Critical undercoolings for different primary phase formation in Nd15Fe77.5B7.5 alloy

Nd₁₅Fe_77.5B_7.5 alloys were melted and solidified using the electromagnetic levitation technique. Bulk undercoolings of up to 78 K were achieved. Three kinds of primary phase formation were observed depending on the undercooling level. γ-Fe solid solution was solidified as primary phase at low undercoolings. It was replaced by the peritectic φ-Nd₂Fe₁₄B compound upon reaching a critical undercooling of 40 K. The primary phase was changed further to the metastable χ-Nd₂Fe₁₇B_x (x = 0 - 1) compound above a second critical undercooling of 55 K. However, γ-Fe formation reentered the samples with high undercoolings because of a strong recalescence. The χ-Nd₂Fe₁₇B_x compound was found to have decomposed during the post-solidification process, which also gave rise to a small amount of γ-Fe in the solidified samples. Nevertheless, it was found that the total amount of the γ-Fe phase in the samples with high undercoolings has been reduced due to the primary solidification of the two compound phases.