| Summary: | 碩士 === 義守大學 === 材料科學與工程學系 === 91 === The Zr-base amorphous alloy has many significant features in scientific research and application, such as high glass-forming ability and wide super-cooled liquid region under low critical cooling rate. In engineering application, it provides good mechanical properties, such as high tensile strength, high elastic modulus, relatively high impact fracture energy and excellent corrosion resistance. Inoue and Masunoto （Northeast University, Japan） declared that “To increase atomic packing density can improve the thermal stability and mechanic strength in amorphous alloy”. Therefore, this research is focused on studying the thermal stability and glass-forming ability by increasing packing density of ZrNiAlCu amorphous alloy with adding smaller atom Boron. The test film was prepared under low cooling rate.
In the experiment, Zr, Al, Cu and Ni pure element were selected as the main components and different atom percent of Boron was added to produce Zr65-xAl7.5Cu17.5Ni10Bx（X＝0，2，4，6，8，10）. Specimens with 0.5-2 mm thickness were prepared by arc melting under the Argon atmosphere furnace and drop cast in a water-cooled copper mold. Then ribbons of 100~200μm thickness were prepared by single roller melt spinning technology. Different techniques, such as X-Ray Diffraction （XRD）, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Differential Thermal Analysis (DTA), Differential Scanning Calorimetry (DSC) and Hot Stage Optical Microscopy were utilized to study the glass-forming ability and thermal stability of these alloys and to observe their changes in crystallization behavior and microstructure.
From the experimental results, it is shown that Zr65-xAl7.5Cu17.5Ni10Bx can obtain amorphous phase under lower cooling rate. By the microstructure analysis, some nanocrystalline phases with size approximate 10 nm are found in some areas of the amorphous matrix. The nanocrystalline phase is recognized to the tetragonal CuZr2 type crystal. According to the thermal stability analysis, we found Reduced Glass Temperature （Trg）is up to 0.55 in this study. Glass Transition Temperature （Tg） and Crystalline Temperature （Tx） raise with the Boron addition. Not only 4 at. % but also 6 at. % Boron addition, △Tx of this crystal have maximum values, approximately 86℃. They are higher than that of base alloy. Activation energy（Kissinger plot）with the 2 at.﹪of Boron addition （X＝2） reaches the highest value, 360 ± 10 KJ/mol, which is 20% higher than that of base alloy. Obviously, inserting Boron element can improve the glass-forming ability and thermal stability of the Zr-base amorphous alloy. From the crystallization kinetics analysis, we acquired the result of Avrami exponent is between 1.5-2.5. We also verify that the mechanism of crystallization growth behavior is diffusion control model of C.Wagner’s theory with a decreasing nucleation rate.
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