Electron Channeling Contrast Imaging Research Articles

Correlation between the step–terrace structure and the nitrogen doping variation observed on the (0001¯) facet of 4H-SiC crystals

The variation in nitrogen doping concentration on the (0001¯) facet of 4H-SiC single crystals grown by the physical vapor transport (PVT) growth method was investigated using Raman scattering microscopy. The observed variation was well correlated with the terrace-width variation in the step–terrace structure on the facet; regions with unevenly distributed terrace-widths always showed a lower nitrogen concentration compared to those having evenly distributed terrace-widths. It was revealed by low-energy electron channeling contrast imaging that in lower nitrogen concentration regions, terraces with two dangling bonds per silicon atom at the step riser in the step-down direction always became wider. On the basis of these experimental results, the nitrogen incorporation kinetics during PVT growth of 4H-SiC crystals was discussed.

Effect of Heterogeneous Surface Structure on Mechanical Properties of Interstitial-Free Steel Subjected to Laser Surface Treatment

To investigate the effect of heterogeneous gradient surface structure on mechanical properties of metals, a Ti-added interstitial-free steel with coarse-grained matrix and ultrafine-grained matrix were applied and were treated by laser surface treatment using liner-scanning. The distances between the scanning lines were modified to alter the space between the hard-treated zone and soft matrix. The laser-treated zone contains a melting zone, comprised of fine grains, and a heat-affected zone featured by coarse columnar grains grown radically from the laser beam center. For coarse-grained matrix sample, after proper laser surface treatment significant improvement (about 100%) in yield strength and moderate improvement (22.8%) in ultimate tensile strength were achieved with little loss in elongation. For ultrafine-grained matrix sample, gradual decrease in strength together with recovering of elongation was realized. Electron channeling contrast imaging was applied to investigate the structure of the sample which was then compared with the untreated one to reveal the origin of the mechanical behavior of the laser surface-treated steel.

Deformation faulting in a metastable CoCrNiW complex concentrated alloy: A case of negative intrinsic stacking fault energy?

Microscopic crystalline defects are of fundamental importance in unraveling the plastic deformation response and thereby tailoring the macroscopic load-bearing performances of metallic alloys. Especially in the microstructural metastability engineering context of complex concentrated alloys (CCAs), while profuse interest has been focused on phase and twin boundaries as well as their interactions with glissile dislocations, stacking faults, as another essential planar defects, have remained comparatively less-explored and unutilized. In the present work, by investigating a metastable CoCrNiW CCA via the combination of in-situ synchrotron X-ray diffraction and in-situ electron channeling contrast imaging (ECCI), we show that stacking faults formation can also operate as the predominant deformation micro-mechanism, accommodating plastic strain while enabling macroscopic strain hardening. Through the examination of relative phase stability by thermodynamic modeling, we reveal that this sort of deformation faulting response is largely correlated with a negative intrinsic stacking fault energy. The corresponding physical revelation is explored in greater details regarding thermodynamics, structure, and mechanics, followed by in-situ experimental verification of the stacking fault activities.

Microstructural and magnetic characterization of a lean duplex steel: Strain-induced martensite formation and austenite reversion

Strain-induced α′-martensite and austenite reversion in a cold rolled UNS S32304 lean duplex steel were tracked by means of magnetic measurements, with emphasis on both Ms (saturation magnetization) and Hc (coercive field) parameters. Grain-averaged quality metrics derived from EBSD (electron backscatter diffraction) analysis were also used to distinguish the phases during austenite reversion. The material was cold rolled to a true strain (ε) of 1.61 and subjected to isothermal and continuous annealing, the latter conducted in the presence of an external magnetic field. The evolution of the α′-martensite fraction upon straining and after isothermal annealing was monitored by coupling the Ms values and thermodynamic simulations, as well as from EBSD analysis. For the isothermally annealed material (ε = 1.61), the overall behavior of Ms and hardness displayed similar trends with a strong decrease for temperatures higher than 500 °C, suggesting austenite reversion. Results confirmed the occurrence of austenite reversion for the temperature interval investigated here. At 800 °C, austenite reversion is complete, and the steel is fully recrystallized. Besides, from the EBSD analysis, evidence of ferrite transformation into austenite was rather noticeable, in accordance with thermodynamic simulations and magnetic probing. Complementary electron channeling contrast imaging (ECCI) revealed that precipitation reactions mainly occur in the recrystallized austenite at 700 and 800 °C. The Hc behavior of both, the strained and annealed conditions was inferred to be mostly driven by microstructural changes in ferrite.

