Microstructure evolution of peritectic Nd₁₄Fe₇₉B₇ alloy during directional solidification

Bridgman type directional solidification experiments were carried out in Nd₁₄Fe₇₉B₇ alloy. Microstructure evolutions along with growth velocities were investigated. At low velocities from 1 to 200 μm/s, the microstructures consisted of leading α-Fe dendrites and peritectic Nd₂Fe₁₄B phase in the interdendritic region. At medium velocities of 425 and 500 μm/s, an incomplete banded structure formed, containing circles of α-Fe/Nd₂Fe₁₄B-Nd₂Fe₁₄B dendrites. At high velocities of 1, 3 and 5 mm/s, a transition from γ-Fe dendrites to Nd₂Fe₁₄B dendrites occurred. Phase selection phenomena were theoretically studied using the extremum criterion which assumes growth-controlled phase selection. The theoretical results are in agreement with experimental results at low velocities of 1-200 μm/s and high velocities of 3 and 5 mm/s. The incomplete dendritic bands at 425 and 500 μm/s were explained by nucleation of Nd₂Fe₁₄B phase at the γ-Fe dendritic interface, and growth competition between γ-Fe and the nucleated Nd₂Fe₁₄B phase. Phase transition from γ-Fe to Nd₂Fe₁₄B at 1 mm/s was interpreted by abundant nucleation and quick growth of Nd₂Fe₁₄B phase at the γ-Fe dendritic interface.