Research On Magnetic Properties And Microstructure Of Fe-Nd-Al Bulk Amorphous Alloys

The amorphous alloys have many unique characteristics such as high strength, high hardness, excellent corrosion resistance and good wear resistance due to the microstructure of long-range disorder and short-range order. Recently, the discoveries of multi-component bulk amorphous alloys have been one of the hottest topics in the materials science field. Ferromagnetic materials have the widest applications among the functional materials. Therefore, the bulk amorphous alloys with ferromagnetic properties have attracted much attention. In this work, the magnetic properties, glass forming ability and crystallization behavior of Fe-Nd-Al amorphous alloys have been investigated. Especially, the present investigations are mainly focused on the correlations between microstructure and magnetic properties of the as-cast bulk amorphous alloys and the annealed alloys. Pulsed magnetic field is employed when annealing temperature is around the Curie temperature of the alloy. The effects of the microstructure and magnetic properties for the ferromagnetic amorphous alloy have been studied in detail. Some of the main experimental results are listed as follows:For Fe_xNd_90-xAl₁₀ (x=30-55) alloys, the saturation magnetization increases and the glass forming ability decreases with the increasing Fe content. The Fe₅₃Nd₃₇Al₁₀ alloy shows good glass forming ability and soft magnetic property with high saturation magnetization. The Fe₅₃Nd₃₇Al₁₀ melt-spun ribbon exhibits high coercivity, while the bulk alloy prepared by suction casting shows soft magnetic property. The phenomenon is contrary to that of the Nd-based bulk amorphous alloy and ribbons. Ax phases (face-centered cubic structure a=0.549nm) can be observed in the amorphous phase of the Fe₅₃Nd₃₇Al₁₀ ribbon by high resolution transmission electron microscopy (HRTEM). The hard magnetic property of the ribbon results from the magnetic exchange coupling interaction between A_x phases with large random anisotropy. The soft magnetic property of bulk Fe₅₃Nd₃₇Al₁₀ alloy is attributed to the amorphous phase with the fully disordered microstructure. Furthermore, the existence of the spin-glass-like short-range magnetic order in the Fe₅₃Nd₃₇Al₁₀ ribbon and the long-range ferromagnetic order in the bulk Fe₅₃Nd₃₇Al₁₀ alloy can be validated.From the SEM and EDS results of the bulk Fe₅₃Nd₃₇Al₁₀ alloy, a new bulk Nd-rich amorphous alloy (Fe₂₈Nd₅₉Al₁₃) with good glass forming ability is obtained. The bulk amorphous alloy shows hard magnetic property, while the ribbon presents soft magnetic property. The microstructure and magnetic property investigations validate that the origin of hard magnetic property for Nd-based bulk amorphous alloy is the short-range order with large random anisotropy.The glass forming ability significantly deceases with the addition of boron in the(Fe_0.53Nd_0.37Al_0.10)_100-xB_x alloy and there is almost no amorphous phase in the x=5 alloy. The magnetic properties of the alloys change from soft magnetic property to hard magnetic property with the increasing boron content, which is due to the presence of the Nd₂Fe₁₄B phase. The proportion of Nd₂Fe₁₄B phase increases while the alloy annealed over crystallization temperatrue. Therefore, the intrinsic coercivity of the alloys increases markedly.The bulk (Fe_0.53Nd_0.37Al_0.1)₉₆B₄ alloy is annealed in a pulsed magnetic field with two different working processes (Processingâ… andâ…¡). The results demonstrate that the magnetic property of the alloy can be further enhanced with pulsed magnetic field during the furnace cooling (Processingâ…¡). It may be ascribed as the pulsed magnetic field can keep the magnetic moment alignment. The alloy has better magnetic property when annealed in a 5T pulsed magnetic field at 463K and 523K. The optimal magnetic property of the alloy is obtained by annealing at 523K using Processing II. The remanence is enhanced by 25.9% compared with that of sample without magnetic field annealing. It can provide a new way to improve the magnetic properties of ferromagnetic bulk amorphous alloys with pulsed magnetic field annealing below the Curie temperature of the alloy.The proper addition of Zr (6%) is very effective in improving the glass forming ability of [Fe_0.51Nd_0.35Al_0.10B_0.04]_100-xZr_x alloys and the magnetic properties of the alloys change from hard magnetic property to soft magnetic property. From the results of SEM and EDS, a new bulk amorphous alloy (Fe₄₇Nd₃₈Al₁₂Zr₃) with good glass forming ability is prepared by suction casting and it shows soft magnetic property. The addition of Nb (4at%-8at%) could effectively enhance the glass forming ability of the [Fe_0.41Nd_0.35Al_0.10B_0.04]_100-xNb_x alloys. However, the precipitation of hard magnetic phase Nd₂Fe₁₄B would be depressed by the certain addition of Nb (≥4at%). Therefore, the magnetic properties of the alloys change from hard magnetic property to soft magnetic property. The minor Dy addition is effective in improving the magnetic properties of the [Fe_0.41Nd_0.35Al_0.10B_0.04]_100-xDy_x alloys. With the addition of Dy=lat%, the remanence (M_r) changes from 13.42Am²/kg to 20.10Am²/kg and the coercivity (iH_c) changes from 237.4kA/m to 491.0kA/m, respectively.