Light-Element EPMA—Correlations, convolutions, and true concentrations

Analyses of small concentrations of light elements by electron microprobe are complicated by a number of factors, including low signal strength and line overlap. Obtaining reliable data may require techniques that are uncommon in wavelength-dispersive (WDS) x-ray analysis. A case in point is the investigation of alloys that contain small amounts of nitrogen. Nitridation of Al metal and Al alloys with N-bearing gaseous species is being studied as a way to form in situ nitride precipitates, which is of interest for altering the mechanical properties of these materials. It was found, for example, that one specific alloy with 1.7 wt% Ti and 7.7 wt% Y retains much larger N concentrations on nitridation than pure Al, using the same processing conditions for both alloys. Nitrogen may be present in TiAl3 precipitates, YA13 precipitates, or the matrix. Identifying the N-bearing compound by EPMA has proven difficult, however, because of the interference of Lℓ on N Ka, combined with the low peak/background ratio of the N Ka peak.