Microstructural behavior on particle surfaces and interfaces in Sm2Fe17N3 powder compacts during low-temperature sintering

Sm2Fe17N3 sintered compacts were prepared below 450 °C by a high-pressure current sintering technique. The coercivity of the sintered compacts decreased linearly as the sintering temperature increased. Transmission electron microscopic analyses indicated that thin Fe-rich layers containing α-Fe phases were formed just inside the initial oxide layer on the particle surfaces and interfaces in the sintered samples. The generation of α-Fe phases was supposed to cause the coercivity decrease. In addition, X-ray photoelectron spectroscopy analysis revealed that Fe2O3 and FeO contained in the oxide layer of the raw powder disappeared subsequent to heat treatment. These results suggested that the α-Fe phases were generated by the oxidation–reduction reaction between the initial iron oxides and the primary Sm2Fe17N3 phase but not by thermal decomposition or exogenous oxidation during sintering. This mechanism was supported by the fact that extending the sintering time did not result in any further decrease in the coercivity.

► Microstructural changes investigated in Sm2Fe17N3 powder compacts during sintering.
► α-Fe phase causing coercivity decrease, is found on powder surfaces after sintering.
► Results of TEM–EDX and XPS analyses suggest mechanism of α-Fe phase formation.
► α-Fe emerges by oxidation–reduction reaction between initial oxide layer and matrix.