Grain refinements in undercooled DD3 single crystal superalloy

The structure evolution of DD3 single crystal superalloy was investigated by the method of molten salt denucleating combined with thermal cycle. Within the achieved range of undercooling as 0-210 K, the solidification structure undergoes two grain refinements. Because of the dendrite break-up or ripening owing to remelting, the first grain refinement occurs in a certain range of undercooling as 30-70 K. The dendrite growth is dominantly controlled by solute diffusion, and the remelting of primary dendrite evaluated by adopting the calculation of the dimensionless superheating in recalescence is most serious. With the enhancement of undercooling, solute trapping that results from high dendrite growth velocity weakens the effect of solute diffusion on the dendrite growth. The second grain refinement occurs with the decrease of the grain size above the critical undercooling as 180 K. It is attributed to the stress originated from the extremely rapid solidification process, which results in dendrite distortion, disintegration and recrystallization.