The Research On Preparation And Crystallization Kinetics Of Nickel-Based Amorphous, Nanocrystalline Alloys

In this paper, glass forming ability and thermal stability of nickel based amorphous ribbon and powder alloys were researched through chemical composition design and experment, and manufacture process, microstructure, soft magnetic properties were studied. Amorphous/nanocrystlline alloys were analysed and studied by X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscope (SEM), vibrating sample magnetometer (VSM), and the crystallization behavior of the amophous alloys were studied through two modes of non-isothermal crystallization and isothermal crystallization.Ni-Fe-Si-B and Ni-Fe-Nb-Si-B ribbons were prepared by rapid solidification processing. The chemical composition is a key discussed factor to the glass forming ability and thermal stability. Undercooled liquid region (â–³T_x) and reduced glass transition temperature (T_rg) of (Ni_0.75Fe_0.25)₇₈Si₁₀B₁₂ and (Ni_0.75Fe_0.25)₇₃Nb₅ Si₁₀B₁₂ amorphous alloys are 54K, 0.560 and 50K, 0.617, respectively, which indicate that alloys have high glass forming ability and thermal stability. (Ni_0.75Fe_0.25)₇₈Si₁₀B₁₂ alloy have obvious soft magnetic properties and which have changed with the anneal temperature. The nanocrystalline crystallization leads to the occurrence of nanocrystalline particles precipitate in the amorphous matrix and improvement of soft magnetic properties.Ni₇Zr₂, Ni₂₁Zr₈, Ni₆₄Zr₃₆, NiZr and Ni₃₆Zr₆₄ amorphous alloys were prepared by MA after milling 10 hours . Ni and Zr powders can dissolve into each other and the dissolution becomes larger after milling. During the milling process, Zr powders firstly dissolve into the gap of Ni crystal lattice and solid solution with Ni supersaturation is formed. Then, the energy of supersaturation gets large enough to break its stability, and the amorphous alloy is obtained.From the non-isothermal DTA thermogram for (Ni_0.75Fe_0.25)₇₈Si₁₀B₁₂ and (Ni_0.75Fe_0.25)₇₃Nb₅Si₁₀B₁₂ amorphous alloys, T_g, T_xi, T_pi of the alloys are shifted to high temperature with increasing heating rate. The activation energies of the alloys have been calculated using Kissinger plot method and Ozawa plot method based on differential thermal analysis data, respectively. The results have the same tendency, indicating the consistency of those two methods for determining the activation energies for the crystallization process of amorphous alloys. All plots of the crystallization fraction x as a function of temperature at different heating rates showtypical sigmoid curves, and are shifted to high temperature with increasing heating rate.The research of isothermal crystallization behavior for (Nio.7sFeo.25)78SiioBi2 amorphous alloy indicates that the Avrami exponents for different temperatures range from 2.58 to 4.95, which implies that the phase transformation mechanism depends on temperature during isothermal annealing. At 705K and 715K, the values of Avrami exponent are 2.58 and 2.74, respectively,which are sightly higher than 2.5 that is typical for the diffusion-controlled three-dimensional growth at constant nucleation rate. The Avrami exponent value increases to 3.62 when the temperature is 725K, indicating that the crystallization is governed by diffusion-controlled three-dimensional growth with increasing nucleation rate. At 73 5K, the value of Avrami exponent is 4.95, suggesting that the growth mechanism changes from the diffusion-controlled three-dimensional growth to the inface-controlled three-dimensional growth with increasing nucleation rate. So (Nio.7sFeo.25)78SiioBi2 amorphous alloy is annealed at certain temperatures, a single nanocrystalline phase, Y -(Fe, Ni) solid solution precipitates in the amorphous matrix, which improves soft magnetic properties.