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 super alloys 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 p osition within the dendritic structure. A significant change of the ga mma/gamma'-lattice misfit was found within the dendrite in the superal loys with higher refractory element levels SRR99 and CMSX4. The observ ed 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 refle cted from the dendrite periphery was Found to be weaker than that from the dendrite centre because of the mosaicity. Therefore misfit measur ements without knowledge of the X-ray spot position in the dendritic s tructure lead to values that correspond more to the dendrite core. (C) 1997 Acta Metallurgica Inc.