Migration of N element and evolution of microstructure in spark plasma sintered bulk γ′-Fe4N

Magnetic bulk γ′-Fe4N alloys were successfully fabricated by spark plasma sintering (SPS) using high purity γ′-Fe4N powder at 500 °C and 550 °C under 100 MPa and 150 MPa, respectively. The density of the bulk γ′-Fe4N was 7.092 g/cm3 with 98.8 % of the theoretical density, which exhibit the high saturation magnetization 199.6 emu/g. The morphology, microstructure, sintering process and magnetic properties of the bulk samples is explored. It is found that the resistivity and hardness which closely corresponds to the density at the center and margin of the bulk sample are different for the temperature and pressure distribution. With the temperature and pressure increases in the SPS process, N element gradually migrates from the central region of high temperature and high pressure to the margin region of low temperature and low pressure, resulting in the appearance of pores and the decreased density. Moreover, ε-Fe3N phase in the margin region is higher than that in the central region. The phase distribution in the SPS process and the related electrical properties provide insight into the formation and evolution of the phase in the SPS sintering process, which can help to obtain bulk samples with good properties and uniform composition in practical applications. •Bulk γ′-Fe4N soft magnetic alloy was prepared by SPS process, which shows a great potential in the future soft magnetic material for power electronics.•The high-purity γ′-Fe4N powder was first prepared by a simple method, which had high saturation magnetization (199.6 emu/g), high density (7.092 g/cm3) and low coercivity (5.04 Oe)•It is found that in the SPS process, the N atoms will migrate from high temperature and high pressure to low temperature and low pressure region•By analyzing the samples sintered at different pressures and temperatures, we reveal the reasons for the differences in magnetic properties, resistivity, phases and hardness between the center and marginal regions of the bulk sample.