Structure evolution in undercooled DD3 single crystal superalloy

A substantial undercooling up to 210 K was produced in the DD3 single crystal superalloy melt by employing the method of molten salt denucleating combined with thermal cycle, and the microstructure evolution with undercooling was systematically investigated. Within the achieved range of undercooling, 0–210 K, the solidification structure could be classified into four categories. At the lower range of undercooling, 0–25 K, the dendrite growth is dominantly controlled by solute diffusion, so the formed dendritic morphologies are similar to those of the conventional as-cast structure. With the increase of undercooling, the alloy undergoes two grain refinements. The first grain refinement occurs in a certain range of undercooling, 30–70 K, because of the dendrite break-up or ripening owing to remelting. At a higher range of undercooling, 78–150 K, however, the severe solute trapping that results from high dendrite growth velocity weakens the effect of solutal 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, i.e. the formation of fine equiaxed structure above the critical undercooling (ΔT*=180 K), can be attributed to the stress that originates from the extremely rapid solidification process, which results in the dendrite distortion, disintegration and recrystallization finally.