Electron Channeling Contrast Research Articles

High temperature friction and wear behavior of cold-sprayed Ti6Al4V and Ti6Al4V-TiC composite coatings

The poor wear resistance of titanium and its alloys often results in galling and high wear rates. One method to improve their wear resistance is by hard secondary phase reinforcement to create a metal matrix composite, which can be utilized in the bulk or as a coating. In the present study, Ti6Al4V coatings and Ti6Al4V-TiC metal matrix composite coatings were deposited on mild steel substrates using cold spray process and their dry sliding wear behavior was studied over a static temperature range of 25–575 °C. Tests were performed using a unidirectional ball-on-disc tribometer with WC-Co sphere as the counterface at a load of 2.5 N and sliding velocity of 2.1 cm s−1. Wear rate and coefficient of friction (CoF) decreased with an increase in temperature and composite coatings exhibited higher wear resistance at all temperatures. Below 200 °C, abrasive wear characterized by ploughing by wear debris resulted in high wear of Ti6Al4V coatings, whereas formation of tribolayers led to lower wear of composite coatings. At elevated temperatures (> 200 °C), oxide glaze layers formed on both coatings were composed of WO3, TiO2 and CoWO4. Electron channel contrast imaging of the wear track cross-sections showed no splat debonding, less extent of recrystallization and larger grain sizes (at 575 °C) in composite coatings compared to Ti6Al4V.

The influence of powder properties on the adhesion strength and microstructural evolution of cold sprayed Ti6Al4V single splats

The adhesion strength and microstructure of single splats significantly influence the properties of cold sprayed coatings. Here, we compare the adhesion strength and microstructural changes of Ti6Al4V spherical powder (SP) particles with a martensitic microstructure and irregular powder (IP) particles with an equiaxed microstructure deposited by cold spray (CS). Splat adhesion tests were performed to determine the adhesion strength and electron channelling contrast imaging was done for microstructural analysis of splat cross-sections. IP splats formed a continuous bonded interface with the substrate resulting in a greater adhesion strength when compared to SP splats. IP splat cross-sections revealed ultrafine grains (UFG) near the interface followed by a highly deformed microstructure. SP splat cross-sections also showed UFG at the interface but largely retained the initial microstructure in the top portion of the splat due to poor deformability. The irregular morphology of the IP led to more adherent deposits while the equiaxed microstructure resulted in highly deformed post spray microstructures.

Fast Diffusion and Segregation along Threading Dislocations in Semiconductor Heterostructures

Heterogeneous integration of semiconductors combines the functionality of different materials,enabling technologies such as III-V lasers and solar cells on silicon and GaN LEDs on sapphire. However, threading dislocations generated during theepitaxy of these dissimilar materialsremain a key obstacle to the success of this approach due to reduced device efficiencies and reliability. Strategies to alleviate this and understand charge carrier recombination at threading dislocations now needan accurate description of the structure of threading dislocationsin semiconductor heterostructures. We show that the composition around threading dislocations intechnologically important InGaAs/GaAs/Ge/Si heterostructures areindeed different from that of thematrix. Site-specific atom probe tomography enabled by electron channeling contrast imaging reveals this at individual dislocations.We present evidence for the simultaneousfast diffusion of germanium and indium up and down adislocation, respectively, leading to unique compositional profiles. We also detect the formation of clusters of metastable composition at the interface between Ge and GaAs, driven by intermixing in these two nearly immiscible materials.Together,our results have important implications for the properties of dislocations and interfaces in semiconductors andprovide new tools for their study.

Low angle boundary migration of shot-peened pure nickel investigated by electron channeling contrast imaging and electron backscatter diffraction.

Study on recrystallization of deformed metal is important for practical industrial applications. Most of studies about recrystallization behavior focused on the migration of the high-angle grain boundaries, resulting in lack of information of the kinetics of the low angle grain boundary migration. In this study, we focused on the migration of the low angle grain boundaries during recrystallization process. Pure nickel deformed by shot peening which induced plastic deformation at the surface was investigated. The surface of the specimen was prepared by mechanical polishing using diamond slurry and colloidal silica down to 0.02 μm. Sequential heat treatment under a moderate annealing temperature facilitates to observe the migration of low angle grain boundaries. The threshold energy for low angle boundary migration during recrystallization as a function of misorientation angle was evaluated using scanning electron microscopy techniques. A combination of electron channeling contrast imaging and electron backscatter diffraction was used to measure the average dislocation density and a quantitative estimation of the stored energy near the boundary. It was observed that the migration of the low angle grain boundaries during recrystallization was strongly affected by both the stored energy of the deformed matrix and the misorientation angle of the boundary. Through the combination of electron channeling contrast imaging and electron backscatter diffraction, the threshold stored energy for the migration of the low angle grain boundaries was estimated as a function of the boundary misorientation.

