Electron Channeling Contrast Research Articles

Damage Mechanism of Low Cycle Fatigue in an Advanced Heat Resistant Austenitic Stainless Steel at High Temperature

Sandvik Sanicro 25 is a newly developed heat resistant austenitic stainless steel grade for the next generation of coal fired advanced ultra-super critical (AUSC) power plants. In this paper, low cycle fatigue behavior and damage mechanisms of the material were studied. The low cycle fatigue test was performed in air at room temperature, 600°C to 700°C. The microstructures were studied using electron back scatter diffraction and electron channeling contrast image techniques. At room temperature, the material shows a conventional hardening and softening behavior as most metal materials. At high temperatures, however, it shows only a cyclic hardening behavior. Dynamic strain ageing is found to be one of the mechanisms. The damage and fatigue crack initiation mechanisms due to cyclic loading at different temperatures and loading conditions have been identified. The interactions between dislocations or slip bands with grain boundary or twin boundary are the main damage mechanism at low temperature or at high temperature with large strain amplitudes. Strain localization due to dislocation slipping is the main mechanism for the fatigue damage in grains.

Analysis of Subsurface Microstructure and Residual Stresses in Machined Inconel 718 with PCBN and Al2O3-SiCw Tools

Subsurface microstructural alterations and residual stresses caused by machining significantly affect component lifetime and performance by influencing fatigue, creep, and stress corrosion cracking resistance. Assessing the surface quality of a machined part by characterizing subsurface microstructural alterations and residual stresses is essential for ensuring part performance and lifetime in aero-engines and power generators. This comparative study characterizes and analyzes subsurface microstructural alterations and residual stresses in Inconel 718 subjected to high-speed machining with PCBN and whisker-reinforced ceramic cutting tools. Effects of cutting tool materials and microgeometry on subsurface deformation, microstructural alterations, and residual stresses were investigated. Surface and subsurface regions of machined specimens were investigated using X-ray diffraction, electron channeling contrast imaging, and electron back-scatter diffraction to characterize microstructural alterations and measure deformation intensity and depth.

Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature

In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel (AISI 316L) and one nickel-base alloy (Alloy 617) have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromechanisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650°C.

Fatigue behaviour of hot pressed austenitic TWIP steel and TWIP steel/Mg-PSZ composite materials

This research work presents the fatigue behaviour of a high-alloyed austenitic TWIP steel as well as metal matrix composites based on this steel grade as matrix material and MgO partially stabilized zirconia (Mg-PSZ) as reinforcing phase. The specimens were produced using the hot pressing technique. The unreinforced steel and MMCs containing 5 and 10vol.% Mg-PSZ were investigated under total strain control in order to clarify the influence of the ceramic reinforcement. The cyclic deformation behaviour in terms of cyclic softening and/or cyclic hardening as well as fatigue lives of the different materials are discussed. Microstructures were examined to study the deformation and dislocation structures as well as phase transformations using electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD), respectively.

Coincident Electron Channeling and Cathodoluminescence Studies of Threading Dislocations in GaN

We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black-white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.

Multi-Scale Correlative Microscopy Investigation of Both Structure and Chemistry of Deformation Twin Bundles in Fe–Mn–C Steel

A multi-scale investigation of twin bundles in Fe-22Mn-0.6C (wt%) twinning-induced plasticity steel after tensile deformation has been carried out by truly correlative means; using electron channelling contrast imaging combined with electron backscatter diffraction, high-resolution secondary ion mass spectrometry, scanning transmission electron microscopy, and atom probe tomography on the exact same region of interest in the sample. It was revealed that there was no significant segregation of Mn or C to the twin boundary interfaces.

A novel, single phase, non-equiatomic FeMnNiCoCr high-entropy alloy with exceptional phase stability and tensile ductility

A non-equiatomic FeMnNiCoCr alloy is introduced and characterized at multiple scales employing various characterization techniques (e.g. atom probe tomography, electron channeling contrast imaging, electron backscatter diffraction, etc.) to elucidate (i) the role of configurational entropy and (ii) the intrinsic tensile ductility of high-entropy alloys. Results reveal that the new material is a true high-entropy alloy with a stable random solid solution despite its comparably low configurational entropy, and that it has excellent tensile ductility irrespective of the substantial lattice distortion.