Novel -75°C SEM cooling stage: application for martensitic transformation in steel.

Microstructural changes during the martensitic transformation from face-centred cubic (FCC) to body-centred cubic (BCC) in an Fe-31Ni alloy were observed by scanning electron microscopy (SEM) with a newly developed Peltier stage available at temperatures to −75°C. Electron channelling contrast imaging (ECCI) was utilized for the in situ observation during cooling. Electron backscatter diffraction analysis at ambient temperature (20°C) after the transformation was performed for the crystallographic characterization. A uniform dislocation slip in the FCC matrix associated with the transformation was detected at −57°C. Gradual growth of a BCC martensite was recognized upon cooling from −57°C to −63°C.

Microstructural characteristics and hardness of CoNiTi medium-entropy alloy coating on pure Ti substrate prepared by pulsed laser cladding

A ternary CoNiTi medium-entropy (MEA) alloy coating was successfully fabricated on pure Ti substrate by utilizing pulsed laser cladding. Phase constitutions and microstructural characteristics of the coating were analyzed by combined use of X-ray diffraction, electron channeling contrast imaging, secondary electron imaging, energy dispersive spectroscopy and electron backscatter diffraction techniques. Results show that three zones with distinct microstructures are formed after the pulsed laser cladding treatment: cladding zone (CZ), bonding zone (BZ) and heat-affected zone (HAZ). Dendritic (BCC solid-solution phase)-interdendritic (Ti2Ni type phase) structures are formed in the CZ with many nanoparticles (Ti2Co type phase) dispersed inside the interdendritic structures. The BZ consists of fine acicular grains while the HAZ below the BZ is composed of irregular-shaped bulk grains. Hardness measurements reveal that hardness of the MEA coating reaches 571 ± 46 H V, which is ∼5 times that of the substrate (114 ± 4 H V). Comprehensive analyses show that such high hardness can be attributed to the combined contributions from solid-solution hardening of the BCC phase and second-phase hardening from the Ti2Ni and Ti2Co type intermetallic compounds.

Microstructure and mechanical properties of (CrCoNi)97Al1.5Ti1.5 medium entropy alloy twisted by free-end-torsion at room and cryogenic temperatures

In this study, the swaged (CrCoNi)97Al1.5Ti1.5 medium entropy alloys (MEAs) were twisted by free-end-torsion at room and liquid nitrogen temperatures. Microstructure in different strained layers of the twisted sample was characterized by combined use of energy dispersive spectroscopy, electron channeling contrast imaging, and electron backscatter diffraction techniques. The microhardness and overall tensile properties of the twisted sample were analyzed. The results show that the MEAs exhibit higher torque and lower torsion angle during low-temperature torsion. The deformation mechanism during torsion is dominated by dislocation slip, and gradient dislocations and lamellar dislocation substructures gradually increasing from core to surface are formed in the twisted samples. The twin content gradually decreases in the surface layer, especially when twisted at room temperature or at liquid nitrogen temperature but with a small torsion angle. The reduction of twins is related to the addition of Al and Ti, the critical shear stress of twinning, and the unique strain state of torsional deformation. The twisted samples have gradient distributions of microhardness. Torsion can effectively improve the tensile strength of the MEAs at the expense of ductility.

