Research Of Structure And Magnetic Properties Of TbCu₇-type Melt Spun Sm-Fe-B Alloys

In 1991, the hexagonal TbCu₇-type SmFe9 phase was firstly discovered by M.Katter et al in melt spun ribbons of binary Sm-Fe system. After nitrogenization, the magnetic properties of SmFe9 compounds were greatly impoved and SmFe9 Nx intermetallic compounds showed excellent permanent magnetic properties, high Curie temperature of 743 K, the saturation polarization raised up to 14 k Gs, which has attracted much attention of material researchers. However, there are two difficulties for the preparation of SmFe9 Nx intermetallic compounds. One is the surface velocity of the preparation of single metastable TbCu₇-type SmFe9 phase higher than 50m/s. The other is the long nitriding time and SmFe9 Nx compounds decomposing very easily during the process of nitrogenization. The two difficulties may be the main reason of hindering the industrial application of SmFe9 Nx permanent magnetic materials. Therefore, the purpose of this paper is to introduce the little boron atom whose atomic radius is very similar to that of the nitrogen into the TbCu₇-type SmFe9 phase and study the effect of boron addition on structure and magnetic properties of melt spun Sm-Fe alloys without nitrogenization.The structure and magnetic properties of SmFe12 Bx and SmFe12-x-yCoxNbyB alloys prepared by the melt spinning technique were systematically studied using powder X-ray diffraction（XRD）, vibrating sample magnetometer（VSM）, transmission electron microscopy（TEM）, Mossbauer spectroscopy（MS）, and so on.（1）The addition of boron atom not only inhibits the appearance of soft magnetic phase α-Fe, but also enhances the ability of amorphous formation for melt spun Sm-Fe ribbons. At the same time, the introduction of boron is also beneficial to reduce the wheel speeds that prepare Sm-Fe permanent magnet materials with single metastable phase TbCu₇-type structure. The surface velocity of preparing single phase TbCu₇-type materials is reduced to 27m/s for SmFe12B1.0 alloys. Moreover, there is a solubility limit（x=1.0, namely 7.14 at%） for boron atom in melt spun Sm-Fe alloys with TbCu₇-type structure. However, the boron content that exceeding the solubility limit（x>1.0） results in the appearance of α-Fe and metastable phase Sm2Fe23B3, which is against the magnetic properties. The average of grain size calculated from the TEM bright dark field images for as-annealed melt spun SmFe12B1.0 ribbons is around 22 nm. However, the grain size of as-annealed SmFe12 ribbons is almost more than 200 nm. In addition, the introduction of boron atom greatly refines the grains of Sm-Fe ribbons.（2）The magnetic properties of single phase TbCu₇-type Sm-Fe-B alloys are relatively lower than the SmFe9 Nx intermetallic compounds. The best magnetic properties of（BH）max=2.19 MGOe, Hcj=2.36 kOe, and Br=4.8kGs were obtained in the SmFe12B1.0 alloys at the surface velocity 40m/s. In order to explain the weak magnetic properties of TbCu₇-type Sm-Fe alloys after introducing the boron atom, the experiments of Mossbauer spectroscopy were performed. The results of Mossbauer spectroscopy show that the hyperfine field of 2e site is almost constant for SmFe12Bx（x=0.0, 0.5, 1.0） compounds. However, the values of H（6l） and H（3g） increase slightly with the increase of boron content. It is supposed that the addition of boron almost has no effect on the distance Fe dumbbells. This behavior explains that the magnetic properties of Sm-Fe-B alloys with TbCu₇-type structure are not greatly improved because the excellent magnetic properties of SmFe9 Nx are mostly attributed to the distance enlargement Fe dumbbells（2e sites）. Moreover, the following sequence of the hyperfine field H（6l） < H（3g） < H（2e） and the isomer shift δ（3g） < δ（6l） < δ（2e） are also obtained by the analysis of 57 Fe Mossbauer spectra.（3）The compound addition of Co and Nb effect on structure and magnetic properties of melt spun SmFe12-x-yCoxNbyB alloys on the basis of the research of ternary Sm-Fe-B system was studied as well. The surface velocities that prepare single TbCu₇-type structure for SmFe9.875Co1.875Nb0.25 B alloys were reduced up to 18m/s. Moreover, the Nb addition greatly enhances the crystallization temperature of amorphous phase transforming into TbCu₇-type structure. The crystallization temperatures of varieties of phases in SmFe12-x-yCoxNbyB alloys gradually moves towards the direction of high temperature. The metastable TbCu₇-type structure already decomposed into α-（Fe, Co） and Sm2（Fe, Co）14B phase after annealing at 680℃ for 1.5h. The best magnetic properties of（BH）max=6.38 MGOe, Hcj=4.9kOe, and Br=6.8kGs were observed in the SmFe10.5Co1.25Nb0.25 B alloys after annealing at 680℃ for 1.5h at the surface velocity 20m/s.