Secondary dendrite arm coarsening and peritectic reaction in NdFeB alloys

Directional solidification experiments were performed on hyperperitectic Nd_13.5Fe_79.75B_6.75 alloy using a Bridgman type furnace. The relationship between secondary dendrite arm spacing (λ₂) of primary γ-Fe phase as a function of growth velocity (V) was established. Experimental results show that λ₂ decreases with increasing V and give a constant value λ₂V^0.33 of 5×10^-7 m^1.33 s^-0.33. Using arm coarsening theory, secondary arm spacing of γ-Fe dendrites was calculated, during which temperature gradient changing with growth velocity in the experiment was taken into account. The calculation gives a constant value λ₂V^0.31 of 8×10^-7 m^1.31 s^-0.31. Theoretical predictions are in reasonable agreement with experimental results. In peritectic reaction, the dissolution rate of γ-Fe dendrite arms varied from 0.0061 to 9.1 μm/s, as the growth velocity increased from 1 to 100 μm/s. The dissolution rate of γ-Fe dendrites was extremely small in low-velocity specimens with plane front peritectic interfaces.