Typical Microstructural Characteristics of Ti–5Al–5Mo–5V–3Cr–1Fe Metastable β Ti Alloy Forged in α + β Region

In this study, typical microstructural characteristics of a metastable β Ti alloy (Ti–5Al–5Mo–5V–3Cr–1Fe) forged in a dual-phase region (strain of 54% at 820 °C) were investigated in detail by the combined use of X-ray diffraction, energy dispersive spectroscopy, electron channeling contrast imaging and electron backscatter diffraction techniques. Results show that the microstructure of the forged alloy is composed of bulk α grains, α plates and β matrix. The bulk α grains correspond to retained primary α phase (αp, average grain size ~ 2.4 μm), while the α plates are secondary α phase (αs, width ~ 70 nm) precipitated from the β matrix during air cooling. During forging, the β matrix experiences dynamic recovery with many subgrains and significant orientation gradients formed. Analyses of the orientation relationship between the α and β phases show that the Burgers orientation relationship is not maintained between some αp and β phases, which should be related to thermal deformation-induced changes of their orientations. In contrast, all of the αs plates are found to maintain well the Burgers orientation relationship with the β phase.

Analysis of nanoscale fluid inclusions in geomaterials by atom probe tomography: Experiments and numerical simulations

The spatial correlation between defects in crystalline materials and trace element segregation plays a fundamental role in determining the physical and mechanical properties of a material, which is particularly important in naturally deformed materials. Herein, we combine electron backscatter diffraction, electron channelling contrast imaging, scanning transmission electron microscopy and atom probe tomography on a naturally occurring metal sulphide in an attempt to document mechanisms of element segregation in a brittle-dominated deformation regime. Within APT reconstructions, features with a high point density comprising O-rich discs stacked over As-rich spherules are observed. The combined microscopy data allow us to interpret these as nanoscale fluid inclusions. Our observations are confirmed by simulated APT experiments of core-shell particles with a core exhibiting a very low evaporation field and the shell emulating a segregated layer at the inclusion interface. Our data has significant trans-disciplinary implications to the geosciences, the material sciences, and analytical microscopy.

Effects of Mo on the mechanical behavior of γ/γʹ-strengthened Co-Ti-based alloys

We investigated the flow behavior of γ/γʹ-strengthened Co-12Ti and Co-12Ti-4Mo (at.%) alloys at room and elevated temperatures (up to 900°C) by electron microscopy and density functional theory. The Mo-added alloy exhibited an enhanced compressive yield strength and strain hardening behavior as compared to the reference binary alloy. This behavior could be attributed to a ~25% larger γʹ volume fraction and ~7% higher planar fault energies in Co-12Ti-4Mo. Using electron channeling contrast imaging, we observed interrupted slip bands in the Co-12Ti-4Mo alloy deformed to a strain of 6%, which led to enhanced strain hardening, in contrast to extended slip bands along {111} planes in the Co-12Ti alloy. Interrupted slip band formation in Co-12Ti-4Mo could be explained by rapid exhaustion of dislocation sources and a higher energy barrier required to cut the γʹ precipitates. These effects are due to a reduced γ channel width and substantial hardening effect of γʹ-Co3(Ti,Mo) in the ternary alloy as well as due to the large shear modulus difference between γʹ and γ.

On the Pile-Up and Sink-in Behavior of Indented Nickel Considering Dislocation Structures

