Stress-induced recrystallization mechanism for grain refinement in highly undercooled superalloy

Microstructure transition from the coarse-grained dendrites to the grain-refined equiaxed microstructures is observed in DD3 superalloy melt with an undercooling above ΔT*. An analytical approach to stress development in the coherent dendrite network during rapid solidification of the undercooled melt is described. With the increment of undercooling, production of large stresses is predicted as a result of shrinkage-induced interdendritic fluid flow originating from rapid solidification (ΔT > ΔT*). A comparison of the calculated stress with the measured shear strength of mushy zones shows that it is very probable for the stresses to exceed the strength, resulting in distortion, fragmentation or collapse of the dendrite network. Combined with the lattice deformation energy stored in the solid structure during rapid solidification, recrystallization occurring in cooling process could make the grain size decrease further. TEM and SEM techniques are employed to verify the recrystallization mechanism for grain refinement above ΔT*.