SEM/EDX Spectrum Imaging and Statistical Analysis of a Metal/Ceramic Braze
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SPECTRUM IMAGING AND STATISTICAL ANALYSIS OF A METAL/CERAMIC BRAZE
PAUL G. KOTULA,' MICHAEL R. KEENAN' AND IAN M. ANDERSON2 2'Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185-1405 Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6376 ABSTRACT Energy dispersive x-ray (EDX) spectrum imaging has been performed in a scanning electron microscope (SEM) on a metal/ceramic braze to characterize the elemental distribution near the interface. Statistical methods were utilized to extract the relevant information (i.e., chemical phases and their distributions) from the spectrum image data set in a robust and unbiased way. The raw spectrum image was over 15 Mbytes (7500 spectra) while the statistical analysis resulted in five spectra and five images which describe the phases resolved above the noise level and their distribution in the microstructure. INTRODUCTION Spectrum imaging, where a complete x-ray spectrum is collected at every pixel in a scanned image, is a powerful new tool for materials characterization. Spectrum imaging differs from mapping, where only windows around pre-selected energy ranges are acquired, in that the entire spectrum is collected at each pixel, as the beam is rastered across the specimen. This procedure allows even unanticipated elements to be mapped after the data has been collected. However, mapping by itself is potentially fraught with errors [1]. Additionally, the spectrum image cannot readily be visualized in its entirety. For these reasons, a robust and unbiased method is needed to extract all of the relevant information from these large raw data files. Ideally, one would like a spectrum describing the elemental distribution, and a corresponding image describing the distribution, for each phase in the microstructure. In the present work we describe the application of multivariate statistical analysis (MSA) to a spectrum image of a metal/ceramic braze. EXPERIMENTAL The braze characterized in this work joins polycrystalline alumina to Kovar (primarily Fe, Ni and Co) with a copper-silver eutectic alloy containing some titanium (an 'active' metal). The specimen geometry consisted of a sandwich of two pieces of alumina, two braze layers and a Kovar filler layer in the middle. The entire assembly was heat-treated to bond the interfaces and then a polished cross-section was prepared. The interface between the alumina and the coppersilver alloy was characterized in this work. The analysis was performed in a Philips XL30-FEG SEM operated at 10 kV, equipped with an Oxford super-ATW detector and XP3 pulse processor. The specimen was normal to the electron beam and the take-off angle was 35'. EDX spectrum images were acquired with an EMiSPEC Vision integrated acquisition system. A spectrum image of 100 x 75 pixels with 1000 channels per spectrum (10 eV/channel) was acquired from the specimen with a sampling density of 200 nm/pixel and a dwell time of I live second per pixel, for a total acquisition time of -2 hours. The binary data file was imported into MATLAB where the sta
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