Background: Surface topography of pile-up and sink-in is an issue of strain-hardening behavior around pyramidal and spherical indentations. The relationship between the surface profile and the activated dislocations is not clearly understood. Objective: This study combined electron channeling contrast imaging (ECCI), electron backscatter diffraction-based (EBSD) techniques and 3D laser microscopy to visualize the stress field and understand the influence of activated dislocations on the surface topography around an indent. Methods: The dislocation structures were identified using ECCI and geometrically necessary dislocation (GND) analysis. The stresses and GND densities were calculated to characterize the plastic deformation in terms of activated dislocations. The 3D laser microscopy was applied to reveal the surface topography. Results: The activated slip systems were identified as screw-type on $$ \left(1\overline{1}\overline{1}\right)\left[110\right] $$ , $$ \left(1\overline{1}\overline{1}\right)\left[101\right] $$ , $$ \left(\overline{1}1\overline{1}\right)\left[011\right] $$ , $$ (111)\left[\overline{1}10\right] $$ and $$ (111)\left[10\overline{1}\right] $$ slip systems, and edge-type on $$ \left(1\overline{1}\overline{1}\right)\left[101\right] $$ , $$ \left(\overline{1}1\overline{1}\right)\left[\overline{1}01\right] $$ and $$ \left(\overline{1}\overline{1}1\right)\left[\overline{1}0\overline{1}\right] $$ slip systems by combining ECCI and GND techniques. Furthermore, the surface morphology reveals a combination of pile-up and sink-in patterns around the indent, as observed by 3D laser microscopy. According to GND analysis, pile-up is generated from the $$ \left(1\overline{1}\overline{1}\right)\left[\overline{1}\overline{1}0\right] $$ , $$ (111)\left[1\overline{1}0\right] $$ , $$ \left(1\overline{1}\overline{1}\right)\left[101\right] $$ and $$ \left(\overline{1}\overline{1}1\right)\left[101\right] $$ slip systems, and sink-in is caused by the $$ \left(\overline{1}1\overline{1}\right)\left[0\overline{1}\overline{1}\right] $$ , $$ \left(1\overline{1}\overline{1}\right)\left[\overline{1}0\overline{1}\right] $$ , and $$ \left(\overline{1}1\overline{1}\right)\left[10\overline{1}\right] $$ . Conclusions: The surface profile reveals a combination of pile-up and sink-in patterns resulting in the activated dislocations, where the deformation around the indent is dominated by screw-type dislocations.

The combined and interactive effects of orientation, strain amplitude, cycle number, stacking fault energy and hydrogen doping on microstructure evolution of polycrystalline high-manganese steels under low-cycle fatigue

We studied the combined and interactive effects of crystallographic orientation, strain amplitude, cycle number, stacking fault energy (SFE) and hydrogen doping on the microstructure evolution of polycrystalline high-manganese steels (HMnSs) under low-cycle fatigue (LCF). An integrated experimental approach combining digital image correlation (DIC), electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) at interrupted cycles was performed in the same region of interest on the bulk shear samples, which enables us to systematically compare the dislocation patterns of grains with defined loading conditions at a much larger field of view and less artefacts compared to transmission electron microscopy (TEM). We found that Taylor factor (M) works well with describing the effect of crystallographic orientation, which was further proved by the crystal plasticity finite element method (CPFEM). In detail, grains with a medium M value (3.3–3.9) tend to form more complex dislocation pattern, which is defined as sensitive value. Generally, increasing strain amplitude and cycle number both promote the evolution of dislocation pattern while their efficiencies depend strongly on grain orientation. The promotion effect of SFE on dislocation evolution becomes obvious at larger strain amplitudes (>0.4%) and non-sensitive M value. Hydrogen can strongly assist the formation of ε-martensite and reduce its critical resolved shear stress (CRSS), while it retards the evolution of dislocation pattern. The number of activated martensite variants in individual grain can be well predicted by its M value. With increasing strain amplitude, the fraction of ε-martensite increases in a manner of thinner but denser plates.

Advances in electron channelling contrast imaging and electron backscatter diffraction for imaging and analysis of structural defects in the scanning electron microscope

In this article we describe the scanning electron microscopy (SEM) techniques of electron channelling contrast imaging and electron backscatter diffraction. These techniques provide information on crystal structure, crystal misorientation, grain boundaries, strain and structural defects on length scales from tens of nanometres to tens of micrometres. Here we report on the imaging and analysis of dislocations and sub-grains in nitride semiconductor thin films (GaN and AlN) and tungsten carbide-cobalt (WC-Co) hard metals. Our aim is to illustrate the capability of these techniques for investigating structural defects in the SEM and the benefits of combining these diffraction-based imaging techniques.

