Search

Rare-earth-free MnAl-C-Ni permanent magnets have been produced for the first time by extruding powders milled from bulk. The resulting materials, fabricated using different conditions, contained a large volume fraction (> 0.92) of the desired τ-phase. In terms of the maximum energy product, the best performance obtained for a whole, transverse section of the extruded material was (BH) max = 46 kJm−3, and was (BH) max = 49 kJm−3 for a sample taken from near the edge of this section. Analysis showed that this material was comparable to the long-established benchmark, comprising MnAl-C-based magnets extruded in industry from bulk or from gas-atomised powder. Such materials are no longer available. The microstructure of the materials produced here consisted of fine, recrystallised grains, which exhibited an <001> fibre texture with intermediate texture quality and of larger, non-recrystallised regions, which contained hierarchical twinning and a high density of defects. The volume fraction and size of the non-recrystallised regions was greatly reduced by decreasing the size of the initial powder particles. This led to a large increase in the squareness factor of the demagnetisation curve and consequently to the high (BH) max values observed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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 NdFe 4 B 4 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 NdFe 4 B 4 phase is as ductile as the disproportionated matrix. It has been observed that, when the NdFe 4 B 4 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 NdFe 4 B 4 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 NdFe 4 B 4 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. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a process that develops texture in nanocrystalline permanent magnet alloys is presented. An originally isotropic material is passed through a high up-temperature gradient, inducing directional grain growth. Texture development by directional annealing of melt-spun Sm12Co88, (Sm12Co88)99Nb1, (Sm12Co88)99C1, and (Sm12Co88)98Nb1C1 alloys was examined. Samples directionally annealed were compared with conventionally annealed samples. Strong (006) in-plane texture was observed by X-ray diffraction in Sm12Co88 and (Sm12Co88)99Nb1 alloys and the anisotropy was corroborated by magnetic measurements (magnetic texture ∼20–53%). Directional annealing produced only slight texture in the (Sm12Co88)99C1 and (Sm12Co88)98Nb1C1 alloys. The development of texture is critically dependent on annealing temperature, the up-temperature gradient, translational velocity, and alloy composition. The activation energy for anisotropic grain growth was estimated to be ∼28 and ∼42kJmol−1 for Sm12Co88 and (Sm12Co88)99Nb1, respectively. These results indicate that directional annealing as a route to texture development in nanocrystalline permanent magnet alloys is a feasible process. [Copyright &y& Elsevier]