High Spatial Resolution X-ray Microanalysis Research Articles

Low Voltage Scanning Electron Microscopy

Abstract Low Voltage Scanning Electron Microscopy (LVSEM), defined as operation in the energy range below 5 keV, has become perhaps the most important single operational mode of the SEM. This is because the LVSEM offers advantages in the imaging of surfaces, in the observation of poorly conducting and insulating materials, and for high spatial resolution X-ray microanalysis. These benefits all occur because a reduction in the energy Eo of the incident beam leads to a rapid fall in the range R of the electrons since R ∼k.E0 1.66. The reduction in the penetration of the beam has important consequences.

Low Voltage Scanning Electron Microscopy

Low Voltage Scanning Electron Microscopy (LVSEM), defined as operation in the energy range below 5keV, has become perhaps the most important single operational mode of the SEM. This is because the LVSEM offers advantages in the imaging of surfaces, in the observation of poorly conducting and insulating materials, and for high spatial resolution X-ray microanalysis. These benefits all occur because a reduction in the energy E0 of the incident beam leads to a rapid fall in the range R of the electrons since R ∼ k.E01.66. The reduction in the penetration of the beam has important consequences. Firstly, volume of the specimen that is sampled by the beam shrinks dramatically (varying as about E05 ) and so the information generated by the beam is confined to the surface of the sample. Secondly, the yield 8 of secondary electrons is increased from a typical value of 0.1 at 20keV to a value that may be in excess of 1.0 at 1keV.

High-Spatial-Resolution Low-Energy Electron Beam X-Ray Microanalysis

Performing X-ray microanalysis at beam energies lower than those conventionally used (< 10 keV) is known to significantly improve the spatial resolution for compositional analysis. However, the reduction in the beam energy which reduces the X-ray interaction diameter also introduces analytical difficulties and constraints which can diminish the overall analytical performance. This paper critically assesses the capabilities and limitations of performing low beam energy, high spatial resolution X-ray microanalysis. The actual improvement in the spatial resolution and the reduction in the X-ray yield are explored as the beam energy is reduced. The consequences for spectral interpretation, quantitative analysis and imaging due to the lower X-ray yield and the increased occurrence of X-ray line overlaps are discussed in the context of currently available instrumentation.

AEM of the Solute Redistribution During Discontinuous Dissolution

High spatial resolution X-ray microanalysis of thin foils in a transmission electron microscope has been used for the first time to examine the residual solute content profiles resulting from the discontinuous dissolution reaction in a Ni-4 at. % Sn alloy. The sharp change in the Sn content, detected performing the line scans across the reaction front, indicated that the solute redistribution occurred just at the reaction front. The Sn profiles measured along the reaction front were the reverse of those typical for the discontinuous precipitation reaction, with a relatively higher excess amount of tin stored close to the position where the Ni3Sn lamellae had previously existed. The obtained results also suggest that the chemistry and shape of profiles are not only dependent on the dissolution temperature, but that the initial state of the single cell just before the beginning of the discontinuous dissolution reaction plays an important role.

High Spatial Resolution X-ray Microanalysis of Radiation-Induced Segregation in Proton-Irradiated Stainless Steels

AbstractThe spatial redistribution of alloying elements and impurities near grain boundaries in several stainless steel alloys arising from non-equilibrium processes have been measured by analytical electron microscopy (AEM) in a field emission scanning transmission electron microscope. Radiation-induced segregation (RIS) has been shown to result in significant compositional changes at point defects sinks, such as grain boundaries. The influence of irradiation dose and temperature, alloy composition, prior heat treatment, and post-irradiation annealing on the grain boundary composition profiles have been investigated. Understanding the importance of these microchemical changes relative to the radiation-induced microstructural change in irradiation-assisted stress corrosion cracking (IASCC) of the irradiated materials is the primary goal of this study.

The Impact of EDS In Materials Science Microanalysis

Since its invention in 1968, the EDS has played an essential role in X-ray analysis of materials, at the micrometer level, in the electron probe microanalyzer (EPMA). In the EPMA, the characteristic X-ray intensity from bulk specimens is sufficient that, despite its very small collection angle, the wavelength dispersive spectrometer (WDS) can also be used. Given the excellent energy resolution of the WDS it has often been the spectrometer of choice for bulk quantitative X-ray microanalysis. Therefore, the most important role of the EDS has been in X-ray microanalysis of thin specimens in the analytical electron microscope (AEM) because, in an AEM, the limited confines of the stage mean that EDS is the only viable spectrometer. Since the pioneering work of Cliff and Lorimer in the 1970s, EDS has been the method by which all high spatial resolution X-ray microanalysis of thin foils has been performed.