Mechanism for embedded in-plane self-assembled nanowire formation

We report a novel growth mechanism that produces in-plane [1-10] oriented ErSb nanowires formed during codeposition of Er0.3Ga0.7Sb via molecular beam epitaxy (MBE). Nanowires are characterized by in-situ scanning tunneling microscopy (STM), as well as ex-situ transmission electron microscopy (TEM) and electron channeling contrast imaging (ECCI). We show that complexes of macrosteps with step heights on the order of 7 nm form during nanowire growth. The macrosteps are shown to be part of the in-plane nanowire growth process and are directly responsible for the observed stratified distribution of in-plane nanowires. TEM indicates that initial growth results in out-of-plane nanowires transitioning to in-plane nanowires after a critical film thickness. A surface energy model is put forward that shows the critical thickness is due to minimization of the GaSb{110} surfaces formed during out-of-plane nanowire growth. Kinetics of the transition are discussed with respect to observed features in STM, along with the material parameters needed to achieve in-plane nanowire growth.

Direct observations of collinear dislocation interaction in a Fe-17.4 Mn-1.50 Al-0.29 C (wt.%) austenitic steel under cyclic loading by in-situ electron channelling contrast imaging and cross-correlation electron backscatter diffraction

In this study we present an integrated experimental approach combing in-situ electron channelling contrast imaging under controlled diffraction conditions (cECCI) and cross-correlation based electron backscatter diffraction (CC-EBSD) to explore the dislocation pattern evolution in a high-Mn austenitic steel under low-cycle fatigue. We experimentally observed that the interactions between dislocations of same Burgers vectors gliding on intersecting slip systems led to the annihilation of the screw parts and the formation of edge dislocation dipoles. This collinear dislocation interaction facilitated the construction of edge dislocation walls in the studied material deforming in planar slip mode.

Effects of Defect Development During Displacive Austenite Reversion on Strain Hardening and Formability

Martensite that is mechanically induced from metastable austenite can be reversed to austenite upon annealing. The reversion transformation can be either diffusive or displacive, and the defect substructure development, in either case, has mechanical consequences. Here, to better understand the effects of microstructure development during displacive phase transformations, we focus on the influence of the initial plastic deformation on the austenite reversion (α′ → γ) in a transformation-induced plasticity-maraging steel. The phase transformation kinetics and the developing defect structure within the reversed γ phase are characterized by carrying out differential scanning calorimetry measurements, electron-backscattered diffraction, and electron channeling contrast imaging analyses. The resulting mechanical behavior is investigated by uniaxial and biaxial tension experiments. These investigations demonstrate that the defect development during sequential deformation-annealing treatments can help increase the overall strain hardening capacity of the alloy, which in turn increases the accumulative uniform elongation, and the formability. While the necking can be progressively delayed to higher strain levels following such treatments, the local fracture strain apparently cannot be, due to damage accumulation.

Strengthening the FeCoCrNiMo0.15 high entropy alloy by a gradient structure

In present work, a gradient structured FeCoCrNiMo0.15 high entropy alloy (HEA) is processed by shot peening treatment. Microstructures of the gradient structured HEA are characterized by electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). Complex microstructures consisting of nanograins, nanotwins, with high densities of dislocations are formed on the surface of the shot peening treated HEA. With the increase of depth from surface to center, a planar dislocation slip occurs while no deformation twin is observed. Un-deformed grains were observed in the regions of ∼100 μm–150 μm depth from the surface. Such gradient structured HEA shows an increase of yield strength from 418 MPa to 486 MPa when compared to the one with homogeneous structure, with a slight decrease in tensile elongation from 50.9% to 46.8%. During tensile deformation, strong deformation twinning including the formation of hierarchical nanotwins occurs in the gradient structured HEA. These unique deformation microstructures are associated with high strain hardening and show increased ductility, which creates excellent combination of strength and tensile elongation. Our work offers valuable insight into achieving high-performance HEAs.

