Complete ductility in NdFeB-type alloys using the Hydrogen Ductilisation Process (HyDP)

Abstract The work outlined in this paper demonstrates significant improvements in ductility during the Hydrogen Ductilisation Process (HyDP) [1] of a Neomax-type alloy and, for the first time, complete ductility of a sub-stoichiometric alloy. In previous studies, the uniformity of the ductilisation transformation was limited by the presence of the NdFe4B4 phase not undergoing complete disproportionation, which left extremely brittle, micron-scale islands in a highly ductile disproportionated matrix. The present works investigated the influence of a significantly longer (18 h) hydrogen treatment of the Neomax-type alloy to observe whether the completely disproportionated NdFe4B4 phase is as ductile as the disproportionated matrix. It has been observed that, when the NdFe4B4 phase is completely disproportionated, it exhibits similar ductility to that of the disproportionated matrix, showing no sign of cracking under a compressive load. However, inhomogeneity in the book mould material leads to varying quantities of NdFe4B4 phase and thus, varying levels of disproportionation. Despite this, significant improvements were observed in the mechanical behaviour of the Neomax-type alloy. The longer disproportionation treatment also results in larger grains and thus, a lower final coercivity in the recombined state than that of the previous 5 h treatment. For comparison, the mechanical behaviour of a sub-stoichiometric alloy, which did not contain any NdFe4B4 phase, has also been studied. The compression behaviour of this sub-stoichiometric alloy in the disproportionated state was found to be completely ductile. It is clear from this work that it is possible to create a completely ductile NdFeB material directly from the solid cast alloy which, in the future, could have significant implications for NdFeB magnet production.