Philippe T. Pinard

Characterization of dual-phase steel microstructure by combined submicrometer EBSD and EPMA carbon measurements.

Electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) measurements are combined to characterize an industrial produced dual-phase steel containing some bainite fraction. High-resolution carbon mappings acquired on a field emission electron microprobe are utilized to validate and improve the identification of the constituents (ferrite, martensite, and bainite) performed by EBSD using the image quality and kernel average misorientation. The combination eliminates the ambiguity between the identification of bainite and transformation-induced dislocation zones, encountered if only the kernel average misorientation is considered. The detection of carbon in high misorientation regions confirms the presence of bainite. These results are corroborated by secondary electron images after nital etching. Limitations of this combined method due to differences between the spatial resolution of EBSD and EPMA are assessed. Moreover, a quantification procedure adapted to carbon analysis is presented and used to measure the carbon concentration in martensite and bainite on a submicrometer scale. From measurements on reference materials, this method gives an accuracy of 0.02 wt% C and a precision better than 0.05 wt% C despite unavoidable effects of hydrocarbon contamination.

Secondary fluorescence in electron probe microanalysis of material couples

We describe a semi-analytical method for the fast calculation of secondary fluorescence in electron probe microanalysis of material couples. The calculation includes contributions from primary K-, L- and M-shell characteristic x-rays and bremsstrahlung photons. The required physical interaction parameters (subshell partial cross sections, attenuation coefficients, etc) are extracted from the database of the Monte Carlo simulation code system PENELOPE. The calculation makes use of the intensities of primary photons released in interactions of beam electrons and secondary electrons. Since these intensities are not readily available and do not allow analytical calculation, they are generated from short Monte Carlo simulation runs. The reliability of the proposed calculation method has been assessed by comparing calculated, distance-dependent k-ratios with experimental data available in the literature and with results from simulations with PENELOPE. Numerical results are found to be in close agreement with both simulated and experimental data.

Electron Probe Microanalysis of Ni Silicides Using Ni-L X-Ray Lines

We report electron probe microanalysis measurements on nickel silicides, Ni5Si2, Ni2Si, Ni3Si2, and NiSi, which were done in order to investigate anomalies that affect the analysis of such materials by using the Ni L3-M4,5 line (Lα). Possible sources of systematic discrepancies between experimental data and theoretical predictions of Ni L3-M4,5 k-ratios are examined, and special attention is paid to dependence of the Ni L3-M4,5 k-ratios on mass-attenuation coefficients and partial fluorescence yields. Self-absorption X-ray spectra and empirical mass-attenuation coefficients were obtained for the considered materials from X-ray emission spectra and relative X-ray intensity measurements, respectively. It is shown that calculated k-ratios with empirical mass attenuation coefficients and modified partial fluorescence yields give better agreement with experimental data, except at very low accelerating voltages. Alternatively, satisfactory agreement is also achieved by using the Ni L3-M1 line (Lℓ) instead of the Ni L3-M4,5 line.

Towards Reliable Quantification of Steel Alloys at Low Voltage

Whether it be inclusions, precipitates or segregation at grain boundaries, the development of new steel alloys and processes hinges on the chemical quantification of submicrometer features. The combined requirements for high spatial resolution and accurate measurements of several elements make field emission electron microprobes suitable instruments for studying steel alloys. For quantitative analyses, the spatial resolution is determined by the x-ray emission volume the dimensions of which depend on the sample composition, the beam energy and the overvoltage ratio. For most elements, lowering the beam energy directly translates into the necessity of using lower energy x-ray lines. However, the improvement of the spatial resolution comes with additional challenges, mainly a lower intensity (worst statistics), a larger influence of contamination and oxidation, and inaccuracies of the quantification.

Electron probe microanalysis of Ni-silicides at low voltage: difficulties and possibilities

Interest in the use of EPMA at low voltage has grown considerably in recent years, mainly because of the availability of electron-beam instruments equipped with field-emission guns. However, EPMA at low voltage is marred by both experimental and analytical problems which may affect the accuracy of quantitative results. In the case of the analysis of transition elements, both the emission and absorption of X-rays are still poorly understood when they originate from electron transitions involving the partially filled 3d-shell. This is the case for the most intense Lα (L3-M5 transition) and Lβ (L2-M4 transition) lines. In this communication, we point out anomalies which appear to afflict the accuracy of EPMA of Ni-silicides using the Ni-Lα X-ray line and we discuss possible solutions.