In Situ Synchrotron Studies of Reversible and Irreversible Non-elastic Strain in a Two-Phase TiAl Alloy

This paper contrasts the cyclic tensile loading of two-phase titanium aluminide Ti-44Al-8Nb-1B microstructures, namely fully lamellar (FL) and duplex (DP). The former, in contrast to the latter, shows premature yielding and hysteresis loops on cycling. These phenomena were studied by in situ cyclic loading of these specimens using high energy synchrotron X-ray diffraction. The results show significant differences in the micromechanics of deformation. Load transfer between the γ and α 2 phases has been identified at stresses below the macroscopic yield point in the FL microstructure, while the DP microstructure showed no signs of such load transfer before its well-defined macroscopic yield point. A partially reversible pseudo-plastic deformation mechanism seems to be operating at relatively low stresses in the FL specimen. This mechanism is believed to be twinning/partial reversible twinning of the γ phase. The presence of twins at relatively low applied stresses has been confirmed for the FL microstructure by electron channeling contrast imaging. Further support of twinning/partial reversible twinning in FL but not DP microstructures below the macroscopic yield point was obtained by following the evolution of the integrated intensity of particular diffraction peaks measured during the in situ synchrotron X-ray experiments.

Characterization of twin boundaries in an Fe–17.5Mn–0.56C twinning induced plasticity steel

A twinning-induced plasticity steel of composition Fe–17.5wt.% Mn–0.56wt.% C–1.39wt.% Al–0.24wt.% Si was analyzed for the purpose of characterizing the relationship between tensile strain and deformation twinning. Tensile samples achieved a maximum of 0.46 true strain at failure, and a maximum ultimate tensile strength of 1599MPa. Electron backscatter diffraction (EBSD) analysis showed that the grain orientation rotated heavily to <111> parallel to the tensile axis above 0.3 true strain. Sigma 3 misorientations, as identified by EBSD orientation measurements, and using the image quality maps were used to quantify the number of twins present in the scanned areas of the samples. The image quality method yielded a distinct positive correlation between the twin area density and deformation, but the orientation measurements were unreliable in quantifying twin density in these structures. Quantitative analysis of the twin fraction is limited from orientation information because of the poor spatial resolution of EBSD in relation to the twin thickness. The EBSD orientation maps created for a thin foil sample showed some improvement in the resolution of the twins, but not enough to be significant. Measurements of the twins in the transmission electron microscopy micrographs yielded an average thickness of 23nm, which is near the resolution capabilities of EBSD on this material for the instrumentation used. Electron channeling contrast imaging performed on one bulk tensile specimen of 0.34 true strain, using a method of controlled diffraction, yielded several images of twinning, dislocation structures and strain fields. A twin thickness of 66nm was measured by the same method used for the transmission electron microscopy measurement. It is apparent that the results obtain by electron channeling contrast imaging were better than those by EBSD but did not capture all information on the twin boundaries such as was observed by transmission electron microscopy.

Influence of Dry Cut and Tool Wear on Residual Stresses in High Speed Machining of Nickel-Based Superalloy

Machining induced residual stresses were investigated in Inconel 718 prepared by high speed turning under dry cut condition. The influence of cutting tool wear and the use of cutting fluid were studied. By x-ray diffraction measurements, characteristic residual stress distributions with tensile stresses in the top layer and compressive stresses in the layer below were found in all the investigated samples. The magnitude of surface tensile stresses and size of the tension as well as compression zones varied depending on the cutting condition. The application of cutting fluid for turning using new tool has a minor effect, giving a somewhat larger subsurface compressive zone but reducing the surface tensile stresses. Tool flank wear has shown a much stronger effect. While a flank wear of VBmax=0.15 mm enhanced mostly the surface tensile residual stresses, a severer wear of VBmax=0.3 mm greatly increased the thickness of the subsurface compression zone and at the same time resulted in strong stress anisotropy. Microstructural study by electron channelling contrast imaging shows that the observed influence of tool flank wear or cutting fluid on residual stresses are related to different contributions from increased plastic deformation and cutting heat, which changed with the cutting conditions.