Hydrogen-enhanced fatigue crack growth behaviors in a ferritic Fe-3wt%Si steel studied by fractography and dislocation structure analysis

The effect of hydrogen (H) on the fatigue behavior is of significant importance for metallic structures. In this study, the hydrogen-enhanced fatigue crack growth rate (FCGR) tests on in-situ electrochemically H-charged ferritic Fe-3wt%Si steel with coarse grain size were conducted. Results showed strong difference between the H-charged and the non-charged conditions (reference test in laboratory air) and were in good agreement with the results from literature. With H-charging, the fracture morphology changed from transgranular (TG) type to “quasi-cleavage” (“QC”), with a different fraction depending on the loading frequency. With the help of electron channeling contrast imaging (ECCI) inside a scanning electron microscope (SEM), a relatively large area in the failed bulk specimen could be easily observed with high-resolution down to dislocation level. In this work, the dislocation sub-structure immediately under the fracture surfaces were investigated by ECCI to depict the difference in the plasticity evolution during fatigue crack growth (FCG). Based on the analysis, the H-enhanced FCG mechanisms were discussed.

Temperature dependence of strengthening mechanisms in a twinning-induced plasticity steel

The temperature dependence of strengthening mechanisms in twinning-induced plasticity (TWIP) steels is of fundamental importance because of the broad range of environmental temperatures during manufacturing and applications. In this study, the temperature dependence of dislocation, twin and dynamic strain aging strengthenings, was investigated over a wide range of temperatures ranging from 298 to 973 K. Uniaxial tensile tests and microstructural characterization via electron backscatter diffraction and electron channelling contrast imaging were carried out. In addition, synchrotron X-ray diffraction was used to measure the dislocation density at various temperatures. Based on the experimental observations, the strengthening mechanisms at various temperatures are elucidated with the quantification of the respective strengthening contributions from dislocations, twins and dynamic strain aging. These experimental results shed light on the temperature-dependent plasticity of TWIP steels and provide guidance for developing temperature-sensitive constitutive models.

Effects of laser surface alloying with Cr on microstructure and hardness of commercial purity Zr

Pulsed laser was utilized to treat a commercial purity Zr (CP Zr) sheet coated with Cr by brush plating to achieve Cr alloying on its surface. Microstructural characteristics and hardness of the laser-treated surface were investigated by use of electron channeling contrast imaging, electron backscatter diffraction, energy dispersive spectrometry techniques and microhardness measurement. Results show that after the laser surface alloying (LSA) with Cr, five zones with distinctly different microstructural characteristics are presented from the surface to the matrix: two melted zones (MZs), two solid-state phase transformation zones (SSPTZs) and the unchanged matrix. The MZ-1 and the MZ-2 are featured by equiaxed and columnar structures with internal substructures inside them, respectively. The SSPTZ-1 is fully comprised of martensitic plates transformed from complete β domains while the SSPTZ-2 contains both martensitic plates and bulk α grains due to cooling from α+β domains. The Cr alloying mainly occurs in the two melted zones (MZ-1 and MZ-2) while essentially not in the SSPTZs. Compared with the initial microstructure, many low angle boundaries appear in untransformed bulk α grains in the SSPTZ-2, which is related to thermal stresses induced by the laser surface treatment. Hardness measurements show that the LSA with Cr can effectively increase the surface hardness of the CP Zr, with the maximum value (468 HV) to be ∼2.4 times that of the matrix. Such hardening could be mainly attributed to solid-solution strengthening of Cr in α-Zr and significant grain refinement.

