Low Voltage EDX

Low-voltage EDX Spectrum-imaging of Complex Solid-state Reacting Systems

Background:

X-ray microanalysis of bulk planar specimens in the electron probe microanalyzer (EPMA) and of electron transparent specimens in the analytical transmission electron microscope (AEM) provides quantitative characterization of the composition of small volumes with µm- and nm-scale spatial resolutions, respectively. Although AEM holds a clear advantage in spatial resolution, the bulk specimen geometry of the EPMA provides numerous advantages relative to electron transparent specimens regarding versatility and robustness of analysis, including: ease of specimen preparation, particularly for composite materials comprised of hard and soft phases; substantially wider field of view (cm² vs. µm²), which is critical for characterizing low-volume fraction features and for statistical sampling; many fewer specimen preparation artifacts; and a standard specimen geometry with well developed quantification methods. X-ray microanalysis in the scanning electron microscope at low operating voltages (<5 kV) extends the spatial resolution for analysis of bulk specimens by an order of magnitude relative to EPMA, allowing acquisition of characteristic spectra from 100-nm-scale phases and detection of 10-nm-scale phases. When combined with energy-dispersive X-ray (EDX) spectrum-imaging and multivariate statistical analysis techniques that have been developed in our laboratory, low-voltage X-ray microanalysis provides a powerful phase mapping capability for bulk specimens with 100-nm-scale spatial resolution.

Accomplishment:

Low-voltage EDX spectrum-imaging has been used for phase mapping of complex solid-state reacting systems. The synthesis route is based on "in-place" oxidation of multi-phase alloy systems to form nanophase metal-ceramic or ceramic-ceramic composite functional surfaces. A sample microstructure is shown, which results from the nitridation of a two-phase alloy composed of a chromium-rich matrix and a chromium-niobium Laves phase intermetallic. The intermetallic reacts more quickly than the matrix with the gaseous nitrogen, and provides a preferential diffusion path of this oxidizing specie in the alloy. A 200 x 200 pixel EDX spectrum image was acquired in the vicinity of the chromium — chromium nitride interface using an operating voltage of 3 kV. Four distinct phases are identified in the spectrum-image with the help of multivariate statistical analysis methods: a chromium-rich solid solution (brown), a niobium-enriched nitride phase (red), a chromium nitride phase (green) and chromium oxide particles (blue). Clearly evident are 100-nm-scale secondary precipitates of the niobium-enriched nitride dispersed throughout the chromium and chromium nitride matrix phases. Low voltage EDX spectrum-imaging provided rapid characterization of the near-interface region of this multiphase composite, which could have been achieved neither by EPMA nor AEM.