Phase equilibria in iron-rich Sm–Fe–Ti and Sm–(Fe,Co)–Ti alloys at 1100–1200 °C

Iron-rich corners of ternary Sm–Fe–Ti phase diagram at 1100 °C and quasi-ternary Sm–Fe0.8Co0.2–Ti phase diagrams at 1100 and 1200 °C are constructed based on experimental investigation of equilibrated alloys with electron probe microanalysis, X-ray diffraction and thermomagnetic analysis. In addition, the upper boundaries of the temperature ranges of Sm(Fe,[Co,]Ti)12 and Sm3(Fe,[Co,]Ti)29 phases are determined with differential thermal analysis to update earlier rough estimates. The existence of a high-temperature phase of the Th2Ni17 type, originally reported by Ivanova et al. [J. Alloys Compd. 224 (1995) 29], is confirmed. In the Sm–Fe0.8Co0.2–Ti system, the composition and equilibria of this hexagonal phase are established for 1200 °C; it is Sm-depleted (≈9.8 at% Sm) compared to the 2:17 stoichiometry and it coexists with the rhombohedral 2:17 phase. The magnetic anisotropy of the cobalt-substituted Th2Ni17-type phase is planar, with the easy magnetization direction parallel to [100]. Equilibrium between a Sm-rich liquid phase and the 1:12 phase, which is important for the development of new high-performance permanent magnets, is absent up to 1000 °C, but does exist at 1100 °C (for the 1:12 phase with at least 8.7–8.9 at% Ti) and at 1200 °C (for the 1:12 phase with as little as 7.4 at% Ti). The development of magnets may be complicated, however, by an observed tendency of the high-temperature liquid to solidify into ferromagnetic phases including the Th2Ni17-type phase. The Curie temperatures of the α-(Fe,Ti), Sm(Fe,Ti)12, Sm3(Fe,Ti)29 and rhombohedral Sm2(Fe,Ti)17 phases are not only increased by the partial Co substitution for Fe, but their dependence on the Ti concentration is changed by this Co substitution from positive (or, for the 1:12 phase, zero) to negative values.