Compositional inhomogeneity in electroless-deposited CoNiP films studied by spin-echo 59Co nuclear magnetic resonance

A compositionally inhomogeneous state in electroless-deposited CoNiP films with perpendicular magnetic anisotropy is investigated using spin-echo 59Co nuclear magnetic resonance (NMR), focusing an its correlation with magnetic properties. By adjusting pH of the electroless-deposition bath, films were prepared which possess almost the Same average composition (Co 42Ni 50P 8) and crystal orientation, but the differing values of coercivity ( H c). By analyzing the shift in peak resonance frequency of NMR spectra of the CoNiP films with respect to that of bulk CoNi alloys, we observed the formation of a compositionally inhomogeneous state in the film which possesses high H c of 2600 Oe, whereas formation of inhomogeneities was not observed for the low- H c films. By assuming that a Co-enriched ferromagnetic component detected by NMR was separated from a non-magnetic amorphous NiP component, we found that the high- H c, film consists of Co 42Ni 26 and Ni 24P 8. Moreover, the results of high spatial resolution X-ray microanalysis an the high- H c film showed a tendency that the Co-enriched ferromagnetic component segregates to the center of grains and the NiP non-magnetic component segregates to grain boundaries. Such evidence indicates that compositional inhomogeneities could be one of the origins of high- H c in CoNiP films.

High spatial resolution x-ray microanalysis of interfaces

X-ray microanalysis in the analytical electron microscope (AEM) refers to a technique by which chemical composition can be determined on spatial scales of less than 10 nm. There are many factors that influence the quality of x-ray microanalysis. The minimum probe size with sufficient current for microanalysis that can be generated determines the ultimate spatial resolution of each individual microanalysis. However, it is also necessary to collect efficiently the x-rays generated. Modern high brightness field emission gun equipped AEMs can now generate probes that are less than 1 nm in diameter with high probe currents. Improving the x-ray collection solid angle of the solid state energy dispersive spectrometer (EDS) results in more efficient collection of x-ray generated by the interaction of the electron probe with the specimen, thus reducing the minimum detectability limit. The combination of decreased interaction volume due to smaller electron probe size and the increased collection efficiency due to larger solid angle of x-ray collection should enhance our ability to study interfacial segregation.

High-spatial-resolution microanalysis in the analytical electron microscope: a tutorial

High spatial resolution x-ray microanalysis in the analytical electron microscope (AEM) describes a technique by which chemical composition can be determined on spatial scales of less than 50 nm. Dependent upon the size of the incident probe, the energy (voltage) of the beam, the average atomic number of the material being analyzed, and the thickness of the specimen at the point of analysis it is possible to measure uniquely the composition of a region 2-20 nm in diameter. Conventional thermionic (tungsten or LaB6) AEMs can attain direct spatial resolutions as small as 20 nm, while field emission (PEG) AEM's can attain direct spatial resolutions approaching 2 nm. Recently, efforts have been underway to extract compositional information on a finer spatial scale by using massively parallel Monte Carlo electron trajectory simulations coupled with AEM measurements. By deconvolving the measured concentration profile with the calculated x-ray generation profile it is possible to extract compositional information at near atomic resolution.

Radiation-induced segregation in austenitic stainless steels

Radiation-induced segregation (RIS) is driven by fluxes of point defects to sinks. RIS can induce composition fluctuations in irradiated stainless steels, which can result in microstructural and property changes, including precipitation, austenite instability, strengthening, embrittlement, and irradiation-assisted sensitization and stress corrosion cracking. Analytical electron microscopy (AEM) provides a powerful technique to study such segregation. RIS in several irradiated stainless steels has been investigated. AEM was performed in a Philips EM400T/FEG equipped with an EDAX 9100/70 analysis system. The specimens were neutron irradiated to 15 displacements per atom (dpa) at 520 ° C in the Fast Flux Test Facility (FFTF) and were only mildly radioactive (&lt;50/μCi = 1.85 MBq), thus permitting high spatial resolution X-ray microanalysis to be employed. Typical acquisitions were performed for 100 s in the STEM mode with &lt;2-nm-diam probes containing &gt;0.5 nA current. Subtraction of “in-hole” spectra from the measured spectra corrected for both the normal “in-hole” counts and those associated with the radioactivity of the specimen.

Determination of the Fe−Ni phase diagram below 400°C

The phase diagram for the Fe−Ni system below 400°C has been determined experimentally in the composition range from 0 to 52 wt pct Ni using analytical electron microscopy techniques. High spatial resolution X-ray microanalysis and electron diffraction were conducted on the Fe−Ni regions of meteorites. Both stable and metastable phase boundaries were defined. Our phase diagram is consistent with the available theoretical diagram in that firm experimental evidence was found for a miscibility gap and an associated, asymmetrical spinodal decomposition region. The spinodal decomposition resulted in a two-phase, isotropic microstructure, as expected. The miscibility gap is a metastable construction arising from the presence of a tricritical point due to magnetic interactions. Our experimental diagram differs from the theoretical diagram in three ways. First, observations of meteorite structures show that Fe−Ni solid solution containing 4.0 wt pct Ni is in local equilibrium with ordered FeNi containing 51.4 wt pct Ni and not Ni3Fe as in the theoretical diagram. Second, our miscibility gap below 400°C, located between 11.7 and 51.9 wt pct Ni at 200°C, is wider than the calculated miscribility gap, especially at the high Ni end. Third, we also find evidence for an ordered structure around ∼25 wt pct Ni. This structure may be either Fe3Ni or a two-phase structure incorporating ordered FeNi.