Electron Channeling Contrast Research Articles

Fast identification of dislocations in semiconductor materials by electron channeling contrast imaging using a scanning electron microscope

Fast identification of dislocations in semiconductor materials by electron channeling contrast imaging using a scanning electron microscope

Strain hardening by dynamic slip band refinement in a high-Mn lightweight steel

The strain hardening mechanism of a high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C (wt%)) is investigated by electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). The alloy is characterized by a constant high strain hardening rate accompanied by high strength and high ductility (ultimate tensile strength: 900 MPa, elongation to fracture: 68%). Deformation microstructures at different strain levels are studied in order to reveal and quantify the governing structural parameters at micro- and nanometer scales. As the material deforms mainly by planar dislocation slip causing the formation of slip bands, we quantitatively study the evolution of the slip band spacing during straining. The flow stress is calculated from the slip band spacing on the basis of the passing stress. The good agreement between the calculated values and the tensile test data shows dynamic slip band refinement as the main strain hardening mechanism, enabling the excellent mechanical properties. This novel strain hardening mechanism is based on the passing stress acting between co-planar slip bands in contrast to earlier attempts to explain the strain hardening in high-Mn lightweight steels that are based on grain subdivision by microbands. We discuss in detail the formation of the finely distributed slip bands and the gradual reduction of the spacing between them, leading to constantly high strain hardening. TEM investigations of the precipitation state in the as-quenched state show finely dispersed atomically ordered clusters (size < 2 nm). The influence of these zones on planar slip is discussed.

Microstructural analysis in the Fe-30.5Mn-8.0Al-1.2C and Fe-30.5Mn-2.1Al-1.2C steels upon cold rolling

Electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were used to examine microstructural changes of the austenitic low-density Fe-30.5Mn-8.0Al-1.2C (8Al) and Fe-30.5Mn-2.1Al-1.2C (2Al) (wt.%) steels during cold rolling. As the strain increased, deformation mechanisms, such as stacking faults, slip, mechanical twinning, and shear banding were activated in both steels cold rolled up to strain of 0.69. Only slip was noted in these steels at low strain (e=0.11) and slip dominance was detected in the 8Al steel at higher strains. Shear banding became active at higher strain (e~0.7) in these materials. An inhomogeneous microstructure formed in both alloys at such strain level. More extensive mechanical twinning in the 2Al alloy than that in the 8Al alloy was observed. Fish bone-like structure patterns were revealed in the 8Al steel and a river-like structure in the 2Al steel. Detailed microstructure features as elongated and fragmented grains along the rolling direction (RD) were found for both steels, as already observed in other high-Mn steels. These deformed structures are composed by lamellar packets which can contain mechanical twins or slip lines and shear bands.

Low cycle fatigue in aluminum single and bi-crystals: On the influence of crystal orientation

Aluminum single crystals with three different double-slip orientations ([2̅32], [3̅3̅2], [130]) and two aluminum bi-crystals – one with a high-angle grain boundary and one with a low-angle grain boundary - were cyclically deformed up to 100 cycles under constant displacement control. The distribution of the local strain and the local strain amplitudes was captured by in-situ digital image correlation (DIC). Dislocation structure analysis was performed by electron channeling contrast imaging (ECCI) and the evolution of local misorientations was recorded by high resolution electron backscatter diffraction (EBSD). The DIC results show a homogeneous strain amplitude distribution in the single crystals while the measured strain amplitude in the low-angle grain boundary bi-crystal sample differs significantly. ECCI observations reveal the presence of dislocation cells elongated along the trace of the primary {111} slip plane in all investigated crystals and the formation of deformation bands parallel to the trace of {110} planes. Deformation bands (DB) were observed in all samples but their frequency and misorientation with respect to the matrix was found to sensitively depend on the crystal orientation and the local strain amplitude. Our results on the bi-crystals show that the grain orientation mainly determines the local stresses and therefore also the formation of the associated dislocation structures rather than the grain boundary character.

On Rotation Contour Contrast in Hot-Compressed Magnesium Alloys in a Scanning Electron Microscope

Two types of electron channeling contrast were observed in backscattered electron micrographs of deformed Mg–0.3Al–0.2Ca (wt%) alloy after the uniaxial hot-compression tests in a scanning electron microscope (SEM). The effects of SEM operation conditions namely, stage tilt and electron beam energy on each type of contrast were investigated. The electron backscattered diffraction crystallographic orientation mapping was used to map the local crystal orientation across the grains. The observed contrast was described as rotation contour contrast (RCC) and found to have crystallographic origin. The appearance of RCC was attributed to the local rotation of the crystal during the deformation of the grain.

