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

Abstract

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.