Microstructure evolution of undercooled DD3 single crystal superalloy

The structure evolution of DD3 single crystal superalloy was systematically investigated using the method of molten salt denucleating combined with thermal cycle, by which a substantial degree of undercooling up to 210 K was produced. Within the achieved range of undercooling 0-210 K, two grain refinements occur. At the lower range of undercooling 0-30 K, the dendrite growth is dominantly controlled by solute diffusion, and the solidified dendritic morphologies are similar to those of the conventional as- cast structure. The first grain refinement occurs in a certain range of undercooling 30-78 K, because of the dendrite break-up or ripening owing to remelting. At the higher range of undercooling 78-150 K, however, solute trapping that results from high dendrite growth velocity weakens the effect of solute diffusion on the dendrite growth. In this case, highly developed fine dendrite is formed as a result of the restrained ripening process. The decrease of the grain size above the critical undercooling (ΔT*=180 K) is attributed to the stress that originates from the extremely rapid solidification process, which results in the dendrite distortion, disintegration and recrystallization finally.