Investigation of microstructures of laser surface-treated Zr702 sheet using electron channeling contrast imaging and electron backscatter diffraction techniques

Abstract In this paper, specimens cut from a fully recrystallized Zr702 sheet were surface-treated by pulsed laser at two different powers (200 and 50 W) and their microstructures were investigated by electron channeling contrast imaging and electron backscatter diffraction techniques. For both the laser-treated specimens, three modified zones with distinctly different microstructural features are presented from the surface to the substrate: melting zone (MZ), solid state phase transformation zone (SSPTZ) and essentially unaffected base metal zone. The MZ is completely comprised of α plates while the SSPTZ could be further divided into the SSPTZ-1 and the SSPTZ-2, according to cooling mainly from β and α + β domains, respectively. With decreasing the laser power, the modification depth and the mean width of α plates decrease. Detailed characterization on microstructural features suggests that the plate structures (MZ and SSPTZ-1) in the 200 W and the 50 W specimens are produced by diffusion and diffusionless transformation, respectively. For those untransformed α grains in the SSPTZ-2 of both specimens, in spite of maintained bulk morphologies, slight subdivisions by subgrains are found inside them due to extensive dislocation activities induced by transformation stresses and/or their recovery.

Spectral TRIP enables ductile 1.1 GPa martensite

Introduction of interlath reverted austenite is an effective method to design ductile lath martensitic steels. The challenge in this concept is that all reverted austenite films have similar mechanical stability, hence, they all undergo transformation-induced plasticity (TRIP) at the same strain level. Here we propose a new thermo-mechanical treatment route to activate the TRIP effect over a broad strain regime and refer to it as ‘spectral TRIP effect’. It aims at spreading the micro-mechanical stability of reverted austenite grains by widening the austenite nucleation barrier in martensite. To validate the proposed thermo-mechanical treatment route, an as-quenched medium-Mn martensitic steel was cold rolled prior to the reversion treatment at 600 °C. Microstructure characterization was carried out by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). Mechanical tests show that the approach is effective. The spectral TRIP effect improves both, the strength and the ductility due to the well dispersed size distribution and the associated size-dependent deformation and phase transformation behavior of the reverted austenite grains, extending TRIP-related work hardening over a broad strain range.

A comparison of EBSD based strain indicators for the study of Fe-3Si steel subjected to cyclic loading

The current work aims at proposing an EBSD-based indicator for fatigue damage of a Fe-3Si steel. At the same time direct observation of dislocation structures is provided by electron channelling contrast imaging (ECCI). The investigation consisted in processing the EBSD data from patterns collected on specimen subjected to low cycle fatigue. It revealed two different regimes depending on the applied total strain variation which is explained by the identification of the dislocations structures and their evolution. At low strain variation, strain accommodation occurs by planar glide of dislocations uniformly distributed throughout the grains. No misorientation evolution is observed. At higher strain variation, the vein-channel structure is observed within the grain and the wall-channel structure in the vicinity of grain boundaries. The misorientation between these two dislocation structures is evaluated at about 0.7° which is detected by the EBSD analyses and explains the increase of the different misorientation based criteria. The EBSD study enables also the prediction of crack initiation mode. Finally, this study points out the limits of the EBSD technique as no misorientation evolution is detected at small strain variation. Indeed, the lattice distortion is too weak to be detected by conventional EBSD.

Hydrogen Embrittlement Susceptibility of Fe-Mn Binary Alloys with High Mn Content: Effects of Stable and Metastable ε-Martensite, and Mn Concentration

To obtain a basic understanding of hydrogen embrittlement associated with e-martensite, we investigated the tensile behavior of binary Fe-Mn alloys with high Mn content under cathodic hydrogen charging. We used Fe-20Mn, Fe-28Mn, Fe-32Mn, and Fe-40Mn alloys. The correlation between the microstructure and crack morphology was clarified through electron backscatter diffraction measurements and electron channeling contrast imaging. e-martensite in the Fe-20Mn alloy critically deteriorated the resistance to hydrogen embrittlement owing to transformation to α′-martensite. However, when e-martensite is stable, hydrogen embrittlement susceptibility became low, particularly in the Fe-32Mn alloys, even though the formation of e-martensite plates assisted boundary cracking. The Fe-40Mn alloys, in which no martensite forms even after fracture, showed higher hydrogen embrittlement susceptibility compared to the Fe-32Mn alloy. Namely, in Fe-Mn binary alloys, the Mn content has an optimal value for hydrogen embrittlement susceptibility because of the following two reasons: (1) The formation of stable e-martensite seems to have a positive effect in suppressing hydrogen-enhanced localized plasticity, but causes boundary cracking, and (2) an increase in Mn content stabilizes austenite, suppressing martensite-related cracking, but probably decreases the cohesive energy of grain boundaries, causing intergranular cracking. As a consequence, the optimal Mn content was 32 wt pct in the present alloys.