Nanoscale Conditions for Ductile Void Nucleation in Copper: Vacancy Condensation and the Growth-Limited Microstructural State

Ductile rupture or tearing usually involves structural degradation from the nucleation and growth of voids and their coalescence into cracks. Although some materials contain preexisting pores, the first step in failure is often the formation of voids. Because this step can govern both the failure strain and the fracture mechanism, it is critical to understand the mechanisms of void nucleation and the enabling microstructural configurations which give rise to nucleation. To understand the role of dislocations during void nucleation, the present study presents ex-situ cross-sectional observations of interrupted deformation experiments revealing incipient, subsurface voids in a copper material containing copper oxide inclusions. The local microstructural state was evaluated using electron backscatter diffraction (EBSD), electron channeling contrast (ECC), transmission electron microscopy (TEM), and transmission kikuchi diffraction (TKD). Surprisingly, before substantial growth and coalescence had occurred, the deformation process had resulted in the nucleation of a high density of nanoscale (≈50 μm) voids in the deeply deformed neck region where strains were on the order of 1.5. Such a proliferation of nucleation sites immediately suggests that the rupture process is limited by void growth, not nucleation. With regard to void growth, analysis of more than 20 microscale voids suggests that dislocation boundaries facilitate the growth process. The present observations call into question prior assumptions on the role of dislocation pile-ups and provide new context for the formulation of revised ductile rupture models. While the focus of this study is on damage accumulation in a highly ductile metal containing small, well-dispersed particles, these results are also applicable to understanding void nucleation in engineering alloys.

Effect of additive manufacturing on fatigue crack propagation of a gas turbine superalloy

Additive manufacturing (AM) offers new possibilities in gas turbine technology by, for example, allowing for more complex internal cooling channels. One such application, where AM can improve the function by new designs, is burners. However, the fatigue performance, especially the fatigue crack propagation of AM gas turbine material, is not fully known. In the present study, an AM adopted nickel-based superalloy Hastelloy X was subjected to low-cycle fatigue (LCF) loading at room temperature. The LCF tests were conducted in strain control on additive manufactured smooth bars with two different build orientations (with an angle of 0° and 90° relative to the building platform). During the the manufacturing process, an AM component often solidifies with a dendritic structure. Initial fractography of the ruptured LCF specimens revealed that the dendritic structure was visible on the fracture surface. It was noted that the dendritic structure could easily be mistaken for regular striations although they represent a different fracture mechanism. The fracture surfaces were therefore cross sectioned and possible correlations between fracture surface characteristics and underlying microstructure were studied using electron backscatter diffraction and electron channelling contrast imaging. The outcome was used to discuss the effect of AM microstructure on the LCF crack propagation.

TWIP and Fracture Behavior in the Superalloy 625 at Room and Cryogenic Temperatures

In this study an austenitic nickel base alloy (Alloy 625) with medium to high stacking fault energy was tensile tested at room temperature and at cryogenic temperature, -196°C, to investigate if twinning was involved in the fracture process. Fracture surfaces and microstructure were characterized using scanning electron microscopy and texture was evaluated with scanning electron microscopy-electron backscatter diffraction at three positions along the tensile test specimens. Features where investigated with electron channelling contrast imaging. At room temperature the predominant deformation mechanism is slip and at cryogenic temperature slip is followed by deformation twinning. At room temperature deformation twinning will make an extra deformation contribution in the localized necking region compared to the cryogenic temperature case where deformation twins already are distributed in the microstructure and in this case will hinder the dislocations to move freely and instead cause stress field build up at the twin boundaries and subsequent microcrack and void formation.

Plasticity in cryogenic brittle fracture of ferritic steels: Dislocation versus twinning

The sub-surface deformation structure after cryogenic (77 K) brittle fracture in a ferritic steel was characterized by scanning electron microscopy (SEM). Twin-like structures were found in many grains below the fracture surface. Electron backscatter diffraction (EBSD) was used to identify the crystallography of the structures, and the twin relation in body-centered cubic (BCC) systems was indexed. The twins in the sub-surface area were characterized further by electron channeling contrast imaging (ECCI). This study clarifies the low temperature brittle fracture behavior in the ferritic Fe-3 wt%Si steel consists of both cleavage and plastic deformation in the form of dislocation activities and twinning.

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.

