Study On Preparing And Crystallzation Kinietics Of Cobalt-Based Amorphous Alloys

The Co-based amorphous alloys exhibit excellent soft magnetic properties, such as high saturation magnetization, large permeability in the high-frequency range, very weak magnetostriction and low coercivity and loss. They are widely applied in magnetic recording heads,thin film inductors and transformer cores . The investigation on Co-based amorphous alloys has been the foreland of condensed physics and design to new materials for its unique physical and chemical properties.In this thesis, the Co-based amorphous alloys had been prepared by mechanical alloying(MA) and rapid cooling on a single cooper wheel. The microstructure and the magnetic properties of the composites had been studied and analyzed. The crystallization behavior of the metallic Co77.5Sil3.5B9and Co68.5Fe4Si10B17.5 were studied by means of transmission electron microscopy (TEM), differential thermal analysis(DTA) and magnetic measurements.Two modes were used to analysis the crystallization kinetics based on DTA measurements has been performed on the exothermic peaks of the sample. One is non-isothermal crystallization kinetics at constant heating rate, which can be analyzed by the Kissinger and Doyle-Ozawa method. The other is the isothermal crystallization kinetics, which can be discussed in terms of the JMA equation and the Arrhenian relationship.A detailed interpretation of the results lead to some interesting conclusions. The important results we obtained are as follows:(1) The nine alloys with four eutectic compositions and five compound compositions based on the Co-Zr phase diagram formed amorphous alloys. Co 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 Co crystal lattice and solid solution with Co supersaturation is formed. Then, the energy of supersaturation gets large enough to break its stability, and the amorphous alloy is obtained.(2) Influencing of milling parameters on Co-Zr, Co-Si-B, Co-Fe-B and Co-Fe-Si-B amorphous alloy prepared by MA has been studied in this paper. Composition constitutes, milling parameters are key discussed factors.(3) Co-Si-B and Co-Fe-Si-B ribbons were prepared through rapid cooling on a single cooper wheel. The Composition constitutes is key discussed factors to the glass forming ability..(4) The Co-Zr powders and the Co-Si-B ribbons have obvious soft magnetic properties and which have changed with the anneal temperature. The Co-Zr powders leads to the occurrence of nanocomposite state and improvement of soft magnetic properties.(5) From the non-isothermal DTA thermogram for amorphous Co77.5Si13.5B9and Co68.5Fe4Sil0B17.5 ribbons, we can see that the value of crystallization peak temperature increases with increasing heating rate. The apparent active energy is variation suggested by Kissinger method with Tg, Tx, Tp. The average active energy suggested by Doyle-Ozawa is higher than which obtained by Kissinger method. The crystallized fraction x as a function of temperature or time at different heating rate is looks like "S".(6) From the isothermal DTA thermogram for amorphous Co68.5Fe4Sil0B17.5 ribbons , the relationship of ln(-ln(l-x)) and lnt for four different annealing temperature is obtained. The date for 10%>x>90% are almost linear, which indicates that JMA equation is suitable for the isothermal crystallization of the Co68.5Fe4Si10B17.5 alloy in this range. The Avrami exponents can also be obtained, which range from 2.11 to 2.58.