Abstract
L10-ordered FeNi alloy (tetrataenite), a promising candidate for rare-earth-free and low-cost permanent magnet applications, is attracting increasing attention from academic and industrial communities. Highly ordered single-phase L10-FeNi is difficult to synthesis efficiently because of its low chemical order–disorder transition temperature (200–320 °C). A non-equilibrium synthetic route utilizing a nitrogen topotactic reaction has been considered a valid approach, although the phase transformation mechanism is currently unknown. Herein, we investigated the basis of this reaction, namely the formation mechanism of the tetragonal FeNiN precursor phase during the nitridation of FeNi nanopowders. Detailed microstructure analysis revealed that the FeNiN precursor phase could preferentially nucleate at the nanotwinned region during nitridation and subsequently grow following a massive transformation, with high-index irrational orientation relationships and ledgewise growth motion detected at the migrating phase interface. This is the first report of a massive phase transformation detected in an Fe–Ni–N system and provides new insights into the phase transformation during the nitriding process. This work is expected to promote the synthetic optimization of fully ordered FeNi alloys for various magnetic applications.
Highlights
L10-ordered FeNi alloy, a promising candidate for rare-earth-free and low-cost permanent magnet applications, is attracting increasing attention from academic and industrial communities
The results indicate a massive phase transformation from the random Fe2Ni2N to the ordered FeNiN phase, which is the first reported experimental demonstration of such an interfacecontrolled phase transformation in a Fe–Ni–N material system
The as ITP synthesized NPs were reduced under a hydrogen gas flow to remove the possible oxide surface
Summary
L10-ordered FeNi alloy (tetrataenite), a promising candidate for rare-earth-free and low-cost permanent magnet applications, is attracting increasing attention from academic and industrial communities. Detailed microstructure analysis revealed that the FeNiN precursor phase could preferentially nucleate at the nanotwinned region during nitridation and subsequently grow following a massive transformation, with high-index irrational orientation relationships and ledgewise growth motion detected at the migrating phase interface. This is the first report of a massive phase transformation detected in an Fe–Ni–N system and provides new insights into the phase transformation during the nitriding process. Current state-of-the-art permanent magnets mostly contain rare earth elements (Nd/Sm/Dy/Pr/La) owing to their high maximum energy product. Such expensive, strategic, and insecure resources have raised severe concerns owing to their availability, extraction difficulties, and national security[3,4]. Blue, and green spheres denote iron, nickel, and nitrogen atoms, respectively
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