Subphase exploration for SmFe12-based permanent magnets by Gibbs energies obtained with first-principles cluster-expansion method

Abstract

In the development of SmFe12-based rare-earth permanent magnets, it is yet to establish a pathway to liquid-phase sintering producing nonmagnetic grain-boundary subphases. Here, phase equilibria of the Sm-Fe-Cu ternary system are examined computationally to explore such a pathway and grain-boundary subphases of SmFe12-based magnets through Gibbs free energies for various phases. A key quantity, the mixing enthalpy is examined from first principles. We show that the B2 Sm-Cu-Fe is a candidate for a nonmagnetic subphase, where the solubility of Fe into Sm-Cu is better described by the cluster-expansion method finding attractive Cu-Fe interaction within the B2 sublattice. In addition, it is found that the liquid phase of Sm-Cu slightly including Fe directly equilibrates with SmFe12, which is consistent with experiments. We point out the possibility of liquid-phase sintering of the SmFe12-based magnets using this liquid phase without significant composition changes by the solidification. The mechanism of attractive interaction between Fe and Cu in B2 Sm(Cu, Fe) is clarified from the viewpoint of electron theory.