Large scatters occur in composition measurements using wavelength-dispersive X-ray spectroscopy (WDS) when only a few precipitates are captured in the X-ray sampling volume, because the measured signal represents an averaged composition of the precipitate and the matrix phase in the sampling volume. The scatters become small when sufficient numbers of precipitates are captured and solidification segregation in the sampling volume is not significant. Monte Carlo simulations were carried out to obtain the X-ray signals from the matrix gamma and precipitated gamma' phases in as-cast Ni-base superalloys using different beam sizes for given gamma' precipitate sizes. The optimum beam size in relation to the precipitate size can be predicted for accurate composition measurements.
- Solidification microstructure
- Wave-length dispersive X-ray spectroscopy
- Nickel alloys
- Volume averaging