Linking recovery and recrystallization through triple junction motion in aluminum cold rolled to a large strain

Recovery mechanisms and kinetics have been studied in commercial purity aluminum (AA1050) cold rolled to a true strain of 5.5 (99.6% thickness reduction) and annealed at low temperatures from 140 to 220°C. Transmission electron microscopy, electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) are used to characterize the microstructural evolution during annealing. The microstructural characterization shows that a deformed lamellar structure coarsens uniformly during annealing by triple junction motion while maintaining the lamellar morphology, leading to a gradual transition into a more equiaxed structure, where recrystallization nuclei start to evolve. The apparent activation energy for the microstructural coarsening is estimated separately for different stages characterized by an increase in the lamellar boundary spacing measured by EBSD and ECC. The apparent activation energy increases during annealing, from 110kJmol−1 at the beginning to 230–240kJmol−1 at the end of uniform coarsening, linking the recovery stages to recrystallization. The increase in activation energy underpins operation of different diffusion mechanisms for migration of boundaries and their junctions during coarsening, and solute drag may become increasingly important as the structure coarsens. These findings form the basis for a discussion of the thermal behavior of a fine lamellar structure produced by cold rolling to a large strain of both scientific and applied interest.

Advanced Electron Backscatter Diffraction Applications and Techniques

Since the first automated electron backscatter diffraction (EBSD) results were attained 22 years ago, EBSD has become a common characterization technique in materials laboratories around the world. This has led to advancements and new understanding in not just crystallography but also in microstructural evolution, phase transformations, defects and dislocations, mechanical behavior, failure, and other materials phenomena. As a complement to scanning electron microscopy (SEM), EBSD is accessible and available to many researchers with little additional sample preparation. With improvements to both SEM and EBSD, it is now possible to analyze large areas for statistically significant data, and as precision increases, higher resolutions and smaller length scales have become accessible. EBSD analyses are also being combined with other characterization and analysis techniques to provide new insights. Although EBSD has matured and measurements have become more commonplace, new techniques, applications, and approaches continue to be introduced regularly. This collection of articles focuses on the versatility of EBSD in the range of materials systems presented, as well as on new techniques and applications of EBSD for materials characterization. Examples of the combination of EBSD with other characterization techniques include the supplementation and enhancement of EBSD data by transmission electron microscopy (TEM), atomic force microscopy (AFM), and electron channeling contrast imaging. Novel approaches to strain measurements, and the analysis of stacking faults, defects, and dislocations are also presented. The versatility of EBSD is demonstrated in the range of materials systems represented in these articles; EBSD analyses of conventional polycrystalline metals and alloys, novel lightweight alloys, second phase precipitates, and the complex structures of thin film solar cells are presented. The article from D. Abou-Ras and his co-workers provides an overview of the application of EBSD on thin-film solar cells. Of particular interest is the integration of EBSD results with several scanning probe microscopy techniques for correlating properties critical to photovoltaic performance with the crystallographic orientation aspects of the observed microstructures. I. Gutierrez-Urrutia, S. Zaefferer, and D. Raabe have reviewed a novel application of EBSD for imaging crystal defects, such as dislocations, cells, and stacking faults. EBSD is used to determine the optimal diffraction conditions for electron channeling contrast imaging to elucidate dislocation substructures similar to those observed using conventional TEM diffraction. Z. Chen and C.J. Boehlert combine EBSD with AFM to quantify deformation in an AZ31 magnesium alloy, noting that grain boundary sliding has a significant contribution. In the article by T. Ben Britton and colleagues, elastic strains are measured using highresolution, cross-correlation-based EBSD, presenting three examples of this exciting technique. Finally, Seiichi Suzuki presents a systematic overview of the transmission EBSD technique, which provides very high spatial resolution in orientation mapping. This collection of articles represents just a sampling of the new and exciting data being collected using EBSD, while also covering a breadth of work that illustrates both the utility and the potential of this important technique. As advancements continue to be made, we anticipate EBSD to fuel further innovative studies and bring exciting results in crystallography, microstructural evolution, materials processing, and microstructure quantification for years to come.

Coupling of Electron Channeling with EBSD: Toward the Quantitative Characterization of Deformation Structures in the SEM

The coupling of electron channeling contrast imaging (ECCI) with electron backscatter diffraction (EBSD) provides an efficient and fast approach to perform ECCI of crystal defects, such as dislocations, cells, and stacking faults, under controlled diffraction conditions with enhanced contrast. From a technical point of view, the ECCI technique complements two of the main electron microscopy techniques, namely, EBSD and conventional diffraction-based transmission electron microscopy. In this review, we provide several application examples of the EBSD-based ECCI approach on microstructure characterization, namely, characterization of single dislocations, measurement of dislocation densities, and characterization of dislocation substructures in deformed bulk materials. We make use of a two-beam Bloch wave approach to interpret the channeling contrast associated with crystal defects. The approach captures the main features observed in the experimental contrast associated with stacking faults and dislocations.

