On the origin of sliver defects in single crystal investment castings

The origin of sliver defects in seeded single crystal castings has been determined to be the deformation of dendrites in the mushy zone through microstructural characterization of a series of castings with a geometry sensitive to these defects. The extent of bending and torsion of the dendrites in the as-cast microstructure and the net misorientation was quantified from electron backscattered diffraction data using a bespoke data analysis method. At the point of initiation of the sliver defects, deformation was localized at the mould wall, indicating that these defects arise as a result of the bending moments generated by differential thermal contraction between the mould and the dendrites. The comparative susceptibilities of dendrites to deformation under converging and diverging growth with respect to the mould wall were distinguished. The experimental observations were supported by continuum finite element simulations using ProCAST™, which confirmed the occurrence of high stresses in the constricted channel where sliver defects form. Contrary to foundry wisdom, ancillary observations demonstrated that the oxide at the seed melt-back interface played no role.