High-Resolution Diffraction Imaging of Misorientation in Ni-Based Single Crystal Superalloys

Abstract

In the present work, the novel high-resolution X-ray diffraction technique for imaging misorientation and mosaicity in single crystal superalloys is introduced. The technique is based on classical X-ray diffraction topography geometries combined with high-resolution diffraction and post-processing of obtained data including color coding and 3D projections of each diffraction images. For the investigations, the single crystal rods were produced from CMSX-4 superalloy at a withdrawal rate of 1, 3, 5, and 7 mm⋅min−1. The high spatial and angular resolution of the method allows to visualize the complex nature of mosaicity present in single crystal superalloys. It was observed that mosaics blocks differ in size and misorientation on the multi-scale level from the small present in dendrite arms, through single dendrites, group of dendrites, up to subgrains. Measurements of misorientation were done on cross and longitudinal sections of the castings. It was proved that increasing withdrawal rate influences the mosaicity structure and mechanisms of its evolution. Solidification at withdrawal rates from 1 to 5 mm⋅min−1 possesses higher misorientation between dendrites. With higher misorientation between dendrites, when the widening of solidification front occurs, the mosaicity spreads across the casting in the form of misoriented dendrite lines as the solidification progress by growing long secondary dendrite arms. Also, it was stated that 3–5 mm⋅min−1 rates possess a higher possibility of creation of subgrains or selector grains defects in the casting.