The phase transformation behavior of Mn-Al rare-earth-free permanent magnets

Rare-earth-free permanent magnet materials based on Mn show great promise for applications in electric motors and devices. The metastable ferromagnetic tau phase of the Mn-Al system has magnetic properties between those of the high-performance Nd-Fe-B magnets and the lower-performance ferrite magnets. However, the hybrid displacive-diffusional pathway of tau formation, from the parent e phase through the intermediary epsilon phase, is still not fully understood. This phase transformation progression was studied in-situ using diffractive, calorimetric, and magnetometric techniques to show that the progression from epsilon to tau in Mn54Al46 at <450 degrees C involves the ordering of epsilon into epsilon'. Density functional theory calculations were performed on each phase and confirmed the experimental observation that the epsilon to epsilon' to tau pathway is energetically favorable. Isothermal annealing of quenched-in e at 350 degrees C demonstrated that epsilon' is ferromagnetic, also in agreement with theoretical results, with a moderate coercivity of at least 50 kA/m. The tau phase was observed to nucleate along the prior e phase grain boundaries and grow into the epsilon' phase regions. A boundary front of epsilon' was observed between the tau and epsilon phases. Both Kissinger and Flynn-Wall-Ozawa methods were used to determine the activation energies for the epsilon' and r phase transformations with values of similar to 140 kJ/mol obtained for both phases. Therefore, the ordering transformation to epsilon' and the hybrid displacive-diffusional transformation to tau were shown to overcome the same magnitude energy barrier. Both activation energies were less than previous tau phase activation energies measured on Mn55Al45 in the absence of a significant epsilon' ordering exotherm, providing a kinetic benefit to the e to epsilon' to tau pathway at 350 degrees C. The results of this study give insight into the phase transformation of L1(0) binary materials as well as materials that undergo a disorder-order transformation followed by displacive shear.