Motion and remelting of dendrite fragments during directional solidification of a nickel-base superalloy

The formation of spurious grains during the directional solidification of a Ni-base superalloy is studied by modeling the movement and remelting of dendrite fragments originating in channels inside the mush. Such channels exist because of thermosolutal convective instabilities during solidification and persist as freckle chains in the solidified material. The fragment model is linked to a phase equilibrium subroutine for multicomponent Ni-base superalloys, as well as to a previously developed solidification and convection code. A parametric study is performed to investigate the effects of initial fragment location and size on the fragment paths and survivability in the melt for one of the channels predicted in a typical directional solidification simulation. It is found that only a small window of initial conditions exists which leads to spurious grain formation. This window corresponds to medium-sized fragments originating near the mouth of the channel. Other fragments either remelt completely or sink into the channel. The need for an accurate fragment generation model is discussed.