R value measurement issues for grain boundaries in nickel single crystal castings

Abstract

Cast single crystal (SX) nickel superalloy components for modern gas turbine engines are rarely free from grain boundaries. The application relevant properties are grain boundary sensitive, so component specific acceptance standards are applied that include limits for the misorientation between grains across a boundary, the R value. R value calculation requires the determination of the orientations of adjacent grains, both measured using the same arbitrary reference system. Comparisons of extant definitions for this parameter revealed both variations and inconsistencies. Uncertainties for R value measurements were established within a European round robin study using surface back reflection X-ray Laue systems. For some boundaries, a position dependency of the value of R was found. In some specimens, electron backscattered diffraction studies revealed continuous changes in orientation within the grains, as the point examined was moved away from the boundary, providing an explanation of the observed position dependency. Further investigations linked spot splitting and smearing in Laue patterns to different intragrain substructures.