Formation of low angle boundaries in Ni-based superalloys

An experimental study has been used to determine the influence of liquid undercooling ahead of dendrite tips on dendrite growth and the accompanying evolution of misorientation resulting in formation of low angle grain boundaries during solidification of a Ni-base superalloy, CMSX-4. The experiment was designed whereby two dendrite envelopes formed by initial branching of the primary front converged along horizontal platforms perpendicular to the withdrawal direction. The magnitude of the undercooling was increased by increasing the length of the platform, ranging from 10 to 60 mm. The position of the dendrite envelope was calculated using a computational thermal model, ProCAST. Misorientation, measured using Electron Back Scattered Diffraction (EBSD), was generally <2.5 degrees, which was consistent with previous studies of dendrite growth in the constrained condition. However, in the longest platform a monotonic increase in misorientation in one dendrite envelope developed leading to formation of a grain boundary of 10 degrees. This monotonic increase was not dependent on the local undercooling ahead of the dendrite envelope and was instead caused by mechanical moments arising from extensive lateral growth of unsupported tertiary dendrite branches growing laterally across the platform.