Tailoring precursor and medium of ball milling to optimize magnetic properties in Mn-Al bonded permanent magnets

Abstract Ball milled Mn-Al powders are ideal raw materials for fabricating bonded Mn-Al permanent magnets, and selecting proper precursor phase and milling medium for ball milling are of great importance to realize optimal magnetic properties in the final magnets. In this work, we studied the effect of different precursors and mediums of ball milling on the magnetic properties in the bonded magnets and the corresponding microstructural origins. e-phase as the precursor leads to a serious decomposition behavior during the subsequent annealing after ball milling, resulting in a strong reduction in magnetization in the powders but just slightly better coercivity, so τ-phase is a superior choice as the precursor. In the case that τ-phase is adopted as the precursor, the milling medium of ethanol (wet-milling) or argon (dry-milling) has a remarkable influence on the morphologies of the milled powders. The former condition can fabricate thin polycrystalline flakes, and form stronger texture inside simultaneously, while the latter condition produces isotropic polycrystalline particles. The bonded magnets prepared using wet-milled powders thoroughly show superior magnetic properties including magnetization, remanence and coercivity than the magnets based on dry-milled particles. This work may promote the understanding of the nanostructures of Mn-Al ball milled powders, and provide the reference for the fabrication of Mn-Al rare-earth-free bonded magnets. Prime Novelty Statement Ball milled Mn-Al powders are ideal raw materials for fabricating bonded Mn-Al permanent magnets, and selecting appropriate precursors and milling mediums for ball milling are important to obtain optimal magnetic properties in bonded magnets. In this work, ball milling with two precursors, e phase (high temperature phase) and τ phase (metastable ferromagnetic phase), are performed. Powders from τ-precursor after annealing treatment showing good properties (M1200kA/m of 73 Am2/kg and HC of 250 kA/m). While for e-precursor, during the subsequent annealing treatment for the transformation from e to τ, obvious decomposition occurs, inducing the distinct magnetization deterioration (36 Am2/kg) as well as slightly higher coercivity (331 kA/m). In comparison, the τ-precursor is superior to e-precursor. In case that τ-phase is adopted as the precursor, the milling medium of ethanol (wet-milling) and argon (dry-milling) has a remarkable influence on the morphologies of the milled powders. Wet-milling condition can fabricate polycrystalline thin flakes with stronger texture, while the dry-milling condition produces isotropic particles. The bonded magnets prepared using wet-milled powders thoroughly show superior magnetic properties including magnetization, remanence and coercivity than the magnets based on dry-milled particles. This work may promote the understanding of the nanostructures of Mn-Al ball milled powders, and provide the reference for the fabrication of Mn-Al rare-earth-free bonded magnets.