L1₀ transformation characteristics of MnAl and FeNi under the influence of strain and magnetic fields

Rare-earth free magnets have received increasing interest due to the complexity in supply chains of rare earth materials. MnAl and FeNi are the two prominent candidates with high magneto-crystalline anisotropy and other intriguing structural and magnetic features from their unique atomically ordered tetragonal L1₀ structure (L1₀ ordered MnAl is also referred to as <!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>τ phase). In this work, an isostructural MnAl was studied to understand the L1₀ ordering in samples prepared by melt spinning followed by annealing in magnetic field. In-situ heating transmission electron microscopy (TEM) was also utilized to understand the mechanistic pathways of the L1₀ ordering in MnAl. In contrast to the relatively easy formation of <!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>τ-MnAl, the L1₀ ordering in FeNi was originally found to exist only in meteorite after the material experienced cosmo-chronological aging to reach the thermodynamic equilibrium. To explore the effectiveness of strain and magnetic field in promoting the L1₀ ordering of FeNi in prolonged low temperature aging, this investigation employed cold-rolled FeNi with 93% deformation which was further annealed in a magnetic field and N₂ environment at 285 °C for 48 days. TEM confirmed the formation of L1₀ FeNi, together with an evolution of fcc FeNi grains.