Nanomechanical Behaviour of Individual Phases in WC-Co Cemented Carbides, from Ambient to High Temperature.

The dependence of the mechanical behaviour of individual phases in WC-Co on microstructural parameters such as grain size and orientation were investigated by means of nanoindentation and electron microscopy. A broad range of WC grain dimensions, from about 1 to 1000 µm2, were selected and subsequently indented to investigate any size effect. A decrease in hardness as a function of grain dimensions was observed, due to an increase in dislocation mobility in larger grains. Whilst the binder phase only exhibits a hardness of about 11 GPa, the hardness of WC grains was measured about 29 and 53 GPa for the prismatic and basal orientations, respectively, in ambient conditions. All WC orientations exhibited a similar decrease in hardness with temperature, up to 700 ˚C. Damage mechanisms occurring in WC-Co during nanoindentation were investigated for the different grain orientations at various temperatures. The damage was visualised using electron microscopy near the residual indent coupled with Focused Ion Beam (FIB) sectioning across the indent. The three-dimensional distribution of plastic deformation across WC grains in the vicinity of an indent was examined using Electron Channelling Contrast Imaging (ECCI). ECCI micrographs enabled the observation of crystal defects, especially dislocations, and slip lines as well as the entire plastic zone. The defect density and spatial distribution in the deformed WC grains were compared to that of untested WC grains to identify the type of deformation originating from spherical indentation. The work provides important information on the relationship between WC-Co microstructure and performance at operating temperatures.

The effect of solid solution and gamma prime on the deformation modes in Ni-based superalloys

Understanding the deformation, strengthening and failure mechanisms in polycrystalline nickel-base superalloys is necessary to develop next generation alloys for application in highly demanding environments. Here, the aim is to examine the various ways in which solution- and γ’ precipitation-strengthening affect the deformation behaviour of three Ni-based superalloys through deformation mapping and investigation at multiple length-scales. This is achieved using high-resolution digital image correlation to quantify local strain, electron backscattered diffraction for lattice rotations and electron channelling contrast imaging to investigate dislocation-mediated mechanisms of deformation. This approach bridges the gap between nano-scale microscopy of dislocation-γ’ interactions and macro-scale measurements of mechanical properties, such as yield stress, flow stress and the strain-hardening rate. Deformation in solution-strengthened alloys progresses by a slip band refinement mechanism, which results in low levels of dislocation pileup at grain boundaries and so better grain deformation compatibility with neighbours. Deformation in the γ’-strengthened alloys evolves through a glide plane softening mechanism and the resulting high strain localisation impinges on grain boundaries, creating diffuse strain regions at the boundary. In the coarse-γ’ variant there is more Orowan looping and cross slip around larger precipitates, more slip planes are active and more grain-scale cross slip takes place, resulting in greater local friction stresses and therefore greater macroscopic flow stresses and strain-hardening rates. We provide evidence of greater interaction between intersecting non co-planar slip bands in the γ’-strengthened alloys, which contributes to the strain-hardening by progressively decreasing the slip distance. This mechanism is not observed in the solution-strengthened alloy.

Grain refinement process in a cold-rolled polycrystalline cobalt

The microstructural evolution of a cold-rolled polycrystalline cobalt (Co) was investigated via electron backscatter diffraction, electron channeling contrast imaging and transmission electron microscope. High fraction of pre-existed phase transformation related boundaries (PBs) with misorientation of ~71°/<112¯0>, together with deformation induced twinning and phase transformation, refine the grain size from 20 μm in the un-deformed Co to nano scale before fracture. The grain refinement arises from the interplays among PBs, twins, and face centered cubic (FCC) plates, which is fundamentally different from grain refinement mechanisms reported in other hexagonal close-packed (HCP) materials.