On the origin of creep dislocations in a Ni-base, single-crystal superalloy: an ECCI, EBSD, and dislocation dynamics-based study

This work investigates the origin of creep dislocations in a Ni-base, single crystal superalloy subject to creep at an intermediate stress and temperature. Employing high angular resolution electron backscatter diffraction (HR-EBSD), electron channeling contrast imaging under controlled diffraction conditions (cECCI) and discrete dislocation dynamics (DDD) modelling, it is shown that low-angle boundaries—which correspond to dendrite boundaries or their residues after annealing—are not the major sources of creep dislocations. At the onset of creep deformation, they are the only active sources. Creep dislocations are emitted from them and percolate into the dislocation-depleted crystal. However, the percolation is very slow. As creep deformation proceeds, before the boundary-originated dislocations move further than a few micrometers away from their source boundary, individual dislocations that are spread throughout the undeformed microstructure become active and emit avalanches of creep dislocations in boundary-free regions, i.e. regions farther than a few micrometer away from boundaries. Upon their activation, the density of creep dislocations in boundary-free regions soars by two orders of magnitude; and the entire microstructure becomes deluged with creep dislocations. ​The total area of boundary-free regions is several times the total area of regions covered by boundary-originated creep dislocations. Therefore, the main sources of creep dislocations are not low-angle boundaries but individual, isolated dislocations in boundary-free regions.

Evolution of microstructure and grain boundary character distribution of a tin bronze annealed at different temperatures

Specimens cut from a rolled tin bronze sheet were annealed at 400–800°C for 1h and evolution of their microstructures was then characterized in details by electron channeling contrast imaging and electron backscatter diffraction techniques. Particularly, statistics on special boundaries (SBs) with Σ≤29 and network connectivity of random high angle boundaries (HABs) in the annealed specimens were examined to probe optimization potentials of grain boundary character distribution (GBCD) for this material. Results show that the deformed microstructure in the as-received material begins to be recrystallized when the annealing temperature increase to 500°C and average grain sizes surge with further increasing temperatures. As a result of the recrystallization, a large number of annealing twins (with Σ3 misorientation) are produced, leading to remarkably increased fractions of SBs (fSBs). Thanks to preexisting dense low angle boundaries, the majority of SBs in the 500°C specimen with only partial recrystallization are Σ3ic (incoherent) boundaries, which effectively disrupt connectivity of random HABs network. Although the fSBs can be further increased (up to 72.5%) in specimens with full recrystallization (at higher temperatures), the Σ3ic boundaries would be replaced to some extent by Σ3c (coherent) boundaries which do not contribute directly to optimizing the GBCD. This work should be able to provide clear suggestions on applying the concept of grain boundary engineering to tin bronze alloys.

Texture evolution in the Fe-30.5Mn-8.0Al-1.2C and Fe-30.5Mn-2.1Al-1.2C steels upon cold rolling

Crystallographic textures of the austenitic low-density Fe-30.5Mn-8.0Al-1.2C (8Al) and Fe-30.5Mn-2.1Al-1.2C (2Al) (wt.%) steels were examined during cold rolling by means of electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). Random oriented grains orient towards Goss- and brass-components along the α-fiber as the strain increased, with activation of slip, mechanical twinning, and shear banding, for both steels. S- and copper-orientations were also observed in the 8Al steel at 50% reduction. The route of Cu-CuT-Goss-brass texture evolution was found in the 2Al alloy. Cu, Goss, and brass textures occur as a dominant texture in the deformed 8Al alloy. Copper-type texture accompanied with slip at low reduction (20%), as well as Cu-type shear bands, and shear banding inside Goss-oriented grains at higher reduction (50%), were observed in the 8Al steel. It is suggested that this copper-type rolling texture may be attributed to the Al addition, which contributes to its low twinning activity compared to that in the 2Al alloy. Cu-CuT-brass f.c.c. rolling texture transition to form the Brass-type texture was observed at higher reduction in the 2Al alloy with strong similarity to that found in other Fe-Mn-C system TWIP steels.

