Containerless solidification of undercooled NdFeZrB alloy droplets in a drop tube

Abstract

Containerless solidification of undercooled Nd13Fe79.9Zr0.1B7 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.