ECCI Characterization of Dislocation Structures at a Non-propagating Fatigue Crack Tip: Toward Understanding the Effects of Mn-C and Cr-N Couples on Crack Growth Resistance

We attempted to clarify the underlying mechanisms of the enhanced small-fatigue-crack resistance of Fe-Mn-C twinning-induced plasticity (TWIP) steel and high-nitrogen austenitic steel. To this end, we performed electron channeling contrast imaging near the tips of non-propagating fatigue cracks in Fe-18Cr-14Ni steel without a significant amount of interstitials, Fe-23Mn-0.5C TWIP steel, and Fe-25Cr-1N austenitic steel. The fatigue crack non-propagation limits of the TWIP steel and high-nitrogen steel were higher than that of the steel without a significant amount of interstitials; the higher limits of the TWIP steel and high-nitrogen steel are attributed to the Mn-C and Cr-N interactions, respectively. The enhanced small-fatigue-crack resistance of the Fe-23Mn-0.5C steel is attributed to local hardening at the crack tip caused by an increase in the dislocation density via dynamic strain aging. The enhanced dislocation planarity of the Fe-25Cr-1N steel, which is a result of the Cr-N interaction, is a significant factor that influences (i.e., increases) the crack resistance. The enhanced dislocation planarity results in dislocation pile-up stress at the crack tip, thereby preventing dislocation emission from the crack tip.

Hydrogen-induced ductility loss of precipitation-strengthened Fe-Ni-Cr-based superalloy

A brittle-like faceted morphology of a precipitation-strengthened Fe-Ni-Cr-based superalloy after charging via exposure to high-pressure hydrogen gas (100 MPa) at elevated temperature (543 K) was interpreted based on multiple electron microscopy observations: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) imaging. The observation results revealed that the brittle-like facets were derived from intergranular cracking accompanied by hydrogen-assisted microvoid nucleation at the grain boundaries (GBs). Deformation twinning also played a crucial role in triggering the final grain boundary separation due to local stress concentration at its intersection with the GBs after severe strain hardening; such a process has not yet been considered to explain the hydrogen-induced ductility loss of this type of alloy.

Magnetic properties of a 17.6 Mn-TRIP steel: Study of strain-induced martensite formation, austenite reversion, and athermal α′-formation

Abstract Strain-induced martensite (SIM) formation was evaluated upon cold-rolling of a 17.6 wt.%Mn-TRIP steel by means of magnetic measurements, X-ray diffraction, and high-resolution electron backscatter diffraction (EBSD). α′-martensite formation was observed to be dependent on the presence of prior e-martensite. Upon deformation, the coercivity of the ferromagnetic α′-martensite is characterized by strong magnetic shape anisotropy. Austenite (γ) reversion was evaluated by means of in situ magnetic measurements during continuous annealing. The experimental results were compared to thermodynamic simulations. It turned out that γ-reversion was not completed in the regime where a γ-single phase field is expected, which suggests the splitting of α′ → γ transformation into two stages. The Curie temperature of remaining α′-martensite was determined as being ∼620 °C. Magnetic properties presented an annealing time-dependence within the temperature range of 500–600 °C, suggesting long-range diffusional α′ → γ transformation. With the aid of electron channeling contrast image technique (ECCI), we noticed that the formation of γ-nanograins in the early stages of reversion is sufficient to induce strong magnetic shape anisotropy in this steel. After full austenitization at 800 °C, further in situ magnetic measurements were also used to track the magnetic response of the material upon controlled cooling. Athermal formation of α′-martensite within the prior athermal e-phase was clearly observed for temperatures lower than 100 °C. Using thermodynamic modeling we also calculated the start temperature for e-formation (Mse). Results showed that e-martensite is indeed expected to form before α′, which was confirmed in all cases by means of EBSD.

