Investigation of phase selection hierarchy in Mn–Al alloys

Primarily attributed to the formation of the ferromagnetic τ-phase, near equiatomic composition of Mn–Al have recently received much attention in permanent magnet industry. Several mechanisms have been proposed in literature for the τ-phase formation but controversy still arises regarding the dominating mode. In the current work, MnAl-based alloys having different compositions in a range between Mn50.5Al49.5 and Mn57Al43 have been studied by means of in-situ high energy X-ray diffraction, differential scanning calorimetry (DSC) and magnetic measurements. Synchrotron and DSC experiments showed the dependence of the τ-MnAl on the intermediate disordered ε′-phase. Alloys having 53.4 at% or less Mn (S1, S2) followed a transformation route of ε+ε'→τ→β+γ2 upon annealing. Alloys having more than 53.4 at% of Mn had only ε-phase. High energy X-ray diffraction patterns showed that ε-phase directly transformed into stable phases in the absence of ε′-phase. It is observed that ε′ not only promoted the ferromagnetic τ-phase but also stabilized it by delaying the nucleation of stable phases.