Grain refinement during rapid solidification of undercooled melts of pure Ni via twinning-assisted recrystallization

Multiscale stresses and microstructure of three samples of pure Ni glass-fluxed and solidified at different undercoolings were investigated using time-of-flight neutron diffraction (ToFND), electron back-scatter diffraction (EBSD), and high-resolution transmission electron microscopy (HRTEM) to understand the mechanism of grain refinement in undercooled melts. ToFND measurements showed that the highest levels of macroscopic stresses in the samples are close to each other whereas the highest levels of intergranular stresses in samples with undercoolings of ΔT = 107 K and ΔT = 196 K are higher than that in a sample with a tiny undercooling. EBSD analysis confirmed grain refinement in the undercooled samples. Grain refinement is incomplete in the sample with ΔT = 107 K as indicated by a bimodal microstructure. In this microstructure, Σ3-type twin boundaries take a higher frequency in a coarse-grained part than in a fine-grained part. HRTEM investigation unveiled dislocation cells, stacking faults and nanotwins near a zigzagged interface. Observations of these defects uncover a twinning-assisted dynamic recrystallization mechanism of grain refinement in undercooled pure Ni. A high level of intergranular stresses in the undercooled samples suggests that dynamic recrystallization is most likely to be driven by thermal stresses accumulated during rapid solidification. Because thermal stresses are more rapidly accumulated in the sample with ΔT = 196 K than in the sample with ΔT = 107 K, they drive a dynamic recrystallization process throughout the sample volume bringing about complete grain refinement. The role of the same mechanism in inducing grain refinement in other materials is discussed.