TMA Study on Phase Evolution During Hydrogen-Assisted Disproportionation of Nd-Fe-B Alloy

Phase evolution during the hydrogen-assisted disproportionation of Nd 12.5 Fe 80.6 B 6.4 Ga 0.3 Nb 0.2 alloy was investigated mainly by using a magnetic balance-type thermomagnetic analyser (TMA). In order to avoid any undesirable phase change in the course of heating for TMA, a swift TMA technique with very high heating rate (around 2 min to reach 800 ℃ from room temperature) was adopted. The hydrided Nd 12.5 Fe 80.6 B 6.4 Ga 0.3 Nb 0.2 alloy started to be disproportionated in hydrogen from around 600 ℃, and the alloy after the early disproportionation (from 600 to 660 ℃) has been partially disproportionated. The partially disproportionated alloy consisted of a mixture of NdH x , Fe₃B, α-Fe, and the remaining undisproportionated Nd₂Fe₁₄BH x -phase. During the subsequent heating to 800 ℃ in hydrogen, two additional phases of Fe₂₃B? and Fe₂B were formed, and the material consisted of a mixture of NdHx, Fe₂₃B?, Fe₃B, Fe₂B, and α-Fe phases. During the subsequent isothermal holding at 800 ℃ for 1 hour, the phase constitution was further changed, and one additional unknown magnetic phase was formed. Eventually, the fully disproportionated Nd 12.5 Fe 80.6 B 6.4 Ga 0.3 Nb 0.2 alloy consisted of NdHx, Fe₂₃B?, Fe₃B, Fe₂B, α-Fe, and one additional unknown magnetic phase.