near the surface in zone I and the lowest value (∼165.2 HV) in zone II. Reasons accounting for the difference are discussed in terms of various microstructural characteristics induced by the pulsed laser surface treatment.

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization.

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities - high spatial resolution and richness of microstructural data - with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

Case studies on the application of high-resolution electron channelling contrast imaging – investigation of defects and defect arrangements in metallic materials

In 1967, Coates discovered the electron channelling contrast of backscattered electrons (BSEs) in scanning electron microscopy, and by this the possibility to investigate arrangements of lattice defects in deformed microstructures of materials. Since that time, a straightforward development of the scanning electron microscopes as well as of the electron channelling contrast technique took place. Nowadays, the performance of scanning electron microscopes is high enough that the resolution of electron channelling contrast imaging (ECCI) micrographs is comparable with conventional bright field transmission electron microscopy (TEM) micrographs. In the first part of the present paper, a historical review on the development of the ECCI technique starting from its discovery more than 45 years ago up to the combination with other advanced methods of scanning electron microscopy like electron backscatter diffraction or high-resolution selected area channelling patterning in the last few years is given. Major important investigations using this technique for the visualization of individual lattice defects like stacking faults (SFs) and dislocations or dislocation arrangements are chronologically summarized. The second part demonstrates that nowadays, ECCI micrographs taken in high-resolution scanning electron microscopes can be called high-resolution ECCI (HR-ECCI). It is shown that the resolution of individual SFs and dislocations in the HR-ECCI micrographs is comparable to that of conventional TEM (about 15 nm defect image width). Furthermore, the paper is demonstrating that HR-ECCI micrographs can be obtained for various types of materials after different mechanical loadings and different grain sizes ranging from large grain size of 500 μm (cast steel) down to less than 2 μm (γ-TiAl).