Complementary use of TEM and APT for the investigation of steels nanostructured by severe plastic deformation

The properties of bulk nanostructured materials are often controlled by atomic scale features like segregation along defects or composition gradients. Here we discuss about the complimentary use of TEM and APT to obtain a full description of nanostructures. The advantages and limitations of both techniques are highlighted on the basis of experimental data collected in severely deformed steels with a special emphasis on carbon spatial distribution

Compositions of Gamma and Gamma Prime Phases in an As-Cast Nickel-Based Single Crystal Superalloy Turbine Blade

The core and the interdendritic regions of an as-cast nickel based single crystal turbine blade were observed by electron microscopy to understand the microstructural development during an investment casting process. The dendrite core region shows an irregular morphology of gamma prime in gamma due to a relatively short casting time, which prevented the development of gamma prime expected in a solution heat-treated microstructure. By comparison, the interdendritic region comprises three different regions composed of: several elongated gamma prime particles, relatively tiny and irregular gamma prime, and gamma prime with relatively regular morphology. The chemical analysis of these phases showed that, regardless of the analysis point in the core or the interdendritic region, almost the same compositions were acquired in the regular type of gamma and gamma prime phases. This result suggests that if the gamma prime forms in the gamma matrix, the composition of gamma prime is almost uniform regardless of the region and prevailing general chemical composition. In contrast, the composition of the elongated gamma prime in the interdendritic region was slightly different depending on the analysis point even within the same elongated particle

Routine characterization and interpretation of complex alkali feldspar intergrowths

Almost all alkali feldspar crystals contain a rich inventory of exsolution, twin, and domain microtextures that form subsequent to crystal growth and provide a record of the thermal history of the crystal and often of its involvement in replacement reactions, sometimes multiple. Microtextures strongly influence the subsequent behavior of feldspars at low temperatures during diagenesis and weathering. They are central to the retention or exchange of trace elements and of radiogenic and stable isotopes. This review is aimed at petrologists and geochemists who wish to use alkali feldspar microtextures to solve geological problems or who need to understand how microtextures influence a particular process. We suggest a systematic approach that employs methods available in most well founded laboratories. The crystallographic relationships of complex feldspar intergrowths were established by the 1970s, mainly using single-crystal X-ray diffraction, but such methods give limited information on the spatial relationships of the different elements of the microtexture, or of the mode and chronology of their formation, which require the use of microscopy. We suggest a combination of techniques with a range of spatial resolution and strongly recommend the use of orientated sections. Sections cut parallel to the perfect (001) and (010) cleavages are the easiest to locate and most informative. Techniques described are light microscopy; scanning electron microscopy using both backscattered and secondary electrons, including the use of surfaces etched in the laboratory; electron-probe microanalysis and analysis by energy-dispersive spectrometry in a scanning electron microscope; transmission electron microscopy. We discuss the use of cathodoluminescence as an auxiliary technique, but do not recommend electron-backscattered diffraction for feldspar work. We review recent publications that provide examples of the need for great care and attention to pre-existing work in microtextural studies, and suggest several topics for future work

Corrosion behaviour of nitrided ferritic stainless steels for use in solid oxide fuel cell devices

Plasma nitriding was applied to ferritic stainless steel substrates to improve their performances as interconnects for solid oxide fuel cell devices. The samples underwent electrical conductivity test and SEM/EDS, TEM/EDS, environmental-SEM analyses. The first stages of corrosion were recorded in-situ with the e-SEM. Nitriding is effective in limiting the undesired chromium evaporation from the steel substrates and accelerates the corrosion kinetics, but its influence of the electrical conductivity is ambiguous. No intergranular corrosion is found in the steel substrate after long time operation. Nitriding helps commercially competitive porous coating to improve chromium retention properties of metal interconnects

Advanced Focused Ion Beam: Preparation Optimization and Damage Mitigation

Focused Ion Beam (FIB) is an important analytical and sample modification technique in the field of electron and ion microscopy. It has been widely used in different kinds of applications including semiconductor device failure analysis, material science research, nanoscale 3D tomography, as well as microstructure prototyping and surface modification. Recent developments from the rapid growing industry and our frontier research have posted new challenges on the FIB technology itself. Higher resolution has been realized by state-of-the-art hardware infrastructures and less sample destruction has been achieved by efficient operation recipes. In this doctoral thesis, a study of advanced Focused Ion Beam sample preparation is presented, with the goal to prepare samples with low or no damage. The study is divided into two aspects according to various aspects in the FIB applications: sample damage and in-situ preparation. In the first aspect, sample damage, namely amorphization, ion implantation and FIB milling rate are investigated on crystalline silicon specimens with a gallium FIB tool. To study the ion-beam induced amorphous layer thickness under certain conditions, silicon specimens were prepared by FIB into specific geometry, so that the induced amorphous layer can be imaged and the thickness can be determined quantitatively using Transmission Electron Microscopy (TEM). Atom Probe Tomography (APT) was carried out to study the implanted ion concentration of gallium FIB prepared silicon specimens. In addition, the gallium FIB milling rate was also studied for a silicon substrate using Scanning Electron Microscopy (SEM). These experimental results provide detailed information of beam-sample interactions from the FIB sample preparation. In order to gain a systematic understanding of the processes, as well as to be able to predict the outcome of a specific FIB recipe, a physics model and an adapted algorithm (TRIDYN) based on Binary Collision Approximation (BCA) were used for the simulation of FIB processes. The predicted results based on simulations were compared with experiments. The proposed model was successfully validated by the experimental results, i.e., the TRIDYN algorithm has the capability to provide predictions for the multi- step FIB sample preparation process and the respective recipes. The other aspect involves a novel design of a hardware configuration of a SEM/FIB system add-on to perform in-situ surface modification tasks such as argon ion polishing of specimens. This Beam Induced Polishing System (BIPS) overcomes the disadvantages that some of the ex-situ methods have, and it completes some of the advanced FIB recipes for extremely thin and pristine specimens. In the thesis, the functionality of a BIPS system is explained in detail, and first experimental results are shown to demonstrate the proof of concept of the system. To summarize, this doctoral thesis presents an adapted algorithm, which is validated by experiments, to simulate the multi-step Focused Ion Beam process for recipes of low- damage sample preparation; A novel in-situ experiment system BIPS is also introduced, providing an option to complement SEM/FIB systems for advanced FIB sample preparation recipes

