Containerless solidification of undercooled NdFeZrB alloy droplets in a drop tube

Containerless solidification of undercooled Nd₁₃Fe_79.9Zr_0.1B₇ alloy droplets was investigated using a 3-m drop tube. It was found that the solidification mechanism of droplets depends strongly on their sizes. Due to small undercoolings and high cooling rates, large droplets (≥300 μm) are solidified via an incomplete peritectic reaction, thus leading to a large amount of preserved primary iron dendrites. However, most of small droplets (<300 μm) are solidified by the primary crystallization of Nd₂Fe₁₄B phase owing to enhanced undercooling levels. As a result, a microstructure consisting of coarse dendritic Nd₂Fe₁₄B grains is formed. It was therefore suggested that large undercooling prior to solidification is able to avoid the formation of primary free iron in near-stoichiometric NdFeB alloys, thus providing a promising way to prepare master alloys for the fabrication of magnets with enhanced energy products.