L10 ordered FeNi alloy is considered as a promising candidate for rare-earth-free and environment-friendly permanent magnet. Highly ordered single-phase L10 FeNi is challenging to synthesize due to its low chemical order-disorder transition temperature. Recently, a nonequilibrium synthetic route utilizing a nitrogen topotactic reaction was considered as a valid approach although the detailed phase transformation mechanism is missing.1 In this work, we investigated the formation mechanism of the tetragonal FeNiN precursor phase during the nitridation of FeNi nanopowders. 2
A low oxygen induction thermal plasma system was applied to synthesize the FeNi nanopowders (NPs). The processed FeNi NPs were annealed under a hydrogen gas flow at 1 L/min for 2 h at 400 °C. Then, the NPs were nitrided at 350 °C under ammonia with gas flowing at 2 L/min for 16 h. The detailed microstructure analysis was performed with an atomic resolution analytical electron microscope JEM ARM200F.
High-resolution TEM observation reveals intensive nanotwins in the nitrided FeNi NPs which results in a distorted lattice Interestingly Fe segregates at the TBs was confirmed with EDS mapping. Thus, the nanotwins region may provide preferential nucleation sites for the FeNiN product phase in the Fe2Ni2N parent matrix Furthermore, detailed microstructure characterization shown in Fig.1 revealed that the growth of the FeNiN product phase followed a massive transformation with high index irrational orientation relationships and ledgewise growth motion characteristics detected at the FeNiN/Fe2Ni2N migrating interface. Based on the results, we delineated a potential formation route of the FeNiN precursor phase in the FeNi NPs during nitridation.
Acknowledgment:
This work was partially supported by the project Development of Magnetic Material Technology for High Efficiency Motors which was commissioned by Japan's New Energy and Industrial Technology Development Organization (NEDO).
References:
1) G oto, S. et al. Synthesis of single-phase L10 FeNi magnet powder by nitrogen insertion and topotactic extraction. Sci Rep 7, 13216, doi:10.1038/s41598 017 135622 (2017).
2) Wang, J., Hirayama, Y., Liu, Z. et al. Massive transformation in FeNi nanopowders with nanotwin assisted nitridation. Sci Rep 12, 3679, doi.org/10.1038/s41598-022-07479-8 (2022).

Fig. 1. ABF-STEM images of nitrided FeNi NPs with terraces/ledges demonstrating the ledgewise growth motion of the FeNiN/Fe2Ni2N interface (a & b). (c) schematizes the representative high-index orientation relationship present in (b).2