Crystallization And Microstructure Studies In Cu-based Metallic Glasses

Metallic glasses, which are also called amorphous alloys, are prepared by quenched the melting alloys with a fast rate. Due to the very short time for the atoms to arrange to the regular crystalline structure, the quenched alloys keep the disorder atomic structure in liquid state. This structure will be stable in a temperature range. The atom distribution in amorphous solid is short-range ordered and long-range disordered.Because of the unconventional and unique structural characteristics, metallic glasses especial the bulk metallic glasses (BMG) have numerous excellent mechanical, physical, and chemical properties. They have many potential applications in industrial and defense fields. Furthermore, metallic glasses will exhibit the super-plastic property in supercooled liquid region, which is very beneficial for casting the device with complicate infrastructure. Recently, Metallic glasses were widely investigated by material scientists and engineers, however, there are also many questions and problems both in the theoretical and applying field of this kind alloys. They can be summarized mainly in follows:1. The nature of the glass forming ability (GFA) cannot be explained clearly. None of the effective rules can guide us to find the composition with high glass forming ability accurately.2. There are no appropriate physical molds to describe short-range ordered and long-range disordered atomic arrangement in metallic glasses. And the interaction of the components in metallic glasses is also puzzling.3. How to understand the glass transition and crystallization occurred in metallic glasses4. How to prepare the super-size BMG and reduce the requirements in preparing processes.5. Speed up the application studies and develop the application potentials of metallic glassesIn recent years, Cu-base BMG that qualified many advantages as an new engineering material, such as low cost, high glass forming ability and excellent mechanical properties were developed in many systems. Last year, it is reported that surprising GFA shows in several Cu-Zr binary alloys and BMG can be produced. This discovery will substantially promote the development of theoretical research and industrial application of metallic glasses.This thesis reviews the amorphous physics and researching history, summarizes the special sample preparing processes, prefect properties and application potentials, and introduces some advance measuring techniques in researching this field.The thermal stability, atomic short-range order, and the kinetics in glass transition and crystallization in Cu-Zr-Al-Ag, Cu-Zr-Al and Cu-Zr metallic glasses were investigated systemically by differential scanning calorimetry (DSC), x-ray diffraction (XRD), Scanning electron microscopy (SEM), X-ray energy dispersion spectroscopy (EDX), positron lifetime spectroscopy, x-ray diffraction and extended x-ray absorption fine structure (EXAFS) using synchrotron radiation source. The results obtained in this thesis may enrich the theories in relevant researching fields and promote the application of this system in industry effectively.Our results show that as the Ag content increases in Cuso-xAgxZfysAls (x=0-25, subscripts indicate atomic %) system, the width of supercooled liquid region ATX increases accordingly, and shows a maximum of about 110K at 15% Ag. This composition may exhibit the maximal supercooled liquid region in all Cu-base metallic glasses. The Ag addition improves the packing efficiency and increases the complexity of the system, finally improves the thermal stability. The results of the EXAFS measurements reveal that each Zr atom is surrounded by about 5 Cu atoms at a distance of 2.70 A, and about 5 Zr atoms at 3.14 A in the as-prepared CusoZ^sAls metallic glass. It is found that the first coordination shell of CU50ZT45AI5 metallic glass is mainly contributed by Zr-Zr, Zr-Cu and Cu-Cu atomic correlations. CusoZ^sAls metallic glass crystallizes simultaneously into tetragonal CuZr2 and orthorhombic CuioZr7 phases at 783 K, which indicates an eutectic type transformation that might lead to a wide supercooled region and high values of Trg and y. In the new reportedCu-Zr bulk metallic glasses, the GFA for the alloy is very sensitive to the composition, but the results indicate that there are no marked differences in microstructures between the composition with the best GFA and neighboring ones. It is found that the first coordination number for five Cu-Zr metallic glasses (Cuioo-xZrx (x=34.5, 35.5,o36, 37.2, 38.2)) calculated from the interatomic range of 1.74 to 3.76A, was found to be N" 14. The results of EXAFS measurements reveal that Cu atoms are surrounded by about 7 Cu atoms at 2.53 A and about 5 Zr atoms at 2.70 A, and Zr atoms areosurrounded by 4 Cu atoms at a distance of 2.70 A and Zr atoms with uncertain numberoZr atoms at a distance of 3.05 A.The last part of this thesis is about the influence of the vacuum condition on the microstructure and mechanical properties of CuCoAl alloy. This project has been found accidentally during the researching process of above Cu-base metallic glasses.