Engineering Hierarchical Microstructures via Advanced Thermo-Mechanical Processing of a Modern HSLA Steel

Advanced thermo-mechanical processing of mild steels in the ferrite phase field has recently achieved breakthrough in grain refinement into the submicron regime. However, these steels often suffer from grain boundary failure and low rates of work hardening. A potential approach to overcome these challenges is to process modern high-strength low-alloy steels with multi-scale hierarchical microstructures. Thus, the applicability of advanced thermo-mechanical processing for achieving such microstructures in a high-strength low-alloy steel was studied. The microstructural evolution during warm deformation of a martensitic/bainitic starting microstructure using a Gleeble 3500 thermo-mechanical simulator at 600 °C followed by a direct aging step was investigated. The strain rate of 10 s−1 led to strain localization and, therefore, the formation of a macroscopic shear band. High-resolution characterization techniques such as electron channeling contrast imaging, electron backscatter diffraction, and transmission electron microscopy were used to reveal the ultrafine grain sizes (~ 0.5 μm) in this shear band. The mechanism behind this refinement is continuous dynamic recrystallization, as the initial grains subdivided into smaller crystallites that are confined by a mix of subgrain and high-angle grain boundaries. Two populations of precipitates were formed. Larger precipitates (mean diameter ~ 150 nm) decorate grain boundaries, whereas smaller precipitates (~ 15 nm) nucleate on dislocations and subgrain boundaries.

Direct observation of dislocation nucleation in pyrite using combined electron channelling contrast imaging and electron backscatter diffraction

AbstractCrystal‐plastic deformation is one of the main mechanisms that can accommodate large amounts of strain within the lithosphere. Despite the requirement of understanding dislocation nucleation and arrangement, the only accepted method for direct observation of dislocations in geological materials so far is transmission electron microscopy. Herein, we present a study using a combination of electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) to visualize and analyse crystal defects in pyrite deforming close to the crystal plastic to brittle transition zone. Structures in focus include (a) dislocation nucleation at crack‐tips and (b) the reactivation of mode I cracks accompanied by the nucleation of dislocations and crystal‐plastic behaviour resulting in the development of complex dislocation structures and low‐angle grain boundaries. EBSD maps reveal an increase in misorientation towards micro‐cracks, consistent with a greater dislocation density along cracks observed by ECCI.

Imaging basal plane stacking faults and dislocations in (11-22) GaN using electron channelling contrast imaging

Taking advantage of electron diffraction based measurements, in a scanning electron microscope, can deliver non-destructive and quantitative information on extended defects in semiconductor thin films. In this work, we have studied a (11-22) semi-polar GaN thin film overgrown on regularly arrayed GaN micro-rod array templates grown by metal organic vapour phase epitaxy. We were able to optimise the diffraction conditions to image and quantify basal plane stacking faults (BSFs) and threading dislocations (TDs) using electron channelling contrast imaging (ECCI). Clusters of BSFs and TDs were observed with the same periodicity as the underlying micro-rod array template. The average BSF and TD densities were estimated to be ≈4 × 104 cm−1 and ≈5 × 108 cm−2, respectively. The contrast seen for BSFs in ECCI is similar to that observed for plan-view transmission electron microscopy images, with the only difference being the former acquiring the backscattered electrons and the latter collecting the transmitted electrons. Our present work shows the capability of ECCI for quantifying extended defects in semi-polar nitrides and represents a real step forward for optimising the growth conditions in these materials.

Deformation mode-determined misorientation and microstructural characteristics in rolled pure Zr sheet

In this work, commercially pure Zr sheets were subjected to β air cooling and then rolled to different reductions (10% and 50%) at room temperature. Microstructures of both the β-air-cooled and the rolled specimens were well characterized by electron channelling contrast imaging and electron backscatter diffraction techniques, with special attentions paid to their misorientation characteristics. Results show that the β-air-cooled specimen owns a Widmanstatten structure featured by lamellar grains with typical phase transformation misorientations. The 10% rolling allows prismatic slip and tensile twinning ({11-21} and {10-12} ) to be activated profusely, which produce new low-angle (~3°–5°) and high-angle (~35° and ~85°) misorientation peaks, respectively. After increasing the rolling reduction to 50%, twinning is suppressed and dislocation slip becomes the dominant deformation mode, with the lamellar grains highly elongated and aligned towards the rolling direction. Meanwhile, only one strong low-angle misorientation peak related to the prismatic slip is presented in the 50%-rolled specimen, with all other peaks disappeared. Analyses on local misorientations reveal that hardly any residual strains exist in the β-air-cooled specimen, which should be related to their sufficient relaxation during slow cooling. Residual strains introduced by 10% rolling are heterogeneously distributed near grain/twin boundaries while heavier deformation (50% rolling) produces much larger residual strains pervasively existing throughout the specimen microstructure.

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.