Stress distribution of GaN layer grown on micro-pillar patterned GaN templates

High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 μm thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template.

Study on Cyclic Strain Localization and Fatigue Fracture Mechanism in High Manganese Twinning-Induced Plasticity Steels

The fatigue cracking mechanisms of two high Mn TWinning-Induced Plasticity (TWIP) steels are investigated in detail using electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Furthermore, the fracture surfaces of the fatigued steels have been studied by employing a field emission gun scanning electron microscope (FEG-SEM). The fine details of the fatigued surface topography are verified using an atomic force microscope (AFM). The results indicate that the fatigue crack embryos nucleate at an early stage of the fatigue life as a result of local straining at grain and annealing twin boundaries at sites, where persistent slip bands create dislocation piled-ups that impinge on boundaries. The EBSD measurements showed that unlike in monotonic straining, the formation of deformation twins is not observed under cyclic straining.

A comparative study of clock rolling and unidirectional rolling on deformation/recrystallization microstructure and texture of high purity tantalum plates

Clock rolling, which rotates a sample 135° with respect to the previous rolling direction, was used to study the effect of strain path change on deformation and recrystallization microstructure and texture in high purity tantalum plates in the present work. High purity tantalum plates were unidirectional and clock rolled to 87% reduction then annealed at various temperatures. The texture and microstructure in the surface layer of the rolled and annealed plates were characterized using X-ray diffraction (XRD), light microscopy (LM), electron backscatter diffracting (EBSD) and electron channeling contrast imaging (ECCI). The hardness values of the annealed plates were measured using Vickers hardness testing. The results show that the unidirectional- and clock-rolled materials exhibit different textures. The unidirectional rolled plate recrystallized more quickly than that of the clock rolled plate. After annealing at 1200°C, pronounced γ-fiber in the unidirectional rolled plate while strong cube texture mixed with less strong γ-fiber in the clock rolled plate was found. The annealed clock rolled plate had smaller grain size and less variation than that of the annealed unidirectional rolled plate. The change of strain path plays an important role in the recrystallization kinetics and microstructure of high purity tantalum plates.

Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging

We investigated the hydrogen embrittlement of a Fe–18Mn–1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7×10−6s−1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe–Mn–C twining-induced plasticity steel.

Twinning during recrystallization cooling in α-Zr alloy

Recently, twinning induced by intergranular thermal residual stresses (ITRSs) has been first reported in a Zr alloy after β→α furnace cooling. To further explore this kind of twinning behavior, microstructures and textures of the Zr alloy during α annealing between 650 and 750°C after β quenching were investigated in the present work, by use of X-ray diffraction, electron backscatter diffraction and electron channeling contrast imaging techniques. Results indicate that the ITRS-induced twins could also occur during cooling after α-recrystallization annealing in the Zr alloy, closely related to both the grain size and the texture. When the recrystallization texture is relatively random, high tensile ITRSs up to ~200MPa along c-axis could be accumulated. Tensile twins can only be induced by the tensile ITRS in the nearly texture-free Zr specimen with an average grain size larger than ~20μm.

Electron channeling contrast imaging studies of nonpolar nitrides using a scanning electron microscope

Threading dislocations, stacking faults, and associated partial dislocations significantly degrade the optical and electrical properties of materials such as non-polar III-nitride semiconductor thin films. Stacking faults are generally difficult to detect and quantify with existing characterization techniques. We demonstrate the use of electron channeling contrast imaging in the scanning electron microscope to non-destructively reveal basal plane stacking faults terminated by partial dislocations in m-plane GaN and InGaN/GaN multiple quantum well structures grown on γ-LiAlO2 by metal organic vapor phase epitaxy.

Kinetics of Thermal Grooving during Low Temperature Recrystallization of Pure Aluminum

The migration of a recrystallization boundary in pure aluminum was followed during in situ annealing in a scanning electron microscope. The microstructure was characterized using the electron channeling contrast technique and a typical stop-go grain boundary motion was observed during annealing. The thermal grooving associated with boundary migration on the inspected free surface was characterized after the in-situ experiment using atomic force microscopy. The results show that new thermal grooves develop at places where the recrystallization boundary segments remain stationary for a relatively long time. The kinetics of thermal grooving are determined and effects hereof on the boundary migration are discussed.