Abstract: L1₀-ordered FeNi alloy with a high uniaxial magnetic anisotropy and large magnetic moment is a promising candidate for rare-earth-free permanent magnets applications. However, the synthesis of this chemically ordered phase remains a longstanding challenge because of its low chemical order-disorder transition temperature (200-320 °C). Although a non-equilibrium synthetic route based on a nitrogen topotactic reaction has been proposed as a valid approach, the volume fraction and degree of chemical ordering of the product phase are limited. Herein, we propose a promising approach that promotes the efficient formation of L1₀-ordered nitride phase in FeNi nanopowders by introducing a quenching treatment during a low-oxygen induction thermal plasma process. The quenched FeNi nanopowders possessed much smaller powder sizes (40.4 vs 74.0 nm), exhibited higher number densities of nanotwins (39.8% vs 24.1%) and formed much larger volume fraction (33.6 wt.% vs 0.6 wt.%) of ordered phase than the unquenched nanopowders. Notably, quenching-induced high-density nanotwins led to the dominant coverage of serrated {001} crystal facets over the surfaces of the FeNi nanopowders. Such unique features substantially accelerated the formation of the L1₀-ordered nitride phase in the FeNi nanopowders because the 〈001〉 crystallographic orientation had the highest nitrogen diffusivity. This work provides not only a valid synthetic approach for mass production of the L1₀-ordered nitride phase in FeNi nanopowders but also novel insights into the crystal-defect-assisted nitridation of nanomaterials.

Key words: Rare-earth-free, Permanent magnet, FeNi, Nitridation, Nanotwin