Phase evolution of ThMn12-type Sm-Fe-Co-Ga-Ti alloys with annealing treatment

SmFe12-based permanent magnet materials with ThMn12 structure have received extensive attention in the exploration and development of new rare-earth-lean permanent magnets. However, the coercivity influence mechanism has not been clearly defined, hindering the development of high coercivity. In this work, the synergistic effect of Sm content and annealing process on phase formation and microstructure evolution has been investigated. The results demonstrate that the SmFe12 hard magnetic phase can be obtained massively in ingots with x from 0.95 to 1.2, however, α-Fe can be completely eliminated only when x exceeds 1.0. Simultaneously, continuous grain boundary has been proposed by annealing at 1180 °C for 1 h in the Sm1.1(Fe0.8Co0.2)10.5Ga0.5Ti alloy, accompanied by the purification of ThMn12-type phase. Elemental distribution analysis demonstrated that Sm and Ga prefer to enter grain boundary during the annealing process, providing the possibility of obtaining continuous non-ferromagnetic grain boundary. However, the distribution of Sm and Ga in the grain boundary is not uniform, and there are some other regions with more Fe and Co, which is difficult to achieve effective demagnetization coupling between grains, resulting in the easy de-magnetization of the grains. TEM results show that nano-sized weak magnetic 1:9 phases are observed and exist around the 1:12 phase, which may be a region prone to demagnetization in an opposing magnetic field. Overall, this work reveals some new and important factors restricting the development of high performance in ThMn12-type magnets.