An Investigation Of Properties And Crystallization Kinetics Of Fe-Si-B-Cu-Nb-Al Amorphous Alloys

Fe-based amorphous alloys have very excellent soft magnetic properties. Nanocrystalline alloys can be obtained by adding other elements to it and crystallizing from the amorphous precursor, e.g., FINEMET. Because of the outstanding, combined soft magnetic properties, the extensive application of FINEMET-type nanocrystalline materials is quite promising. In this thesis, a new kind of FINEMET-type alloys are designed by Al substitute for Nb. The glass forming ability, soft magnetic properties, thermal stability and crystallization kinetics of Fe-Si-B-Cu-Nb-Al amorphous alloys are studied.The alloy ingots were prepared by arc melting under the titanium-gettered argon atomsphere. A single-roller melt spinning method is employed to prepare Fe_73.5Si_13.5B₉Cu₁Nb_3-xAl_x (x=0, 0.5, 0.8, 1.0, 1.5) amorphous ribbons. Variant ribbons of different thickness were obtained by using different jet presures in single-roller melt spinning method. The microstructure and magnetic properties were investigated by X-ray diffractometer(XRD), differential scanning calorimetry(DSC), vibrating sample magnetometer(VSM). The thickness data and XRD results show that jet presures influence the thickness and the amorphous state of the ribbons obviously. The glass forming was assessed by the relation between thickness data and XRD results. With Al addition to the original alloy, the glass forming ability decrease a little. The magnetic properties of the as-cast ribbons were measured by VSM. The results show that saturation induction densities Bs of the ribbons first decrease and then increase with increasing Al content. On the other hand, Al can enhance soft magnetic properties by decreasing coercivity. The combined soft magnetic properties of Fe_73.5Si_13.5B₉Cu₁Nb2Al1 amorphous ribbon are best.The isothermal and non-isothermal crystallization kinetics of Fe-Si-B-Cu-Nb-Al amorphous alloy were investigated by DSC. The crystallization activation energies calculated by Kissinger method are similar with those by Ozawa method, which are 427kJ/mol, 443.7kJ/mol, 401.5kJ/mol, 461.4kJ/mol for Fe73.5Si13.5B9Cu1Nb3-xAlx (x=0, 0.5, 1.0, 1.5) respectively. These results imply the high thermal stability of this kind of amorphous alloys. The variation of crystallized fraction with time is a typical sigmoid figure for Fe73.5Si13.5B9Cu1Nb3 amorphous alloy under the isothermal temperature of 765K⁷80K. Avrami exponents n calculated from JMA plots are in the range of 1.85².65, which indicates primary crystallization behaviors of diffusion controlled growth with a decreasing or constant nucleation rate.