X-ray powder diffraction evidence for the incorporation of W and Mo into M 23 C 6 extracted from high-temperature alloys
X-ray powder diffraction patterns of M23C6 where M=(Cr, Mo) and M=(Cr, W) have been recorded accurately using a focusing Guinier camera and a diffractometer. The improved resolution of the modern focusing diffraction equipment made it possible to detect new features in the powder diffraction patterns, showing that M23C6 carbides extracted from W- and Mo-containing alloys differ from pure Cr23C6 and (Cr, Fe)23C6 met in more conventional stainless steels. The distinctions concern the lattice parameters, as well as the intensities of specific diffraction maxima. These differences can be explained in terms of the theoretical considerations of Goldschmidt who predicted that W and Mo, when incorporated in Cr23C6, will preferentially occupy specific positions in the unit cell. Diffraction intensity calculations indeed confirm that the introduction of W and Mo atoms on these particular positions will modify the diffracted intensities in the same way as was observed experimentally.