Phase identification of microfeatures using EPMA methods, especially high-resolution X-ray spectroscopy

Methods of electron-probe microanalysis (EPMA), with some input from scanning and transmission electron microscopy (SEM/TEM), are applied for the identification of micro-scale constituents in a solid matrix. The subject of the study is a magnesium alloy composite, which contains silicon carbide-based fibres made by a liquid metal infiltration process. Backscattered electron imaging of the composite in the SEM showed that during composite manufacture, fibres were chemically attacked by the metal, many of the fibres exhibiting three distinct grey levels, indicative of different reaction zones, and others appearing uniformly black. EPMA measurements showed that each region contained approximately 12wt.% oxygen and that penetration of the fibre by magnesium was accompanied by a reduction in the concentration of silicon and carbon. From studying the position and shape of specific X-ray lines it was shown that magnesium penetration involved a chemical reaction with silicon oxycarbide, established in earlier EPMA studies as one of the fibre constituents. Also, in the outermost region, aluminium in the alloy reacted with free carbon in the fibre to form aluminium carbide and with magnesium to produce a Mg-Al intermetallic. The composition of black fibres was quite different from the grey ones, with negligible silicon and only a small amount of aluminium. Oxygen levels in black fibres were consistent with complete oxidation, indicating these fibres were subjected, locally, to severe oxidising conditions during composite manufacture. In the metal matrix itself, particles of a mixed magnesium/aluminium oxide, silicon carbide and magnesium silicide were observed, the latter two phases forming as silicon and carbon were ejected from fibres.