Voids in kesterites and the influence of lamellae preparation by focused ion beam for transmission electron microscopy analyses

Kesterite solar cells based in Cu2ZnSnS4 and Cu2ZnSnSe4 are potential future candidates to be used in thin film solar cells. The technology still has to be developed to a great extent and for this to happen, high levels of confidence in the characterization methods are required so that improvements can be made on solid interpretations. In this study we show that the interpretations of one of the most used characterization techniques in kesterites, scanning transmission electron microscopy (STEM), might be affected by its specimen preparation when using focused ion beam (FIB). Using complementary measurements based on scanning electron microscopy and Raman scattering spectroscopy, compelling evidences show that secondary phases of ZnSe mixed in the bulk of Cu2ZnSnSe4 are the likely cause of the appearance of voids in the STEM lamellae. Sputtering simulations support this interpretation by showing that Zn in a ZnSe matrix is preferentially sputtered compared with any metal atom in a Cu2ZnSnSe4 matrix.publishe

Microstructure and mechanical properties of cold drawn steel wires

Cold drawn eutectoid steel wires have been widely used for a variety of applications, such as suspension bridges, steel cords for automobile tires, and springs. Much research has been done to increase their mechanical strength. With advances in modern production technology, both the drawing speed and the quality of drawn steels have been enhanced. After a careful literature survey, it is obvious that some issues are still controversial. As Y.S. Yang, J.G. Bae and C.G. Park mentioned, the lamellar spacing, thickness and volume fraction of cementites have all reached the nanometer regime, and the conventional theory is not enough to explain it. Besides, the cementite dissolution is a huge problem to the performance, according to Y.S. Yang and C.G. Park. To address the above issues, a systematic study has been taken on the wire drawing process under the conditions of the industrial production. Through the morphology, microscopic, mechanical and comprehensive analysis, a clear understanding of the microstructures and associated professing conditions of the high-strength carbon steel wires has been obtained. This project aims to clarify why the mechanical properties improve with the increasing strain. The project will be carried out in four stages: 1) characterisation of the microstructure of the cold drawn steel wires; 2) measurements of the modulus, hardness and toughness of steel wires; 3) modelling the deformation behaviour of the cold drawn steels. The techniques involved in the project include X-ray diffraction (XRD), focus ion beam (FIB), scan electron spectrum (SEM), Nanoindentation. A deep understanding of the relationship between composition, structure and performance will be achieved in this project. The results may provide the basis for improving cold-drawn steel wire designs

Accurate measurement of absolute experimental inelastic mean free paths and EELS differential cross-sections

Methods are described for measuring accurate absolute experimental inelastic mean free paths and differential cross-sections using DualEELS. The methods remove the effects of surface layers and give the results for the bulk materials. The materials used are VC0.83,TiC0.98,VN0.97and TiN0.88but the method should be applicable to a wide range of materials. The data were taken at 200 keVusing a probe half angle of 29mradand a collection angle of 36mrad. The background can be subtracted from under the ionisation edges, which can then be separated from each other. This is achieved by scaling Hartree-Slater calculated cross-sections to the edges in the atomic regions well above the threshold. The average scaling factors required are 1.00 for the non-metal K-edges and 1.01 for the metal L-edges (with uncertainties of a few per cent). If preliminary measurements of the chromatic effects in the post-specimen lenses are correct, both drop to 0.99. The inelastic mean free path for TiC0.98 was measured as 103.6±0.5 nm compared to the prediction of 126.9 nm based on the widely used Iakoubovskii parameterisation

Atomic-scale investigation of hydrogen distribution in a Ti–Mo alloy

Ingress of hydrogen is often linked to catastrophic failure of Ti-alloys. Here, we quantify the hydrogen distribution in fully β and α + β Ti–Mo alloys by using atom probe tomography. Hydrogen does not segregate at grain boundaries in the fully β sample but segregates at some α/β phase boundaries with a composition exceeding 20 at.% in the α + β sample. No stable hydrides were observed in either sample. The hydrogen concentration in β phases linearly decreases from ~13 at. % to ~4 at. % with increasing Mo-content, which is ascribed to the suppression of hydrogen uptake by Mo addition