The Influence of the Disproportionated Microstructure on the Hydrogen Ductilisation Process (HyDP) for NdFeB Alloys

Recently, researchers at the University of Birmingham have been investigating a technique for inducing ductility in solid NdFeB alloys at room temperature, termed the Hydrogen Ductilisation Process [1,2]. The process utilises the high temperature (>650ºC) solid hydrogen disproportionation [3] reaction, to transform the Nd₂Fe₁₄B matrix phase into a mixture of NdH₂, Fe₂B and Fe. It has been shown that once in this condition, the disproportionated mixture can be significantly deformed at room temperature. Once deformed, this material can be recombined, with evidence that the deformation in the intermediate step produces a degree of magnetic alignment after recombination.

HyDP was initially limited by the presence of a minority NdFe4B4 phase, which failed to fully disproportionate during hydrogen processing. Previous work has been done to alter initial microstructures to limit the quantity of this phase [4,5] with some success. However, this had an influence on the disproportionated microstructures that formed during hydrogen processing, and mechanical behaviours at room temperature.

The work presented here uses a multi-stage disproportionation process to significantly alter the morphology of the disproportionated microstructures, creating a series of samples ranging from fine lamellar structures to coarse spherical. It is observed that, with increased coarseness of the disproportionated microstructure the yield stress required to deform the material can be significantly improved, decreasing by ~30%, greatly improving the ductile behaviour. Furthermore, deformation caused the coarsened spherical NdH₂ to elongate in the disproportionated microstructure, influencing the kinetics for recombination and the final magnetic anisotropy.