Solidification of levitated Nd–Fe–B alloy droplets at significant bulk undercoolings

Abstract

Nd–Fe–B alloys with composition of Nd 16Fe 76B 8, Nd 18Fe 73B 9 and Nd 22Fe 67B 11 were melted and solidified using an electromagnetic levitation technique. Two types of solidification behavior were observed depending on the bulk undercooling achieved prior to solidification. The samples with small undercoolings were solidified by the predominant primary formation of the Nd 2Fe 14B compound, whereas those with large undercoolings were solidified by the primary formation of the metastable Nd 2Fe 17B x compound ( x∼1) plus the subsequent formation of Nd 2Fe 14B. The critical undercooling for the primary Nd 2Fe 17B x formation was determined to be 40, 70 and 130 K in the three alloy compositions, respectively. The liquidus temperature of the metastable Nd 2Fe 17B x compound was estimated to be 1423, 1393 and 1283 K, respectively. The Nd 2Fe 17B x compound was found to decompose into a mixture of Fe plus Nd 2Fe 14B due to the slow cooling rates of the samples. It was suggested that the metastable Nd 2Fe 17B x compound may have a lower interfacial energy than that of the stable Nd 2Fe 14B compound, hence being favored in nucleation-controlled phase selection at large undercoolings.