L10 ordered τ-MnAl near the equiatomic composition was mentioned in the past as a possible rare earth free compound for permant magnet applications due to its high saturation magnetisation and uniaxial anisotropy. This phase is metastable and forms from the disordered hexagonal ɛ-phase during quenching from high temperatures or annealing at low temperatures.
Our research on this system is focused on the interplay of crystal structure and chemical composition on the intrinsic magnetic properties (saturation magnetisation and Curie temperature) as well as the impact of microstructure and processing on the extrinsic magnetic properties (coercivity, remanence).
In the binary Mn-Al system pure τ can only be synthesised with over-stoichiometric Mn contents. The saturation magnetisation (Fig. 1) and hence the theoretical limit for the maximum energy density is highly sensitive to the Mn content. Therefore the search for alloying elements which improve the intrinsic magnetic properties, for example by allowing τ formation at a Mn:Al ratio of 1, is an important factor in the development of rare earth free permanent magnets with energy densities in the range of 100 kJ/m3.
Fig. 1: Impact of Mn content on the saturation magnetisation for binary τ-MnAl.
A detailed knowledge of the influence of microstructure on the extrinsic magnetic properties is critical for the design of permeant magnets with improved permenant magnetic performance. At IFW we study the microstructure of MnAl-based alloys at different stages during processing to identify potential weak points, which may limit the development of well textured MnAl-based magnets with high coercivity. Using electron backscattered diffraction (EBSD) twin-like defects were observed which are the dominant feature in the microstructure (Fig. 2). The magnetically easy c-axis in the subgrains on either side of a twin-like defect has a large misorientation. This limits the formation of a high quality fibre texture and thus limits the remanence of the material. Metallurgical strategies for the elimination of this kind of defect are therefore likely to result in an increase in performance.
Fig.2: Inverse pole figure orientation map derived from EBSD measurements (left) and interface distribution of Mn54Al46 (right). Defects with a large misorientation of the c-axis are marked red in the right image.