Gas-atomized Nd-Fe-B droplets of non-peritectic chemical compositions Nd16Fe76B8, Nd17Fe74.5B8.5, and Nd18Fe73B9 were undercooledand solidified in an 8mdrop tube filled with helium. The microstructure of the received particleswas investigated by means of scanning electronmicroscopy and powder X-ray diffractometry. It was found that in each alloy composition, in excess of 71% of the particle population showsa microstructure comprising primary Nd2Fe17Bx dendrites plus interdendritic Nd2Fe14B grains plus intergranular Nd1.1Fe4B4 and Nd-richgrains, where the metastable Nd2Fe17Bx dendrites are partially or completely decomposed into a mixture of α-Fe plus Nd2Fe14B depending onparticle size. There is often a certain amount of Nd2Fe14B dendrites in these particles, resulting in a dual microstructure. On the other hand, therest of the particle population shows a microstructure consisting of primary Nd2Fe14B grains, either columnar or dendritic, plus intergranularNd1.1Fe4B4 and Nd-rich phase grains. Additionally, a tiny amount of α-Fe dendrites is formed in some of the particles independent of thetype of the microstructure. They are exclusively distributed near the particle surface, and are surrounded either by Nd2Fe14B grains or byNd2Fe17Bx grains in individual particles.

Nd14Fe69Co10B7 alloys were levitated, melted and solidified using the electromagnetic levitation technique for the purpose of studying theeffect of cobalt addition on phase selection in undercooled Nd-Fe-B alloy melts. Bulk undercoolings of up to 80K were achieved in thelevitated melt droplets. Similar to cobalt-free alloys, λ-(Fe, Co) solid solution, χ-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 theprimary formation of the ф-phase and of the x-phase were determined to be 50 and 65 K, respectively. Compositional analysis showed thatthe λ-phase has a lower cobalt concentration than that of the bulk alloys, whereas the ф-and λ-phases have a cobalt concentration close tothat of the bulk alloys. However, the cobalt concentrations of all the three phases do not change significantly with the bulk undercooling ofthe samples.

Nd–Fe–B alloys with composition of Nd16 Fe76 B8, Nd18 Fe73 B9 and Nd22 Fe67 B11 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 Nd2 Fe14 B compound, whereas those with large undercoolings were solidified by the primary formation of the metastable Nd2 Fe17 Bx compound (x～1) plus the subsequent formation of Nd2 Fe14 B. The critical undercooling for the primary Nd2 Fe17 Bx formation was determined to be 40, 70 and K in the three alloy compositions, respectively. The liquidus temperature of the metastable Nd2 Fe17 Bx compound was estimated to be 1423, 1393 and 1283K, respectively. The Nd Fe B compound was found to decompose into a mixture of Fe plus Nd Fe B due to the slow cooling rates of the samples. It was suggested that the metastable Nd Fe B compound may have a lower interfacial energy than that of the stable Nd Fe B compound, hence being favored in nucleation-controlled phase selection at large undercoolings.

Bulk Nd14Fe79B7 alloy droplets were processed using electromagnetic levitation technique for the purpose of studying their metastable solidification behavior at significant melt undercoolings. The results show that γ-Fe solid solution, Nd2Fe14B compound and the metastable Nd2Fe17Bx compound were solidified as primary phase in sequence of increasing bulk undercooling level. The critical undercoolings were determined to be 45 K and 60 K, respectively. Following primary γ-Fe formation, the Nd2Fe17Bx compound was solidified peritectically prior to the Nd2Fe14B compound. However, primary Nd2Fe14B formation was quite predominant over the whole sample volume. In case of primary Nd2Fe17Bx formation, the Nd2Fe14B compound was solidified also in a peritectic manner. The metastable Nd2Fe17Bx compound was found to decompose into α-Fe plus Nd2Fe14B during the post-solidification process. The phase selection mechanisms were discussed in terms of in-situ observations on the solidification process.