Microstructural and Magnetic Evolution of α″-Fe16N2 Bulk Magnets Consolidated by Spark Plasma Sintering

The development of rare-earth-free permanent magnets represents a strategic direction in advanced magnetic materials research. Among the most promising candidates, the metastable α″-Fe16N2 phase stands out due to its exceptionally high saturation magnetization. In this work, α″-Fe16N2 powders produced by gas atomization followed by nitriding were consolidated via Spark Plasma Sintering (SPS). The effects of sintering temperature (498–598 K) and pressure (40–80 MPa) on phase evolution, densification, microstructure, and magnetic properties have been systematically investigated. Optimal processing conditions were identified at 548 K and 60 MPa, providing a balance between densification (~80% of the theoretical density), phase stability, and magnetic performance. X-ray diffraction revealed that the α″-Fe16N2 phase remains stable up to ~523 K, while its decomposition into α-Fe and γ′-Fe4N becomes significant at higher temperatures. The consolidated samples exhibited a saturation magnetization of ~230 Am2/kg, a maximum coercivity of ~86.5 kA/m, and a Mr/Ms ratio of 0.42. δM curve analysis indicated a transition from magnetostatic interactions (at low pressures) to exchange-dominated coupling (at intermediate and high pressures). These findings demonstrate the potential of SPS processing to preserve the α″-Fe16N2 phase and produce rare-earth-free magnetic compacts with competitive magnetic performance, providing a basis for further process optimization.