High-temperature hydrogen-induced phase transformation in Nd-Fe-B alloys

The disproportionation of Nd-Fe-B alloys has been thoroughly studied in connection with its importance in the well-known Hydrogenation Disproportionation Desorption Recombination (HDDR) process. However, little attention has been paid to the Dy containing Nd-Fe-B alloys. We present here a study of the evolution of the microstructure according to the disproportionation temperature on both Dy-rich and Dy-free Nd-Fe-B based alloys. Thanks to a combined implementation of XRD, SEM, TEM and WDS-EDS techniques, we could evidence that, under a hydrogen pressure of 80 kPa, a partial recombination occurs at high temperature prior to the full desorption of hydrogen and the complete recombination of the alloy. A phase transformation of the type ${\mathit{REH}}_{2\pm x}+\mathit{Fe}+{\mathit{Fe}}_{2}B\to {\mathit{RE}}_{1+\epsilon }{\mathit{Fe}}_4{B}_4+{\mathit{RE}}_2{\mathit{Fe}}_{14}B$${\mathit{REH}}_{2\pm x}+\mathit{Fe}+{\mathit{Fe}}_{2}B\to {\mathit{RE}}_{1+\epsilon }{\mathit{Fe}}_4{B}_4+{\mathit{RE}}_2{\mathit{Fe}}_{14}B$ is evidenced at high temperature, where RE (Rare Earth) stands for either Nd, Pr or Dy. To the best of our knowledge such a phase transformation has not been previously reported in the literature. This phase equilibrium occurs simultaneously with a coarsening of the microstructure. The phase transformation temperatures are identified by a TGA/DTA analysis. It points to a dependence of the extension of the stability domain of this newly referenced phase equilibrium on the Dy-content in the initial alloy. This study paves the way for the determination of phase diagrams under hydrogen for Nd-Fe-B alloys.