MnAl and MnAlC permanent magnets produced by mechanical alloying

Abstract

The ferromagnetic {tau} phase in the MnAl system, first reported by Kono and Koch et al., has the potential to replace anisotropic ferrites as a permanent magnet material. The {tau} phase is metastable and is usually produced by a rapid quench of the high temperature {var_epsilon} phase followed by isothermal annealing at temperatures between 400 and 700 C. Prolonged annealing results in decomposition of the {tau} phase to the equilibrium {gamma} and {beta}-Mn phases. The addition of carbon to the alloy stabilizes the {tau} phase against decomposition and improves both the magnetic properties and ductility. Mechanical alloying has been used successfully to synthesize a number of rare earth permanent magnet alloys, including Nd{sub 2}Fe{sub 14}B and SmCo{sub 5}. This processing technique can be used to produce a nanocrystalline microstructure, with a grain size below 10 nm, similar to some rapidly quenched structures. This small grain size can have beneficial effects upon the magnetic properties. The mechanical alloying of manganese and aluminum with compositions near 34 wt% Mn has been previously reported, however, no work appears to have been carried out in the region of the {tau} phase near 70 wt% Mn. In this paper the authors report the results ofmore » a study of the structure and magnetic properties of MnAl and MnAlC prepared by mechanical alloying and heat treatment.« less