Investigation On Crystallization Behavior And Glass-forming Ability For Co-based Metallic Glasses

By employing the X-ray diffraction, differential scanning calorimetry (DSC), scanning electron microcopy and vibrating sample magnetometer, the crystallization behaviors of the amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy under different conditions, the effect of annealing on the kinetics of glass transition and crystallization for this alloy and the effect of the crystallization on the magnetic properties of Co-(43)Fe₂₀Ta_5.5B_31.5 amorphous alloy have been systematically investigated. The effect of cooling rate on crystallization and/or magnetic properties for the amorphous alloys Co₄₃Fe₂₀Ta_5.5B_31.5, Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ and Co_74.4B_25.6 has also been investigated. Besides these investigations, the currently used criteria and newly proposed criteria for evaluating the glass-forming ability (GFA) of alloys have been studied detailed, and a new dimensionless criterion has been proposed, which is statistically better than other criteria in reflecting the GFA. At last, the effect of minor additions of the rare earth element Y on the glass-forming ability of Co₄₃Fe₂₀Ta_5.5B_31.5 alloy has been investigated, and A bulk metallic glass (Co₄₃Fe₂₀Ta_5.5B_31.5)₉₇Y₃ with higher GFA has been developed. Based on above-mentioned investigations, the following results can be obtained:The heating rate has an important influence on the activation energy for crystallization of the metallic Co₄₃Fe₂₀Ta_5.5B_31.5 glass, and its non-isothermal crystallization mechanism is composed of the processes of two types, namely the Johnson-Mehl-Avrami (JMA) nucleation-and-growth mode and the mode which has been well described by the normal grain growth kinetic law (NGG). Though the proportion between the JMA-like and the NGG-like modes is related to the heating rate, the main crystallization process is completed through NGG-like kinetics. The phase transformation mechanism depends on temperature during isothermal annealing. Along with increasing temperature, the nucleus growth mechanism changes from the diffusion-controlled growth to the interface-controlled growth. The calculation of local activation energy for crystallization indicates that the non-isothermal crystallization becomes easier, whereas the isothermal crystallization becomes harder, along with the development of crystallization.The structural relaxation caused by annealing near glass transition temperature diminishes the kinetic effect of glass transition, whereas the kinetics effect of crystallization was not significantly affected by the structural relaxation. The glass transition is delayed by the structural relaxation, but the effect of the structural relaxation on crystallization is twofold, namely promotion and delay.Crystallization has significant influence on the magnetic properties of the amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy. The saturation magnetization M_s and the coercivity H_c y increase with the annealing time when isothermal annealing at 928K is applied to the alloy. When the annealing time is smaller than 30min, the values of H_c and M_s gradually increases with the annealing time. The values of H_c and M_s increase sharply when annealing time is equal to 30min. After then, the variety of M_s is small, whereas the variety of H_c is large. Under the non-isothermal crystallization conditions, the value of M_s increases monotonously with the stopping temperature, but the increase is very small. The value of H_c varies substantially with the stopping temperature, and the varying is non-monotonous. Within the temperature range of the first crystallization peak, the value of H_c increases monotonously with the stopping temperature. When the stopping temperature is higher then the end temperature of the second crystallization peak, the value of H_c decreases drastically.In the case of non-isothermal crystallization of the amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy, the thermodynamic parameters and the precipitation phases are not affected by the cooling rate. During the isothermal annealing, the cooling rate also does not change the crystallization kinetics of metallic Co₄₃Fe₂₀Ta_5.5B_31.5 glass, but the nucleation rate in the whole crystallization process. The magnetic hysteresis loops measuring indicates that the values of saturation magnetization and coercivity increase with decreasing of cooling rate.The cooling rate had significant influence on the crystallization kinetics of metallic Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ glass. In case of non-isothermal crystallization process, the glass transition temperature and the activation energy for glass transition decreased with decreasing of cooling rate. The activation energy for crystallization also decreased with decreasing of cooling rate, in despite of the peak temperature of crystallization was independent of cooling rate. Although the isothermal crystallization was governed by diffusion-controlled three-dimensional growth with decreasing nucleation rate for Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ glasses under different cooling rates, the nucleation rate decreased, while the incubation time decreased, with increasing cooling rate. The magnetic properties of Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ glass were significantly affected by cooling rate. The sample obtained at high cooling rate was paramagnetic, whereas the sample obtained at low cooling rate was ferromagnetic. The saturation magnetization increased with decreasing cooling rate. The preparation conditions affect the crystallization kinetics and the precipitation phases of the binary Co-B amorphous alloy. Though the amorphous melt-spun ribbon has the almost identical composition with the amorphous powder prepared by chemical reduction, their precipitation phases are very different. The amorphous powders obtained by chemical reduction with different preparation condition also present different crystallization kinetics.The glass forming ability of Co₄₃Fe₂₀Ta_5.5B_31.5 alloy can be improved by minor addition of Y. The glassy rod with diameter of 2 mm can be easily prepared with (Co₄₃Fe₂₀Ta_5.5B_31.5)₉₇Y₃ alloy. But compared with the amorphous Co₄₃Fe₂₀Ta_5.5B_31.5 alloy, the magnetic property of amorphous (Co₄₃Fe₂₀Ta_5.5B_31.5)₉₇Y₃ alloy is worsened.A new dimensionless criterionβhas been proposed, which is statistically better than some previous criteria in reflecting the GFA and more sensitive to the GFA of alloy. It can be anticipated that theβcriterion will be used as a useful and efficient guideline for exploring new BMG formers.