An investigation of the grain refinement in the bulk undercooled DD3 single-crystal superalloy is performed. The application of a molten salt denucleating technique combined with thermal cycle enables such investigation over a wide range of undercooling up to 210K. The microstructure formation has been discussed with respect to undercooling, nucleation and recalescence as well as recrystallization during recalescence and post-recalescence, respectively. Furthermore, TEM and SEM techniques are employed to reveal the recrystallization mechanism for grain refinement occurring at the critical undercooling DeltaT*, and to explain the influence of Y' precipitation and solute segregation on the recrystallization? i.e. grain-boundary migration process.