Microstructure Selection Involving Nucleation in Directionally Solidified Nd-Fe-B Peritectic Alloys

Directional solidification experiments were performed on Nd14Fe79B7 alloys to investigate microstructure formation related to nucleation. It was found that a small amount of α-Fe dendrites with equiaxed morphology existed in specimens grown at 200 and 500 μm/s. A banded structure showing alternate growth of α-Fe and Nd2Fe14B dendrites, was formed in the specimen at 500 μm/s. The formation of the equiaxed morphology microstructure or the banded structure is attributed to the phase nucleation ahead of the solidifying interface, since an undercooled zone corresponding to γ-Fe liquidus would appear ahead of the growing γ-Fe interface at the growth velocity of 200 and 500 μm/s. However, at growth velocity of 500 μm/s, only an undercooled zone corresponding to Nd2Fe14B liquidus would appear. These undercooled zones cause the nucleation of γ-Fe and Nd2Fe14B phases, resulting in the different microstructure patterns. The theoretical predictions are in good agreement with the experimental results.