Microstructural characterization of α-Fe/Sm–Fe–N nanocomposite hard magnets

Abstract The microstructural evolution of melt-spun Sm8Fe88−xCo4Zrx (x=0, 2, 3.5 at%) ribbons obtained at a wheel speed about 75 m/s has been investigated by means of three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). As-quenched, heat treated and nitrogenated samples mainly consist of SmFe7 phase and α-Fe, with a small amount of Sm2Fe17 phase. In the Zr-free samples, grain sizes were approximately 20 nm for α-Fe and 100 nm for SmFe7 in the as-quenched state, and were 100 nm for α-Fe and 500 nm for SmFe7 in the annealed samples. After nitrogenation, grain refinement was noticed, but the average grain size was still over 100 nm. As-quenched alloy containing 3.5 at% Zr composed mainly of amorphous phase, which reveals that the Zr addition improved the glass forming ability. An ultra-fine (∼20 nm) equiaxed nanocomposite (α-Fe/SmFe7 or α-Fe/SmFe7Nx) microstructure was obtained in the annealed and nitrogenated Zr-containing alloys. The Zr addition was effective in refining the grain size due to the enrichment of Zr atoms at the interfaces between α-Fe and SmFe7 phases, the optimum hard magnetic properties (with coercivity Hc∼8.92 kOe, remanence Br∼8.27 kG and (BH)max∼11.81 MGOe) were obtained in the nitrided powder after the Zr addition.