Formation of pure τ-phase in Mn–Al–C by fast annealing using spark plasma sintering

Mn–Al–C is intended to be one of the ‘‘gap magnets’’ with magnetic performance in-between ferrites and Nd-Fe-B. These magnets are based on the metastable ferromagnetic τ-phase with L1₀ structure, which requires well controlled synthesis to prevent the formation of secondary phases, detrimental for magnetic properties. Here, we investigate the formation of τ-phase in Mn–Al–C using Spark Plasma Sintering (SPS) and compare with conventional annealing. The effect of SPS parameters (pressure and electric current) on the phase formation is also studied. Single τ-phase is obtained for annealing 5 min at 500°C with SPS. In addition, we show that the initial grain size of the τ-phase is influencing the τ-phase transformation and fraction at a given annealing condition, independently of the annealing method used. A faster transformation was observed for smaller initial ϵ-grains. The samples obtained by SPS showed comparable magnetic properties with the conventional annealed ones, reaching coercivity of 0.18 T and saturation magnetization of 114 Am ²/kg in the optimized samples. The similarity in coercivity is related to the microstructure, as we reveal the presence of structure defects like twin boundaries and dislocations in both materials.