High-Pressure Synthesis of High Coercivity Bulk MnAl-C Magnets from Melt-Spun Ribbons

Bulk MnAl-C magnets were prepared via high-pressure compaction of melt-spun ribbons. The e → τ phase transformation (PT) in the melt-spun MnAl and MnAlC ribbons occurs mainly via the massive mode, whereas the displacive mode is not prominent. The massive PT temperatures of the melt–spun MnAl and MnAlC ribbons were measured to be in the range 660–739 K and 700–758 K, respectively. The PT temperatures of the melt-spun MnAlC ribbons are lower than those of water–quenched MnAlC. The annealing process for producing τ-phase from e-phase was optimized based on the measured PT temperature range. As a result, no decomposition of the metastable e/τ-phase MnAl and MnAlC were observed during the optimized high temperature annealing. High-purity τ-phase MnAlC ribbons with high magnetization (122 Am2/kg at 8 T) and low coercivity (0.13 T) were obtained. The coercivity of the bulk MnAlC obtained by high-pressure compaction was significantly enhanced and reached 0.38 T partially due to the grain refinement of the sample during high-pressure deformation.