Study Of The Electropulsing Annealing Preparation And Texture Of Nanocrystalline α-Fe/Nd₂Fe₁₄B

In this paper, the formation of the nanocrystalline structures of the Nd₈Fe₈₄Co₃B₅ amorphous alloy annealed by electropulsing heating and the formation and growth of the crystal texture in α-Fe/Nd₂Fe₁₄B nanocomposites prepared by NdPrFeCoB melt spinning were investigated by employing X ray diffraction (XRD) and transmission electron microscopy (TEM). The α-Fe and Nd₂Fe₁₄B phases are crystallized out when the amorphous alloy was annealed by electropusling heating. When the pulse duration is 0.75 ms, with the increase of pulse voltage from 192 V to 205 V, the average grain size of α-Fe phase keeps invariable about 18 nm, and its volume fraction increases from 6.1% to 25.6%. This indicates that the increase of the volume fraction of α-Fe phase depends on the enhancement of the nucleation rate. And the average grain sizes of both the α-Fe and Nd₂Fe₁₄B phases are smaller than that obtained by annealing the amorphous alloy in a vacuum furnace. At the heating temperature Tp=935 ℃, the average grain size increases from 16.1 nm to 20.2 nm for the α-Fe phase and from 21.0 nm to 55.1 nm for the Nd2Fe14B phase when the pulse duration increases from 0.56 ms to 0.94 ms. The α-Fe/Nd2Fe14B nanocomposites have been prepared by the controlled melt-spinning of NdPrFeCoB at the speed of 10 m/s. The Nd₂Fe₁₄B has a (00l) crystal texture and the α-Fe phase has a (110) texture, which are perpendicular to the ribbon plane. From the free surface to the surface faced to the wheel, the intensity of the crystal textures are gradually weaken, the grain size of the Nd₂Fe₁₄B phase along the normal direction of its (00l) face and that of the α-Fe phase along the normal direction of its (220) face decrease. The crystalline grains have an equiaxed characteristic on the layer touched with the wheel surface, and they have a characteristic of columnar grain at the free surface of the ribbon. This demonstrates that the formation and growth of the crystal texture in α-Fe/Nd₂Fe₁₄B nanocomposites prepared by controlled melt-spinning of NdPrFeCoB result from the large temperature gradient normal to the free surface of the ribbons during the process of rapid solidification of NdPrFeCoB melt.