Solidification defects in directionally solidified nickel base superalloys: problem solving using electron backscattered diffraction

Abstract

The production of high quality turbine blades from nickel base superalloys poses a challenging problem for the aeroengine and power generation industries. The occurrence of defects in the final single crystal component has become more common with the increasing complexity of the components in the aerospace sector and the increasing size of the blades in industrial gas turbines. In parallel, alloy development has improved the operating capabilities of these parts, but these new generation alloys have encountered more casting difficulties. The defects that occur are normally identified by metallographic and X-ray diffraction techniques. These approaches are limited by the subjectivity of determining subtle changes in contrast and by the averaging over the relatively large sampling volumes of the X-ray beam. Electron backscattered diffraction (EBSD) provides a direct approach for identifying the nature of the defects formed in these castings. This paper will present an EBSD driven assessment of problems encountered in directional solidification of nickel base superalloys: the formation of stray grains in seeded single crystals and the effect of alloy composition on the evolution of orientation. The origins of these defects will be considered with respect to classical grain growth theory.