Metastable solidification of NdFeB alloys by drop-tube processing

Metastable solidification of small droplets of Nd_11.8Fe_82.3B_5.9 and Nd₁₄Fe₇₉B₇ alloys was performed in an 8-m drop tube filled with helium. The results showed that the solidification path of the droplets depends on the alloy composition and on the droplet size. For Nd_11.8Fe_82.3B_5.9 alloy, larger droplets are solidified by primary iron formation and subsequent Nd₂Fe₁₄B crystallization from the residual liquid phase, whereas smaller ones tend to be frozen by metastable primary Nd₂Fe₁₇B_x growth (x = 0–1). A similar transition of the solidification path from primary iron formation to primary Nd₂Fe₁₇B_x formation occurred in Nd1₄Fe₇₉B₇ alloy with reducing droplet size. However, metastable primary growth of Nd₂Fe₁₄B was also observed within a wide droplet size range prior to the appearance of the metastable Nd₂Fe₁₇B_x phase. Nucleation and growth of different phases were considered to produce an explanation of the observed phase selection phenomena in these two alloys.