Structural change in Sm2Fe17 alloys during homogenization and HD processing

The evolution of phase compositions, phase change and microstructure in Sm2Fe17 alloys during anneal and hydrogenation–disproportionation processing has been studied systematically using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The results indicate that, after annealing at 1050 °C for 24 h, the main phase in the as-homogenized ingot is Sm2Fe17 with a rhombohedral Th2Zn17-type structure, together with a negligible amount of α-Fe phase. A minor phase, SmFe3/SmFe2, is barely detectable. The Sm2Fe17 alloy first absorbs the hydrogen in the hydrogenation atmosphere with a pressure of 0.1 MPa. Disproportionation begins at T ≥ 500 °C, and large amounts of SmHx and α-Fe phases are formed, partly in microcrystalline or amorphous structures. As the temperature increases, the microcrystals and amorphous structures transform into a fully crystalline structure, the transformation becoming complete at 750 °C. The resultant crystal grain size is about 50–100 nm.