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

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 degrees 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 degrees C, whereas that of the Sm3(Fe,Ti)29 increases from 188 degrees C to 207 degrees 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 (alpha-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 degrees 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 degrees C or at 1087 degrees 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 degrees 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.(c) 2023 Elsevier B.V. All rights reserved.