Low voltage EPMA: experiments on a new frontier in microanalysis - analytical lateral resolution

Field emission (FE) electron gun sources provide new capabilities for high lateral resolution EPMA. The determination of analytical lateral resolution is not as straightforward as that for electron microscopy imaging. Results from two sets of experiments to determine the actual lateral resolution for accurate EPMA are presented for Kα X-ray lines of Si and Al and Lα of Fe at 5 and 7 keV in a silicate glass. These results are compared to theoretical predictions and Monte Carlo simulations of analytical lateral resolution. The experiments suggest little is gained in lateral resolution by dropping from 7 to 5 keV in EPMA of this silicate glass.

Analytical Challenges and Strategies in FE-EPMA

Unquestionably, since their introduction one decade ago, field emission (FE) electron microprobes have pushed the boundaries of electron probe microanalysis (EPMA) by offering new possibilities to characterize smaller features. As with conventional microprobes, the proper selection of the analytical conditions (beam energy, beam current, acquisition time, x-ray line) is essential to obtain an accurate and precise quantification. With FE microprobes, the new challenge is to optimize these parameters to obtain the best spatial resolution. Our previous work [1] has shown that, as each analytical condition has a different influence on the accuracy, precision and spatial resolution, a compromise must be found depending on the characterization goals. In continuation, this work aims at evaluating the influence of instrumental parameters, such as the focusing capability, beam stability and stage reproducibility, on high resolution acquisitions, and proposing strategies to perform reliable and automated measurements.

Application of Monte Carlo Calculations to Improve Quantitative Electron Probe Microanalysis

The generation of x rays from solid samples under electron bombardment is the basis of electron probe microanalysis (EPMA), a widely used technique for materials analysis. The problem of quantitative analysis, i.e. the transformation of the measured x-ray intensities into element concentrations, is generally solved by means of approximate, semi-empirical algorithms. These algorithms provide reliable results for samples that are homogeneous at the micron scale, and, with suitable modifications, they can also be used for thin films. However, when the sample volume from which x-rays are generated is not homogeneous (e.g. small particles, multilayer films, lamellae structures...) or under unconventional measurement conditions (e.g. low voltage, low overvoltage, oblique incidence, standardless analysis...) the legitimacy of the simplifications underlying conventional quantification algorithms is not firmly established and there is a need for more realistic procedures. This need has lead to the use of the Monte Carlo simulation (MC) method.

Electron probe microanalysis of carbon containing steels at a high spatial resolution

Whether it be inclusions, precipitates, segregation at grain boundaries, heterogeneous distribution of phases or constituents with complex morphologies, the development of new advanced high strength steels and manufacturing processes relies on the chemical characterization of their submicrometer features. More than any other alloying element, carbon plays a determining role, controlling the microstructure and the mechanical properties of these alloys. This work establishes acquisition and quantification strategies to measure the carbon concentration in low and high alloy steels by electron probe microanalysis at a high spatial resolution, circumventing many challenges associated with the measurement of this element. The optimal experimental conditions in terms of accuracy, precision and spatial resolution are determined using an optimization algorithm based on Monte Carlo simulations. Applications of the developed method for dual phase, transformation induced plasticity and twinning induced plasticity steels are presented. Out of the challenges, the carbon contamination and the quantification using soft X-rays are further elaborated in order to better understand their factors and mechanisms. The influence of the sample preparation, anti-contamination devices and experimental parameters on the carbonaceous deposits produced by a focused electron beam is studied based on specifically designed experiments and numerical simulations. The results support the hypothesis of the sample as the predominant source of contaminants, the low absorption and desorption rate of organic molecules on/from the sample surface, and the existence of a diffusion limited regime under the typical experimental conditions of a modern electron microprobe equipped with oil-free vacuum pumps. The quantification problems using soft X-rays, especially for the L3–M4,5 X-ray transition of first transition series metals, are investigated using Fe−Ni binary alloys. The effect of chemical bonding on the X-ray intensity is observed even for these metallic alloys. This work shows that the use of empirical mass attenuation coefficients and transition probability ratios, calculated from experimentally measured X-ray intensity vs. accelerating voltage curves, reduces the discrepancies between the experimental and calculated k-ratios, at least for this binary system.

Progress in a New Method of Thickness Measurement by X-ray Analysis in TEM

A new method for measuring mass thickness ( t )sp, of a thin film specimen in a transmission electron microscope (TEM) uses a pre-calibrated thin film reference standard to avoid the need to measure beam current [1]. The beam current just needs to be stable for the duration of the analysis session and a single X-ray spectrum acquired from the reference film allows mass thickness and elemental composition to be determined from all subsequently acquired X-ray spectra. The mass thickness is automatically incorporated into the X-ray absorption correction. Mass thickness for an element is also proportional to the areal density (atoms/m 2 ) and can be used in estimation of beam broadening, modelling of image contrast, EELS quantification and determination of defect density.