Binary Fe-Nd metastable phases in the solidification of Fe-Nd-B alloys

摘要

It has been shown that a binary Fe-Nd metastable phase, A(sub 1), is responsible for coercivity of (approx-equal) 5 k O e in Fe 80 Nd 5B as-cast alloys. It is rapidly transformed into Fe(sup 14) Nd(sub 2) B after a very short annealing at 600 deg C and it is also present in alloys of magnet composition. Thus it may account for the lower coercivity of as-sintered magnets. here we discuss how can this phase can be formed in the solidification of Fe-Nd-B magnets. The microstructure and thermal events of DTA samples of constant 60 at%Nd were used to construct a vertical section of the Fe-Nd-B ternary phase diagram. The solidification of alloys related to magnet compositions did not follow the path predicted by the known phase diagrams which leads to a ternary eutectic. Instead, they pass through a transition reaction, involving Fe(sub 4) Nd B(sub 4), liquid, Fe(sub 14) Nd(sub 2) B and Nd, which may reach the binary Fe-Nd eutectic, giving rise to different metastable phases, depending on the cooling rate: A(sub 1), A(sub 1) or fe(sub 17) Nd(sub 2). A new version of the liquidus projection of the Nd-rich corner of the ternary phase diagram is presented. (author) 12 refs., 3 figs.