A.J. Craven

Complex heterogeneous precipitation in titanium-niobium microalloyed Al-killed HSLA steels - I. (Ti,Nb)(C,N) particles

Abstract Precipitation in Ti–Nb Al-killed microalloyed HSLA steels (Ti/N weight ratio from 4.4 to 1) was investigated in both the as-rolled and the normalised conditions using analytical electron microscopy including parallel electron energy loss spectroscopy (PEELS). An extensive formation of heterogeneously nucleated complex (Ti,Nb)(C,N) particles down to 10 nm in size was observed. The core of such a complex particle is based on TiN and has a spherical, cubic or cruciform shape. The N/(Ti+Nb) atomic ratio in the core is similar to the average value in the steel whereas the Nb/Ti ratio is much smaller than the average value and not proportional to it. Many of the cores have caps in the form of epitaxial overgrowths based on NbC. Their composition changes from Nb(C,N) to (Nb,Ti)C as the N/Ti ratio decreases. The formation of these complex particles and their detailed morphology are controlled by the processing conditions as well as the overall composition.

Solid state metathesis routes to transition metal carbides

Flame initiated (800 °C, 10 s) or bulk thermal (2 days, 1000 °C) reactions of mixed powders of transition metal halides and CaC 2 or Al 4 C 3 produce transition metal carbides (TiC, ZrC, HfC, V 8 C 7 , NbC, TaC, Cr 3 C 2 , Mo 2 C and WC) in good yields. The carbides were characterised by X-ray powder diffraction, SEM/EDX, FTIR, microelemental analysis, TEM, electron diffraction and ELNES.

Near-simultaneous dual energy range EELS spectrum imaging.

A system that allows the collection of the low loss spectrum and the core loss spectrum, covering different energy regions, at each pixel in a spectrum image is described. It makes use of a fast electrostatic shutter with control signals provided by the spectrum imaging software and synchronisation provided by the CCD camera controller. The system also allows simultaneous collection of the X-ray spectrum and the signals from the imaging detectors while allowing the use of the existing features of the spectrum imaging software including drift correction and sub-pixel scanning. The system allows acquisition of high-quality spectra from both the core and the low loss regions, allowing full processing of the EELS data. Examples are given to show the benefits, including deconvolution, absolute thickness mapping and determination of numbers of atoms per unit area and per unit volume. Possible further developments are considered.

Parallel electron energy-loss spectroscopy (PEELS) study of B in minerals; the electron energy-loss near-edge structure (ELNES) of the B K edge

Abstract The B K-edge spectra of a wide variety of minerals have been measured using the technique of parallel electron energy-loss spectroscopy (PEELS) conducted in a scanning transmission electron microscope (STEM) from sample areas of nanometer dimensions. The B K edges of the minerals exhibit electron energy-loss near-edge structure (ELNES) characteristic of B coordination. For threefold-coordinated B ([3]B), the spectra are dominated by a sharp peak at ca. 194 eV because of transitions to unoccupied states of π* character, followed by a broader peak at ca. 203 eV attributed to states of σ* character. The ELNES on the B K edge (B K ELNES) of fourfold-coordinated B ([4]B) consists of a sharp rise in intensity with a maximum at ca. 199 eV followed by several weaker structures. For [4]B, the ELNES is interpreted as transitions to states of antibonding σ* character. Minerals that possess both [3]B and [4]B exhibit an edge shape that is composed of B K edges from the respective BO3 and BO4 units, and we demonstrate the feasibility of quantification of relative site occupancies in minerals containing a mixture of B coordinations. The origins of the B K ELNES are discussed in terms of both molecular orbital (MO) and multiple scattering (MS) theory. We also present the B K-edge spectra of selected nonminerals and show how differences in edge shapes and energy onsets allow these nonminerals to be readily distinguished from borates and borosilicates.

