Properties And Crystallization Kinetics Of Fe-Cu-Nb-Si-B-Ni Amorphous Alloys

In this thesis, the glass-forming ability, soft magnetic properties as well as isothermal and isochronal crystallization dynamics of Fe-Cu-Nb-Si-B-Ni amorphous alloys are investigated with differential scanning calorimetry (DSC), vibrating sample magnetometer (VSM). Through component design, Fe-Cu-Nb-Si-B-Ni amorphous alloys are fabricated by proportional substitution of Nb by Ni in a FINEMET alloy. The alloy ingots are prepared by arc melting under the titanium-gettered argon atmosphere. The amorphous alloys are produced with the method of melt-spinning by using a single roll apparatus. The microstructures and the amorphous nature of the as-spinning alloys are characterized X-ray diffraction (XRD).The XRD patterns of as-spinning Fe_73.5Cu₁Si_13.5B₉Nb_3-xNi_x (x = 0, 0.5, 0.8, 1.0, 1.5, 2) amorphous alloys show that the jet pressure considerably influences the microstructures of alloys. The DSC data indicate that the thermal stability of Fe_73.5Cu₁Si_13.5B₉Nb_2.5Ni_0.5 amorphous alloy is the best, while Fe_73.5Cu₁Si_13.5B₉Nb₂Ni₁ is the lowest. The thermal stability of Fe_73.5Cu₁Si_13.5B₉Nb_1.5Ni_1.5 and Fe_73.5Cu₁Si_13.5B₉Nb₂Ni₁ amorphous alloys is similar to the original composition, which indicates their glass-forming ability is simliar to that of the FINEMET alloy. There is a close relationship between the glass forming ability and the content of Ni. The results of VSM show the alloy with Ni = 2 has the best soft magnetic property, and is better than that of the original composition. This indicates that an appropriate substitution of Nb by Ni enhances the soft magnetic properties. Furthermore, the cost of the FINEMET-type alloys is substantially reduced.The crystallization kinetics of the glassy alloys was investigated with the help of differential scanning calorimetry (DSC) under isochronal and isothermal conditions. The crystallization activation energies of the crystallization process are investigated by using Kissinger and Ozawa methods. The crystallization activation energies of Fe_73.5Cu₁Si_13.5B₉Nb_3-xNi_x (x = 0.5, 1.0, 1.5, 2) amorphous alloys are 380.8,398,363.2,434.6 kJ/mol, respectively, with high thermal stability. The variation of crystallized volume fraction with time is a typical sigmoid type. The Avrami exponents n of these amorphous alloys calculated according to the JMA equation are 1.69～1.92, which indicates primary crystallization behaviors with a decreasing nucleation rate.