Local X-ray diffraction analysis of the structure of dendrites in single-crystal nickel-base superalloys

The structure of the dendrites in the single-crystal nickel-base superalloys SC16, SRR99 and CMSX4 with different refractory element levels (Mo + Ta + W + Re) has been investigated by local X-ray diffraction. A special technique was used to improve the spatial resolution of the X-ray diffraction and to enable the precise control of the X-ray spot position within the dendritic structure. A significant change of the gamma/gamma'-lattice misfit was found within the dendrite in the superalloys with higher refractory element levels SRR99 and CMSX4. The observed misfit change is based on the change of the gamma-lattice parameter due to segregation of W and Re. The intensity of the X-ray beam reflected from the dendrite periphery was Found to be weaker than that from the dendrite centre because of the mosaicity. Therefore misfit measurements without knowledge of the X-ray spot position in the dendritic structure lead to values that correspond more to the dendrite core. (C) 1997 Acta Metallurgica Inc.