Phase transformation and enhanced coercivity in B-N-doped MnAl nanocrystalline bulk alloys prepared by high pressure torsion

High coercivity B-N-doped MnAl alloys were prepared by the traditional induction melting, high temperature homogenization, water-quenching, post-annealing, and severe plastic deformation process. The massive phase transformation of the ε→τ in the B-N-doped ε-MnAl occurs at the temperatures above 680 K and accomplishes at 760 K, both of which are slightly higher than those of the ε-MnAl without elemental doping. The Currie point of the B-N-doped τ-MnAl is measured to be 620 K, which is lower than that of the doping-free τ-MnAl. High-pressure torsion was employed to decrease the crystal size and increase the coercivity of samples. A coercivity up to 0.54 T was achieved in the B/N doped MnAl bulk sample after severe plastic deformation under 6 GPa. The aging processing on the severely deformed samples enhances the remanent magnetization significantly and decreases the coercivity slightly. Our work shows that the high-pressure torsion process is effective in developing coercivity in MnAl-based bulk magnet.