Re-investigation of high-temperature phase equilibria in Fe-rich Sm–Fe–Ti alloys

Abstract

Recently renewed attempts to develop high-performance rare-earth-lean permanent magnets based on the Sm(Fe,Ti)12 compound have drawn attention to the limited knowledge about the high-temperature phase equilibria in the Sm–Fe–Ti system. Experimental investigation of equilibrated alloys with electron probe microanalysis, X-ray diffraction and thermomagnetic analysis revealed several inaccuracies in the currently accepted phase relations at 1000 °C and allowed for a revision of the Fe-rich corner of the Sm–Fe–Ti phase diagram. The Sm(Fe,Ti)12 and Sm3(Fe,Ti)29 phases were found to have more extended Ti ranges of 5.1–9.7 at% and 2.8–6.9 at%, respectively. With increasing of the Ti content, the Curie temperature of the Sm(Fe,Ti)12 remains nearly constant at 306–312 °C, whereas that of the Sm3(Fe,Ti)29 increases from 188 °C to 207 °C. The low-titanium Sm3(Fe,Ti)29 phase equilibrates not only with the Sm(Fe,Ti)12 and Sm2(Fe,Ti)17 phases, but also with (α-Fe) solid solution. Newly demonstrated equilibrium between Sm2(Fe,Ti)17 and TiFe2 phases makes impossible the earlier reported equilibrium between the Sm3(Fe,Ti)29 and Sm(Fe,Ti)11 phases. Because of an invariant reaction at 1000 °C, the revised phase diagram also features a class II four-phase equilibrium Sm3(Fe,Ti)29 + TiFe2 + Sm(Fe,Ti)12 + Sm2(Fe,Ti)17. Peritectic decomposition of the Sm(Fe,Ti)11 phase, which occurs either at 1075 °C or at 1087 °C, was found to have among its products the Sm(Fe,Ti)12 phase. Although no such equilibration was attempted, it must be possible to obtain above 1087 °C a two-phase state composed of the Sm(Fe,Ti)12 phase and a liquid – which is important for manufacturing of the Sm(Fe,Ti)12-based permanent magnets via the liquid-phase sintering.