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

Abstract

The microstructural evolution of melt-spun Sm 8Fe 88− x Co 4Zr x ( 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 SmFe 7 phase and α-Fe, with a small amount of Sm 2Fe 17 phase. In the Zr-free samples, grain sizes were approximately 20 nm for α-Fe and 100 nm for SmFe 7 in the as-quenched state, and were 100 nm for α-Fe and 500 nm for SmFe 7 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/SmFe 7 or α-Fe/SmFe 7N x ) 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 SmFe 7 phases, the optimum hard magnetic properties (with coercivity H c∼8.92 kOe, remanence B r∼8.27 kG and ( BH) max∼11.81 MGOe) were obtained in the nitrided powder after the Zr addition.