Magnetic Property And Crystallinzation Behavior For Ferromagnetic Bulk Amorphous Alloys

Comparing with the crystallization alloys, bulk metallic glasses (BMGs) have many unique characteristics such as high strength, high hardness, excellent corrosion -resistance and good wear-resistance. Recently, the discovery and development of multi-component amorphous alloys have refreshed and been the hottest topics in the materials science field. Ferromagnetic materials have the widest applications among functional materials. Therefore, the bulk amorphous alloys with ferromagnetic property have attracted a great attention. In this work, the magnetic properties and crystallization behavior for several ferromagnetic bulk amorphous alloys have been investigated. Exspecially, the present research activities are mainly focused on the crystallination behavior, the correlations between microstruture and magnetic property for as-cast bulk ferromagnetic alloys. In particully, the relations between microstruture and magnetic property for alloys with hard magnetic property after annealing over crystallization temperature have been studied. In this work, we hope to find methods for improving magnetic property of alloys and could prepare bulk magnetic materials with good performance, which could make good basis for applications for bulk ferromagnetic alloys. Some of main experimental results and conclusions are listed as follows:1. The proper yttrium (2 at%～6 at%) addition is very effective in improving the glass forming ability (GFA) for Fe_67-xCo₁₀Nd₃Y_xB₂₀(x=0, 2, 6, 10) alloys. The structure for as-cast Fe₆₁Co₁₀Nd₃Y₆B₂₀ alloy is composed of major amorphous phase. The as-cast Fe₆₁Co₁₀Nd₃Y₆B₂₀ alloy presents soft magnetic behavior, while shows the hard magnetic behavior due to exchange coupling between soft phase and hard phase after annealing at 948K for 30min. The coercivity (_iH_c) and remanence (M_r) are 101kA/m and 46Am²/kg, respectively. The alloy with fully amorphous structure is obtained using a melt spinning technique at wheel speeds 8m/s. The _iH_c and M_r are 165kA/m and 54Am²/kg, respectively after annealing at 948K. It is noted that the as-cast samples with fully amorphous structure will have better hard magnetic property after annealing above crystalline temperature for Fe₆₁Co₁₀Nd₃Y₆B₂₀ alloy.2. For Fe_75-xZr_xY₄B₂₁ (x=0, 1, 2, 3) alloys, the GFA is extremely sensitive to the zirconium additions. The proper Zr (1 at%～2 at%) addition is very effective in improving GFA for Fe_75-xZr_xY₄B₂₁ (x=0, 1, 2, 3) alloys. The new bulk amorphous Fe₇₃Y₄B₂₁Zr₂ alloy with good GFA and excellent soft magnetic property was found.3. For Fe_73-xZr₂Nd_xY₄B₂₁ (x=0, 2, 5, 7) alloys, the GFA is very sensitive to the addition of neodymium additions. With the small amount of Nd addition changing, the GFA of alloys will be changed. The proper addition of Nd (5at%) could effective in keeping GFA for Fe_73-xZr₂Nd_xY₄B₂₁ (x=0, 2, 5, 7) alloys.4. It is noted that the as-cast Fe₆₈Zr₂Nd₅YaB₂₁ alloy presented soft magnetic behavior, while showed hard magnetic behavior after annealing at temperatures above the crystalline temperature. A bulk nanocomposite magnet with good hard magnetic property was obtained with composing of soft magneticα-Fe phase and Fe₃B phase and hard magnetic Nd₂Fe₁₄B phase. The magnetic property is, M_s=86Am²/kg, Mr=49Am²/kg, iH_c=380kA/m, (BH)_max=43kJ/m³. It could provide one promising way for the bulk magnet produced by the simple process of copper mold casting and subsequent heat treatment.5. For as-cast Fe₆₈Zr₂Nd₅Y₄B₂₁ alloy, the congregation and departure of Fe and B atom is observed by three-dimensional atom probe (3DAP) method. It is noted that the ratio of Fe atom and B atom is about 1:1 in rich B region, 8:1 in poor B region and 7:2 in other regions. A new viewpoint is put forward, that is, the non-magnetic phases such as NdFe₄B₄ and B₃Fe₃Nd phase could first formed in rich B region, while hard magnetic Nd₂Fe₁₄B phase could first formed in B-deplepted region.6. For Nd_60-xDy_xFe₃₀Al₁₀ (x=0, 2, 5) alloys, the GFA is first increased, then decreased with the addition of Dy element. There are two type magnetic phases, that is, soft magnetic phase and hard magnetic phase for as-cast Nd₅₅Dy₅Fe₃₀Al₁₀ alloys. There is a kint on the hystersis curves due to poor exchange coupling effect. The GFA and Curie temperature could be improved by the Co element substitution for Fe element in Nd-Fe-A1 bulk amorphous alloys.7. On the DSC curves for Nd₆₀Fe₂₀Co₁₀Al₁₀ alloy, it is found that there is a wide exothermic reaction peak over the temperature ranging from 600K to 750K, while it disappeared after 2-5 at%B addition. It may be due to the crystallization of chemical inhomogeneity resulting in forming more uniform amorphous structure for residule amorphous phase during the process of copper suction.8. The coercivity is increased with increasing B addition for Nd₆₀Fe₂₀ Co₁₀Al_10-xB_x (x=0, 2, 5) alloys. With B addition, the corresponding structure and amounts for atomic clusters in amorphous phase are changed. The crystallization behavior investigations validate that the origin of hard magnetic behavior for Nd-based bulk amorphous alloys is due to the magnetic exchange coupling interaction among the magnetic clusters with large random anisotropy.