BINARY FE-ND METASTABLE PHASES IN THE SOLIDIFICATION OF FE-ND-B ALLOYS

It has been shown that a binary Fe-Nd metastable phase A1 is responsible for coercivity of congruent-to 5 kOe in Fe80Nd5B as-cast alloys. It is rapidly transformed into Fe14Nd2B after a very short annealing at 600-degrees-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 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 Fe4NdB4, liquid, Fe14Nd2B, and Nd, which may reach the binary Fe-Nd eutectic, giving rise to different metastable phases, depending on the cooling rate: A1, A'1 or Fe17Nd2. A new version of the liquidus projection of the Nd-rich corner of the ternary phase diagram is presented.