Research of grain refinement in undercooled DD3 single crystal superalloy

The structure evolution of commercial DD3^TM1 single crystal superalloy has been systematically investigated within the achieved range of undercooling 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 (τ2*= 180 K). These two kinds of grain refinements arise in completely different ways. Based on current dendrite growth theory, a thermodynamic concept, dimensionless superheating, is adopted to evaluate the tendency of the dendrite remelting. With the increase of undercooling, the dimensionless superheating of the alloy increases first and then decreases, which suggests that the dendrite remelting driven by recalescence superheating should only be responsible for the grain refinement at lower undercooling range. Whereas the decrease of grain size above τ2*is attributed to the high strain energy and lattice deformation energy that originate from the extremely rapid solidification, and lead to the dendrites distortion and disintegration. Subsequently, the recrystallization occuring in cooling could enable the grain size to decrease further. Dislocation morphology evolution in as-solidified structure is also provided by TEM technique to verify the recrystallization mechanism.