Column ratio mapping : A processing technique for atomic resolution high-angle annular dark-field (HAADF) images

An image processing technique is presented for atomic resolution high-angle annular dark-field (HAADF) images that have been acquired using scanning transmission electron microscopy (STEM). This technique is termed column ratio mapping and involves the automated process of measuring atomic column intensity ratios in high-resolution HAADF images. This technique was developed to provide a fuller analysis of HAADF images than the usual method of drawing single intensity line profiles across a few areas of interest. For instance, column ratio mapping reveals the compositional distribution across the whole HAADF image and allows a statistical analysis and an estimation of errors. This has proven to be a very valuable technique as it can provide a more detailed assessment of the sharpness of interfacial structures from HAADF images. The technique of column ratio mapping is described in terms of a [110]-oriented zinc-blende structured AlAs/GaAs superlattice using the 1 angstroms-scale resolution capability of the aberration-corrected SuperSTEM 1 instrument.

Bonding in alpha-quartz (SiO2): A view of the unoccupied states

Abstract High-resolution core-loss and low-loss spectra of α-quartz were acquired by electron energyloss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra contain the Si L1, L2,3, K, and O K core-loss edges, and the surface and bulk low-loss spectra. The core-loss edges represent the atom-projected partial densities of states of the excited atoms and provide information on the unoccupied s, p, and d states as a function of energy above the edge onset. The band structure and total density of states were calculated for α-quartz using a self-consistent pseudopotential method. Projected local densities of Si and O s, p, and d states (LDOS) were calculated and compared with the EELS core-loss edges. These LDOS successfully reproduce the dominant Si and O core-loss edge shapes up to ca. 15 eV above the conduction-band onset. In addition, the calculations provide evidence for considerable charge transfer from Si to O and suggest a marked ionicity of the Si-O bond. The experimental and calculated data indicate that O 2p-Si d π-type bonding is minimal. The low-loss spectra exhibit four peaks that are assigned to transitions from maxima in the valence-band density of states to the conduction band. A band gap of 9.65 eV is measured from the low-loss spectrum. The structures of the surface low-loss spectrum are reproduced by the joint density of states derived from the band-structure calculation. This study provides a detailed description of the unoccupied DOS of α-quartz by comparing the core-loss edges and low-loss spectrum, on a relative energy scale and relating the spectral features to the atom- and angular-momentum-resolved components of a pseudopotential band-structure calculation.

Electron-beam-induced reduction of Mn4+ in manganese oxides as revealed by parallel EELS

Abstract Chemical reduction of Mn 4+ in the mineral asbolan was examined using parallel electron energy-loss spectroscopy (PEELS). Changes on the oxidation state of the manganese were monitored by recording the electron energy-loss near-edge structure (ELNES) for the Mn M 2,3 - and L 2,3 -edges. The edges in the undamaged material were consistent with Mn 4+ . Electron beam irradiation of the mineral caused the ionisation edges to change in shape and energy consistent with the decrease in the proportion of the Mn 4+ and the increase in the proportions of Mn 3+ and Mn 2+ . The final edge is very similar in energy and shape to that of Mn 2+ in rhodochrosite (MnCO 3 ) and manganosite (MnO) and the differences in shape can be accounted for by the presence of a small amount of residual Mn 3+ .

Design considerations and performance of an analytical stem

Abstract A VG Microscope HB5 has been modified in collaboration with the manufacturers. The major modifications are the addition of three post specimen lenses, a second condenser lens and a z -lift on the specimen stage. The requirements of the various parts of the microscope are reviewed and the compromises made necessary their interaction are discussed. The performance of the double condenser system, the z -lift stage and the high excitation objective lens are discussed briefly. A more detailed discussion of the expected performance of the post-specimen optics is given and preliminary experimental results are presented. The discussion is in two parts. The first part considers the recording of diffraction patterns and the use of detectors in the diffraction plane. The major effect is the distortion introduced by the lens system. By a suitable choice of operating conditions, this can be minimised. The second part considers the use of an electron spectrometer. Large chromatic effects can occur if a wide range of electron energies is involved. Provided that the correct lens configuration is used, these may prove to be an advantage since it is possible to produce an increase in collection efficiency with increasing energy loss. Finally, the effect of using an electron spectrometer with lower aberrations in conjunction with the post-specimen lenses is considered briefly.

