P Michaud

Determination of Diffusion Coefficients with Quantitative X-Ray Microanalysis at High - Spatial Resolution

This paper present the determination of concentration profiles of an Mg Al diffusion couple that was obtained with a high resolution field emission scanning electron microscope, the Hitachi SU-8000 equipped with a SDD EDS detector. From these concentration profiles, the inter-diffusion coefficient is determined with the Boltzmann-Matano technique. The advantages and disadvantages of working at high and low beam energy for quantitative x-ray microanalysis are highlighted. The f ratio method is used in this work to convert the x-ray intensities into composition.

MC X-Ray, The Monte Carlo Program for Quantitative Electron Microscopy of Real Materials

MC X‐Ray [1] is a new Monte Carlo program that allows quantitative electron microscopy of real materials. This program is an extension of the Monte Carlo programs Casino [2] and Win X‐Ray [3] since it computes the complete x‐ray spectra from the simulation of electron scattering in solids of various types of geometries. This new program, which has been completely reprogrammed in C++ under a window environment, is a real improvement because Win X‐Ray is only able to compute x‐ray spectra of homogeneous materials and CASINO performs only the computation of net x‐ray intensities in a limited set of geometries. MC X‐Ray allows more than 100 different regions in the materials having shape of spheres, cylinders and combinations of horizontal and vertical planes. All these regions can have a different composition. As an example, simulated 128 X 128 images of a 20 nm thin foil of a 50 (wt.) % B Fe alloy with a 20 nm square W phase (left) and a 20 nm square Cr phase (right) at 200 keV with 50 e per pixels were performed. Figure [1] shows the B  generated intensity map. The length of the x scan was twice the length of the y scan and this explains the rectangular shape of the W and Cr phases. Figure [2] shows the B emitted intensity map and absorption effects are seen towards the x‐ray detector located towards the top of the image with a 20° TOA. Figure [3] shows the Fe  emitted intensity map and absorption effects are negligible because of its higher photon energy. Figure [4] shows a dark field image. Bright field images can also be simulated as well as High Annular Dark Field Images with a choice of collection angles from the user. Details to obtain Mc X‐Ray are given at http://montecarlomodeling.mcgill.ca/.

Improvement in the Characterization of the 2099 Al-Li Alloy by FE-SEM

This paper describes how state-of-the-art Field-Emission Scanning Electron Microscopy (FE-SEM) can contribute to the characterization of the 2099 aluminum-lithium alloy, and metallic alloys in general. Investigations were carried out on bulk and thinned samples. BSE imaging at 3kV and STEM imaging at 30kV along with highly efficient microanalysis permitted to correlate experimental and expected structures. Although our results confirm previous studies, this work points out possible substitutions of Mg and Zn with Li, Al and Cu in the T1 precipitates. Zinc and magnesium are also present in “rice grain” shaped precipitates at the grain boundaries. The versatility of the FE-SEM is highlighted in that it can provide information at the macro and micro scales with relevant details. Its ability to probe the distribution of precipitates from nano-to micro-sizes throughout the matrix makes Field-Emission Scanning Electron Microscopy a suitable technique for the characterization of metallic alloys.

Quantitative X-Ray Microanalysis of Real Materials in Electron Microscopy with Monte Carlo Simulations

Electron microscopy has two main emerging areas of applications, low voltage field emission scanning electron microscopy (FE-SEM) and high voltage field emission transmission electron microscopy (FETEM) with spherical aberration correctors. In the first case, the reduction of the electron beam energy leads to improved spatial resolution for x-ray microanalysis but when the beam energy is too low, the yield of generated x-rays becomes too small. In the second case, a probe diameter of the order of 1 Å with high current can be obtained with a spherical aberration corrector. Coupled with the high energy of the electron beam that penetrates almost in a straight line through the transparent material, high spatial resolution can be achieved. However, beam damage and specimen preparation issues are still of concerns. Focus Ion Beam (FIB) has helped in fixing problems related to specimen preparation for FE-TEM. But, since the FIB allows making thin films of different materials, imaging in FE-SEM in the STEM mode is now possible, as well as x-ray microanalysis in these conditions. Therefore, we now have to see x-ray microanalysis as a characterization technique that can be used between 3 to 300 keV of electron beam energy on complex materials having various shapes and phases and or regions of different composition. This paper presents applications of MC X-Ray, a new Monte Carlo program that allows simulating x-ray spectra in materials of various phases of different composition and size withelectron beam energy between 1 to 400 keV. Results for quantitative x-ray microanalysis obtained in a FE-SEM (Hitachi SU8000) at low beam energy and in a FE-STEM (Hitachi HD-2700 with Cs corrector) at 200 keV will be presented. As an example of the capabilities of MC X-Ray, Figure [1] shows a 400 X 400 C map simulated for a 1 μm W sphere on top of bulk C at 20 keV with 50 e/pixel with a TOA of 40°. The absorption of the C in the W sphere is clearly visible. Since the variation of C intensity is due to absorption variation, not composition variation, correct modeling of x-ray absorption with Monte Carlo simulations is required for accurate quantitative x-ray microanalysis. Figure [2] shows the peak to background ratio of the C line and the Pd and Sn lines for a 2 nm Pd – Sn spherical particulate on top of a 10 nm carbon nanotube as a function of beam energy. Optimum beam energies can be predicted. 256 doi:10.1017/S1431927610054425 Microsc. Microanal. 16 (Suppl 2), 2010