Heat treatment of Metastable Nd₂Fe₁₇B_x Phase Formed from Undercooled Melt of Nd-Fe-B Alloys

Abstract

Alloys of Nd₁₀Fe₈₅B₅, Nd₁₂Fe₈₂B₆, and Nd₁₄Fe₇₉B₇ were melted and then solidified under the containerless state using a drop tube. In the as-dropped sample, a large amount of the Nd₂Fe₁₇B_x was contained as a metastable phase. Particularly in relatively large samples, the metastable phase, which was partially decomposed into the Nd₂Fe₁₄B and <font face="symbol">a</font>-Fe phases, shows a microstructure similar to that of normalized low-carbon steel. The heat treatment of the as-dropped samples induces a dual- stage phase transformation. The 1st stage of the phase transformation, which is dominant in the Nd₁₄Fe₇₉B₇ alloy, is the diffusive phase transformation from Nd₂Fe₁₇B_x and Nd-rich phases into the Nd₂Fe₁₄B phase at 950 K, and the 2nd stage of the transformation, which is dominant in the Nd₁₀Fe₈₅B₅ alloy, is the decomposition of the metastable phase into the Nd₂Fe₁₄B and <font face="symbol">a</font>-Fe phases at 1100 K. Although the rate of the phase transformation in either stage is controlled by the diffusion of Nd atoms, the diffusivity at the 1st stage of the phase transformation, being large due to the existence of the Nd-rich phase adjacent to the Nd₂Fe₁₇B_x phase, results in a decrease in transformation temperature.