Phase precipitation and properties of melt-spun Ho-Fe-B alloys prepared by direct quenching and over-quenching followed by heat treatment

Abstract

Aiming to fully utilize rare earth (RE) resources and develop new high coercivity magnets, the magnetic properties and phase precipitation behavior of Ho-Fe-B alloys were systematically studied. The Ho12+xFe82-xB6 (x = −2, −1, 0, 1, 2) alloys were prepared by melt spinning at various wheel speeds. For the directly quenched Ho12Fe82B6 alloy, the optimum magnetic properties can be obtained at a wheel speed of 22 m/s with the coercivity Hcj = 1.07 × 103 kA/m, remanence Jr = 0.45 T and maximum energy product (BH)max = 36.6 kJ/m3 due to its single Ho2Fe14B phase structure. A maximum Hcj = 2.95 × 103 kA/m can also be obtained at 100 K due to the temperature dependence of anisotropy field and existence of spin reorientation of Ho2Fe14B compound. Jr decreases with decreasing temperature owing to the antiparallel coupling between Ho and Fe moments. The increase of Ho content can enhance the coercivity but deteriorate the remanence. More than 2 at. % reduction of Ho leads to the formation of soft magnetic Ho2Fe17 phase and greatly reduces the coercivity, which is different from many other RE-Fe-B alloys. The phase precipitation behavior of over-quenched amorphous Ho12+xFe82-xB6 alloys during heat treatment were also studied, and the precipitation of Ho2Fe14B phase is confirmed in all alloys. Ho2Fe17 phase precipitates in the Ho10Fe84B6 alloy at the temperature of 882.7 K. The magnetic properties of over-quenched Ho12+xFe82-xB6 alloys after optimally annealed are similar to the directly quenched alloys. In addition, the corrosion resistance of the directly quenched alloys was also investigated. The present results provide reference for developing highly stable RE-Fe-B based magnets with alternative RE resource.