Rare-earth-free MnAl-C-Ni permanent magnets produced by extrusion of powder milled from bulk

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 tau-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. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.