The electron energy-loss near-edge structure (ELNES) on the N K-edges from the transition metal mononitrides with the rock-salt structure and its comparison with that on the C K-edges from the corresponding transition metal monocarbides

This paper presents the shapes of the electron energy‐loss near‐edges structure (ELNES) on the N K‐edge of the group IVA (Ti, Zr, Hf) and group VA (V, Nb, Ta) transition metal mononitrides close to stoichiometry. With the exceptions of NbN and TaN, these compounds have the rock‐salt (B1) structure when close to stoichiometry. NbN exists with both the rock‐salt structure and a hexagonal structure. Two distinct ELNES shapes were observed from it, one of which corresponds closely with previously published data from the rock‐salt structure. Under normal conditions, TaN is considered to exist only in the hexagonal form, the rock‐salt form being a high‐temperature/high‐pressure phase although it has been reported as the result of plasma jet heating of the hexagonal form. Again two distinct ELNES shapes were observed, one of which appeared to fit into the pattern of the shapes from the other compounds with the rock‐salt structure. The systematic changes of shape observed are very similar to those observed in the equivalent carbides and qualitatively follow the behaviour expected from theoretical band structures. The change in the chemical shift of the N K‐edge on going from a group IVA nitride to a group VA nitride is ∼‐0·8 eV while that on going from a group IVA carbide to a group VA carbide is ∼+0·8 eV. This difference in behaviour is explained as the result of differences in the densities of states at the Fermi levels of the compounds. The position of the first peak in the ELNES also shows a systematic change in its energy relative to the core state as the number of valence electrons in the compound increases and also as the transition series of the metal species changes. The energies, Er, of the peaks in the ELNES relative to the threshold follow a relationship similar to that predicted by Natoli, i.e. (Er ‐ V)a = const. where V is the ‘muffin tin’ potential and a is the lattice parameter. The first peak gives a negative constant in the relationship. The value of constant increases for each subsequent peak up to the sixth becoming positive for the fourth and higher peaks but drops slightly on going from the sixth to the seventh peak. Each peak gives a different value of V in the relationship. The data sets for the carbides and the nitrides are systematically different in a similar way for each peak and there are deviations from linearity within each set. The systematic difference is minimized and the linearity significantly improved if the difference in the energies of two prominent peaks is used instead of Er. This systematic variation of peak energy with lattice parameter can be used to predict the lattice parameter. If both the nitride and the carbide data for the energy of a prominent peak relative to the threshold are used, this results in a maximum deviation of 4% (or ∼0·02 nm). However, if the differences in the energies of two prominent peaks are used and the data for the carbides and the nitrides are treated independently, the maximum deviation drops to 0·4% (or ∼0·002 nm). At this level, uncertainties in the lattice parameters themselves come into play and better characterized materials are required to set true limits to the accuracy of the predictions. Finally some applications in the microanalysis of materials are outlined briefly.

Complex heterogeneous precipitation in titanium–niobium microalloyed Al-killed HSLA steels—II. Non-titanium based particles

An analytical electron microscopical investigation of three Ti-Nb Al deoxidized steels with different N:Ti ratios has been undertaken. In each steel, a large number of small (<10 nm) particles were observed. Parallel electron energy loss spectroscopy (PEELS) showed that their compositions in the three steels were consistent with those reported for the caps on the TiN cores in the equivalent steels in Part I, i.e. NbC0.7N0.3, NbC and (Nb0.7Ti0.3)C, respectively. The Nb incorporated in these caps added to that dissolved in the TiN cores results in a significant reduction in the number of small particles which give effective dispersion hardening. The size of this reduction depends on a number of competing factors. AlN precipitation also occurred in the as-rolled steel with highest N content and in the normalized steels with the two higher N contents. AlN is usually expected to control the austenite grain growth. NbC-based material grew on the AlN. A dendritic complex based on the iso-structural compounds MnSiN2 and AlN was observed in the high N steel.