Research of grain refinement in undercooled DD3 single crystal superalloy

The structure evolution of commercial DD3 (TM) (1) single crystal superallo y has been systematically investigated within the achieved range of underco oling 0-210K. The grain structure of the alloy can be refined notably if it solidifies in a range of lower undercooling 30-70 K or above the critical undercooling (DeltaT* = 180 K). These two kinds of grain refinements arise ill completely different ways. Based on current dendrite growth theory, a t hermodynamic concept, dimensionless superheating, is adopted to evaluate th e tendency of the dendrite remelting. With the increase of undercooling, th e dimensionless superheating of the alloy increases first and then decrease s, which suggests that the dendrite remelting driven by recalescence superh eating should only be responsible for the grain refinement at lower underco oling range. Whereas the decrease of grain size above DeltaT* is attributed to the high strain energy and lattice deformation energy that originate fr om the extremely rapid solidification, and lead to the dendrites distortion and disintegration. Subsequently, the recrystallization occurring in cooli ng could enable the grain size to decrease further. Dislocation morphology evolution in as-solidified structure is also provided by TEM technique to v erify the recrystallization mechanism. (C) 2001 Elsevier Science B.V. All r ights reserved.