Changing phase equilibria: A method for microstructure optimization and properties improvement in preparing anisotropic Sm2Fe17N3 powders

Sm₂Fe₁₇N₃ compound possesses excellent intrinsic permanent magnet properties. Sm₂Fe₁₇N₃ is usually produced by nitridation of the Sm₂Fe₁₇ alloy; however, it is difficult to avoid the formation of α-Fe and SmFe₃ phases in the Sm-Fe system, which may have adverse effects on the magnetic properties of the final magnets. In this study, the three-phase region of SmCu + SmCu₂+Sm₂Fe₁₇ was determined experimentally and a partial isothermal section of the Sm-Fe-Cu phase diagram at 450 °C was established. Based on this newly determined three-phase region, a parent alloy free of α-Fe and SmFe₃ phases was obtained and the effects of Sm-Cu assistant phases on the microstructure and magnetic properties of nitrided powder were investigated. Anisotropic Sm₂Fe₁₇N₃ powders free of α-Fe and having a high (BH)_max exceeding 31 MGOe and H_cj of 13.9 kOe were obtained using a parent alloy containing 3 at.% of Sm-Cu assistant phases. The magnetization reversal process of anisotropic Sm₂Fe₁₇N₃ powders with and without α-Fe was investigated and the underlying mechanisms were analyzed.