Electron channelling contrast imaging for III-nitride thin film structures

Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a rapid and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in crystalline materials. In this review, we will demonstrate the application of ECCI to the characterisation of III-nitride semiconductor thin films grown on different substrates and with different crystal orientations. We will briefly describe the history and the theory behind electron channelling and the experimental setup and conditions required to perform ECCI. We will discuss the advantages of using ECCI, especially in combination with other SEM based techniques, such as cathodoluminescence imaging. The challenges in using ECCI are also briefly discussed.

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.

Electron backscatter diffraction investigation of duplex-phase microstructure in a forged Zr-2.5Nb alloy

Microstructural features of a duplex-phase Zr-2.5Nb alloy were investigated in detail using electron channeling contrast (ECC) imaging and electron backscatter diffraction (EBSD) technique in an emission gun scanning electron microscope (FEGSEM). The excellent resolution provided by the FEGSEM promises the combined utilization of both techniques to be quite adequate for characterizing the duplex-phase microstructures. Results show that the microstructure of the Zr-2.5Nb alloy is composed of bulk a grains (majority) in equiaxed or plate shape and thin β films (minority) surrounding the bulk grains, with their average grain size and thickness measured to be 1.4 µm and 72 nm, respectively. Analyses on α-grain boundaries reveal a number of low angle boundaries, most of which belong to deformation-induced dislocation boundaries. Measurements on relative proportions of various Burgers boundaries suggest very weak (if any) variant selection during β → α cooling, which should be related to deformation-induced higher nucleation rate of α phases. Compared to earlier attempts, more satisfactory indexing of fine β phases (down to nanoscale) is attained by the FEGSEM-based EBSD. Examples are presented to clearly reveal well-obeyed Burgers orientation relationships between adjacent a and β phases. Finally, it is deduced that continuing application of the FEGSEM-based EBSD to duplex-phase Zr alloys could help clarify controversies like the deformation priority of the two phases.

Hydrogen-assisted damage in austenite/martensite dual-phase steel

For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1 1 0}M localized slip and {1 1 2}M twin and (2) cracking of martensite–martensite grain interfaces. The former resulted in nanovoids along the {1 1 2}M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking.

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.

Effect of Reduced Extended Defect Density in MOCVD Grown AlGaN/GaN HEMTs on Native GaN Substrates

AlGaN/GaN high-electron mobility transistor (HEMT) structures were grown by metal–organic chemical vapor deposition on SiC, hydride vapor phase epitaxy (HVPE) GaN, and ammonothermal GaN substrates to achieve HEMTs with over five orders of magnitude variation in extended defect density. This enables a direct comparison of the effect of extended defects on device performance to achieve the best possible reliability. As-grown material was characterized by atomic force microscopy, electron channeling contrast imaging, and Raman spectroscopy. Devices were characterized by Hall, dc $I$ – $V$ , and pulsed $I$ – $V$ behavior. Reduced threading dislocation density provides an increased 2-D electron gas mobility, but inhibits ohmic contact formation resulting in high contact resistance. Transistor characteristics were nominally identical, with higher OFF-state leakage in the HEMTs on ammonothermal GaN. The pulsed $I$ – $V$ response indicated significantly reduced current collapse in the HEMT on HVPE GaN due to reduced buffer trapping.

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.

Microscopic and nanoscopic study on subsurface damage and fatigue crack initiation during very high cycle fatigue

“Fish eye” is a typical phenomenon of fatigue crack initiation at a subsurface defect such as inclusion during very high cycle fatigue. The formation of a fine grained area and micro-debonding is believed to cause fatigue crack initiation. This paper provides a basic study on the formation of the fine grained area in a martensitic stainless steel during very high cycle fatigue using scanning electron microscopy, SEM, focused ion beam technique, FIB, electron backscatter diffraction, EBSD, and electron channeling contrast imaging, ECCI. The results show that the formation of a fine grained zone is a local behavior. The fine grained zone is very near the fatigue crack initiation origin. In the transversal direction (cross section), the depth of the fine grained zone is only few micrometers. In the longitudinal direction (crack propagation direction), the depth of the fine grain zone is about one micrometer. ECCI analysis shows that in the fine grained area with high retained strain, high plastic deformation can be found. Dislocation slip bands can be observed. They interact with grain boundaries and cause the formation of damage due to impingement cracking. The results indicate that occurrence of plastic deformation in metallic material during very high cycle fatigue is very localized, mainly near the front of the crack tip or a defect.