Metallographic samples with transverse orientations within the columnar zone about 3 to 4 inches (75– 100 mm) above the chill were selected and prepared for electron microprobe analyses. The selection of transverse orientations allowed observations on dendritic structures with fixed orientations. Structures were well-developed with relatively large dendrite arm spacings at locations where samples were removed. Figure 8 is a backscattered electron image of the dendrites. The arms in the plane of the photomicrograph are secondary arms. Primary arms are perpendicular to the plane of the photomicrograph and are located at the intersections of secondary arms.
The distribution of alloying elements around dendrite arms was qualitatively evaluated with x-ray mapping techniques using wavelength dispersive spectroscopy (WDS). In the images of Figure 9, lighter coloured areas contain higher concentrations of the element being observed, and darker areas are regions of lower concentrations of the element under observation. The images presented in Figure 9 indicate that gold, zinc, and silicon are uniformly distributed throughout the microstructure. The gold and zinc data is reasonably acceptable. However, the silicon is present in such low concentrations that this analytical technique is neither valid nor appropriate. On the other hand, copper was found to be present at the core of dendrites at higher concentrations. Interdendritic locations are leaner in copper. Silver behaves in a reverse manner. Dendrite cores are depleted in silver and interdendritic areas are rich in silver. The x-ray maps correlate well with optical and backscattered images. However, x-ray mapping provides specific information about which elements are in a particular segregation pattern whereas optical and backscattered images do not identify the segregating elements that are responsible for distribution patterns in a particular array of dendrite arms.