The Study On The Formation And Properties Of Co-based Bulk Metallic Glasses

In this paper, The glass forming ability, thermal stability against crystallization, mechanics fracture behavior, magnetic properties under room temperature of Co₄₈Cr₁₅Mo₁₄C₁₅B₆RE₂ (RE=Y,Gd,Er) bulk amorphous alloys have been investigated. Kinetics on non-isothermal crystallization, effect of cooling rate on crystallization and magnetic properties by using differential scanning calorimetry (DSC) of Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ amorphous alloy have studied.On study of glass forming ability, it is found that Co₄₈Cr₁₅Mo₁₄C₁₅B₆RE₂ (RE=Y,Gd,Er) has wide supercooled liquid temperature ΔT_X, which is 85.4K, 83.2K, 89.3K respectively; GFA of Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ bulk amorphous alloy is better than others, suggesting that Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ bulk metallic glass has best GFA and highest thermal stability, because of Er-containing alloy being at or close to the eutectic, having proper atomic size combination, holding bigger negative heats of mixing and electro-negativity. The values of ΔT_X, δ are better than T_rg, γ as expressing GFA.Mechanics fracture behavior, magnetic properties under room temperature of Co₄₈Cr₁₅Mo₁₄C₁₅B₆RE₂ (RE=Y,Gd,Er) bulk amorphous alloys have been investigated. Y-Containing alloy has highest Hv which is 1345, that higher than Er-containing alloy than Gd-containing alloy, which accord with compressive experiment. It is found that the stress-strain of samples before fracture has good linearity approximately, which the characteristics of non-plasticity and fracture angle nearing zero. It is found that SEM images of fracture of bulk glassy Co₄₈Cr₁₅Mo₁₄C₁₅B₆RE₂ (RE = Y,Gd,Er) alloys have clear tearing and step structure, suggesting that fracture is brittleness without exception, attributing to minor quenching structure, oxidation impurity or pore during casting process. In addition, magnetic properties are investigated. The samples belong to half-hard magnetic properties material, which their coercive force are 264.94Oe, 260.7Oe, 408.76Oe respectively. The value of coercive force and saturation magnetization decrease then increase with atomic number of rare elements increasing.The non-isothermal crystallization kinetics of the metallic Co₄₈Cr₁₅-Mo₁₄C₁₅B₆Er₂ glass has been studied. It is found that all the non-isothermal DSC traces of amorphous Co₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ alloy have a single exothermic peak which is asymmetrical, with a steeper leading edge and a long high temperature tail. The heating rate has a significant influence on the shape of the DSC curve, activation energy and transformation mechanism. The activation energy for crystallization are determined as 481.58kJ/mol, 450.92kJ/mol, 406.74kJ/mol for different cooling rates with diameters of 2 mm, 5 mm, 7mm respectively, when using the Kissinger equation; And using the Ozawa equation, the activation energy for crystallization are 472.71kJ/mol, 443.58kJ/mol, 427.15kJ/mol respectively, implying that the value of activation energy of an amorphous alloy depends strongly on cooling rates. For the volume fraction crystallized (x), E_c(x) dependence was obtained by Flynn-Wall-Ozawa (FWO) method, which the conclusion is identical to the result of using the Kissinger equation and the Ozawa equation. Using the Surinach curve fitting procedure, the kinetics was specified. Namely, the crystallization begins with the Johnson-Mehl-Avrami nucleation-and-growth mode and the mode which has been well described by the Normal-Grain Growth kinetic law. These two modes are mutually independent. The JMA exponent, n, initially being equal to 4 and continuously decreases to 1.32, from 3.5 to 1.2 and from 3.35 to 1.25 respectively, along with the development of crystallization, suggesting that growth mechanism of crystalline grain all change from interface-governed to diffusion-controlled three-dimensional growth of nuclei, and the increasing degree of nucleation rate decrease with heating rate increase. The NGG-like mode dominates in the advanced stages of the transformation with the NGG exponent m≈0.5 when the volume fraction crystallized is between 21% and 34%, which are the major and principal kinetic characterristics for heating rate. In addition, magnetic properties are investigated. Their coercive force are 408.76Oe, 354.78Oe, 144.84Oe respectively, suggesting that the samples belong to half-hard magnetic properties material. The value of coercive force decrease with cooling rare decreasing, implying that magnetic properties strongly depend on cooling rare.