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

Methods of electron-probe microanalysis (EPMA), with some input from scanni ng and transmission electron microscopy (SEM/TEM), are applied for the iden tification of micro-scale constituents in a solid matrix. The subject of th e study is a magnesium alloy composite, which contains silicon carbide-base d fibres made by a liquid metal infiltration process. Backscattered electro n imaging of the composite in the SEM showed that during composite manufact ure, fibres were chemically attacked by the metal, many of the fibres exhib iting three distinct grey levels, indicative of different reaction zones, a nd others appearing uniformly black. EPMA measurements showed that each reg ion contained similar to 12 wt.% oxygen and that penetration of the fibre b y 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 sil icon oxycarbide, established in earlier EPMA studies as one of the fibre co nstituents. Also, in the outermost region, aluminium in the alloy reacted w ith free carbon in the fibre to form aluminium carbide and with magnesium t o produce a Mg-Al intermetallic. The composition of black fibres was quite different from the grey ones, with negligible silicon and only a small amou nt of aluminium. Oxygen levels in black fibres were consistent with complet e oxidation, indicating these fibres were subjected, locally, to severe oxi dising conditions during composite manufacture. In the metal matrix itself, particles of a mixed magnesium/aluminium oxide, silicon carbide and magnes ium silicide were observed, the latter two phases forming as silicon and ca rbon were ejected from fibres. (C) 2000 Elsevier Science Ltd. All rights re served.