Phase selection in undercooled Nd–Fe–Co–B alloy droplets

Nd14Fe69Co10B7 alloys were levitated, melted and solidified using the electromagnetic levitation technique for the purpose of studying the effect of cobalt addition on phase selection in undercooled Nd–Fe–B alloy melts. Bulk undercoolings of up to 80 K were achieved in the levitated melt droplets. Similar to cobalt-free alloys, γ-(Fe, Co) solid solution, φ-Nd2(Fe, Co)14B compound, and the metastable χ-Nd2(Fe, Co)17Bx compound (x~1) were solidified primarily in sequence of the increasing bulk undercooling. The critical undercoolings for the primary formation of the φ-phase and of the χ-phase were determined to be 50 and 65 K, respectively. Compositional analysis showed that the γ-phase has a lower cobalt concentration than that of the bulk alloys, whereas the φ- and χ-phases have a cobalt concentration close to that of the bulk alloys. However, the cobalt concentrations of all the three phases do not change significantly with the bulk undercooling of the samples.