In order to measure changes in composition as a function of distance (macrosegregation) in directionally solidified two phase samples, a well collimated incident x-ray beam is required for XRF analysis. This is accomplished using Bragg diffraction of AgKα radiation from a highly perfect Si crystal. Because the incident beam is also monochromatic, additional advantages are realized: a) the backgrounds caused by Compton and thermal diffuse scattering (TDS) of the incident beam are well localized in the energy spectrum and do not interfere with the fluorescent peaks, b) the TDS can be used as a monitor of the incident photon flux and hence eliminates often substantial errors caused by incident beam intensity fluctuations.
Using several prepared standards, the ratio of PbL counts to TDS counts was found to be a function of the total Pb content of the two phase microstructure, with a reproducibility determined only by counting statistics. Furthermore, the function was found to be nearly linear over a wide range of compositions. Standard methods of absorption or enhancement correction can be employed using this ratio. The spatial resolution, determined by profiling a sharp discontinuity between two metals, was 0.5 mm.
Macrosegregation data is presented for Pb-Sn two phase alloys whose compositions range from 35 wt % Pb to 70wt % Pb. Comparison of compositions with those determined by a titration method agrees to within 2 wt % for most of the metallurgical structures present in the work. Somewhat larger deviations were found for samples with high Pb contents with extremely coarse two phase microstructures.