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 tau-phase with L1(0) structure, which requires well controlled synthesis to prevent the formation of secondary phases, detrimental for magnetic properties. Here, we investigate the formation of tau-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 tau-phase is obtained for annealing 5 min at 500 degrees C with SPS. In addition, we show that the initial grain size of the epsilon-phase is influencing the tau-phase transformation and fraction at a given annealing condition, independently of the annealing method used. A faster transformation was observed for smaller initial tau-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. [GRAPHICS]