Explanation of grain refinement by direct observation of dislocation morphology evolution in undercooled DD3 superalloy

The application of a molten salt denucleating technique combined with thermal cycles enables the as-solidified structure of DD3 superalloy to be investigated over a wide range of undercooling up to 250 K. The microstructure formation has been discussed with respect to undercooling, nucleation and recalescence as well as recrystallization during recalescence and post-recalescence, respectively. Microstructure transition from the coarse-grained dendrites to the grain-refined equiaxed microstructures is observed in DD3 superalloy melt when ΔT>ΔT∗. Furthermore, TEM and SEM technique are employed to reveal morphologies of crystal defects such as grain boundary and dislocation, aiming at explaining recrystallization mechanism for grain refinement occurring beyond the critical undercooling ΔT∗, and demonstrating the influence of ϒ′ precipitation and solute segregation on recrystallization.