One step towards MnAl-based permanent magnets - Differences in magnetic, and microstructural properties from an intermediate annealing step during synthesis

The influence of an additional annealing step during synthesis on the preparation of MnAl based permanent magnet alloys has been investigated. Bulk samples of Mn55Al45C2 alloys were synthesized using induction heating (t)hrough drop synthesis from 1400 degrees C. Samples produced using cooling directly from 1400 degrees C (from the melt), and from 1400 degrees C to an intermediate annealing step at 1200 degrees C for similar to 30 min before cooling were compared with respect to differences in phase purity, microstructure and magnetic properties. We found that the phase purity was significantly enhanced using the route with an intermediate annealing step at 1200 degrees C. From XRD the relative content of the tau-phase was improved from similar to 91 wt% for the sample cooled directly from 1400 degrees C to similar to 95.1-99.5 wt% for the sample exposed to an intermediate annealing step before cooling. Additionally, EBSD, and SEM with EDS indicates a clear difference in the phase composition and differences in the distribution of the magnetic tau-phase and the non-magnetic epsilon-, beta-, and gamma(2)-phases. Magnetic properties also indicate an improvement in saturation magnetization for the sample exposed to the extra annealing step during synthesis. Our results suggest that an intermediate annealing step in the production of MnAl based alloys will provide a simple way of achieving better phase purity and magnetic properties in the bulk alloy.