Spectrum imaging of complex nanostructures using DualEELS: I. digital extraction replicas

This paper shows how it is possible to use Dual Electron Energy Loss Spectroscopy (DualEELS) to digitally extract spectrum images for one phase of interest in a complex nanostructured specimen. The specific cases studied here concern Nb or V precipitates, a few nanometres in size, in high manganese steels. The procedures outlined allow the extraction of the precipitate signal from the Fe–Mn matrix, as well as correction for surface oxide and any surface carbon contamination. The resulting precipitate-only spectrum images are then suitable for quantitative analysis of the precipitate chemistry. This procedure results in much improved background shapes under all edges of interest, mainly as a result of the removal of the extended electron loss fine structure (EXELFS) from the elements in the matrix. This allows the reliable extraction of even tiny quantities of elements, such as low levels of nitrogen in some carbide precipitates. As well as being relevant to precipitation in steels, these techniques will be widely applicable to the separation of chemically-distinct phases in complex nanostructured samples, and can be viewed as a digital version of the extraction replica technique

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

Characterisation and absolute quantification of nanosized V and Nb precipitates in high manganese steel using DualEELS

The dispersion strengthening is a process that strengthens the material's matrix with use of the smallest precipitates. The most power comes from the character of the nanostructures, their chemical compositions, crystallographic structures, sizes, shapes and interfaces between matrix and precipitates. Through analytical methods it is possible to study and achieve the optimal mechanical properties of elements produced from high-manganese steels with dispersive nanoparticles such as carbides or carbo-nitrides. So far the most effective research methods are transmission electron microscopy and spectroscopy with use of multiple detectors such as Dual Electron Energy Loss Spectrometer (DualEELS), Energy Dispersion X-ray detector, (EDX) or Gatan Imaging Filter (GIF). The DualEELS, which is mostly developed during this thesis, is a technique which results in spectrum imaging of all elements presented in the analyzed area. The material studied during this Ph.D. is a high-manganese steel with vanadium and niobium carbides or carbo-nitrides with various sizes from few to dozen of nanometers, but the average is less than 20nm. The main goal of this Ph.D. is to advance the current available methods of nano-analysis using DualEELS of the smallest precipitates embedded in the matrix supported by improved and optimized sample preparation method with Focused ion Beam (FIB) lamella. The advancement is made in several fields including development of acquisition conditions, development of sample preparation technique using low voltages at FIB, and development and adaptation of the novel post-processing routines of the acquired data using Gatan DualEELS QuantumER post-column detector. The post-processing routine involved quantitative and qualitative analysis of precipitates, which required new values of partial cross-sections and mean free paths of inelastic scattering events estimated and calculated based on experimental (standards) approach. The results consist of fully quantified precipitates (V,Ti)(N,C) and ((Nb,Ti)C) supported by chemical profiles, comparison discussion between specimen, with a pseudo-3D reconstruction of spectrum imaging of these precipitates, which means that all elements have individually calculated thicknesses. The results gave a glimpse into the nucleation of the smallest precipitates, unexpectedly different than in most literature, showing that in this case there is no core-shell structure in mixed transition metal carbides. The Ti, which is not an intentional part of the alloy, acts as a nucleation sites, but is not built over core for the V or Nb mixed precipitates. These results are shown by four examples of V-steel and three of Nb-steel. They are presented as spectrum imaging signals, separated from matrix, with pseudo 3D representation and quantified in absolute manner with the accuracy down to a few atoms

Spectrum imaging of complex nanostructures using DualEELS: II. Absolute quantification using standards

Nanometre-sized TixV(1−x)CyNz precipitates in an Fe20%Mn steel matrix with a thickness range from 14 to 40 nm are analysed using DualEELS. Their thicknesses, volumes and compositions are quantified using experimental binary standards and the process used to give robust results is described. Precisions of a few percent are achieved with accuracies that are estimated to be of a similar magnitude. Sensitivities are shown to be at 0.5–1 unit cells range in the thinnest matrix region, based on the assumption that a sub-lattice is fully populated by the element. It rises to the 1–2 unit cell range for the metals and 2–3 unit cells for the non-metal in the thickest matrix region. The sensitivities for Ti and N are greater than those for V and C respectively because the O K-edge from surface oxide needs to be separated from the V L2,3-edge, and the C K-edges from C in the matrix and amorphous C on the surface have to be separated from the C in the precipitate itself. Separation of the contributions from the bulk and the surface is demonstrated, showing that there is significant and detectable C in the matrix but no O, while there is significant O but little C in the surface oxide. Whilst applied to precipitates in steel in this work, the approach can be adapted to many multi-phase systems

Linear chemically sensitive electron tomography using DualEELS and dictionary-based compressed sensing

We have investigated the use of DualEELS in elementally sensitive tilt series tomography in the scanning transmission electron microscope. A procedure is implemented using deconvolution to remove the effects of multiple scattering, followed by normalisation by the zero loss peak intensity. This is performed to produce a signal that is linearly dependent on the projected density of the element in each pixel. This method is compared with one that does not include deconvolution (although normalisation by the zero loss peak intensity is still performed). Additionaly, we compare the 3D reconstruction using a new compressed sensing algorithm, DLET, with the well-established SIRT algorithm. VC precipitates, which are extracted from a steel on a carbon replica, are used in this study. It is found that the use of this linear signal results in a very even density throughout the precipitates. However, when deconvolution is omitted, a slight density reduction is observed in the cores of the precipitates (a so-called cupping artefact). Additionally, it is clearly demonstrated that the 3D morphology is much better reproduced using the DLET algorithm, with very little elongation in the missing wedge direction. It is therefore concluded that reliable elementally sensitive tilt tomography using EELS requires the appropriate use of DualEELS together with a suitable reconstruction algorithm, such as the compressed sensing based reconstruction algorithm used here, to make the best use of the limited data volume and signal to noise inherent in core-loss EELS