Development Of Disordered Metals And Temperature-and Pressure-Induced Structural Evolution In Disordered Metals

Metallic glass (MG), as a relative new member of materials family, has been at the cutting edge of material researches since its discovery due to its unique structure. Searching for bulk metallic glasses (BMGs) with high glass forming ability (GFA) and understanding the mechanism for the high GFA is one of the most important issues. The unique nondirectional metallic bonding and closesly packed structure of metallic glass provides an ideal model system for the studies of some fundamental issues in condensed matter physics, e.g. polyamorphism, glass structure, glass transition, supercooled liquid etc. Moreover, because MGs are usually prepared by rapid cooling from metallic melts, the structure of metallic melts largely affects structure and properties of MGs. Thus, the study of the structure of metallic melts is of great important because it is a fundamental issue in materials science and condensed matter physics due to its critical role in understanding the processes of melting, solidification and glass transition. In this thesis, we focus on (ⅰ) the development of new BMG systems with high GFA,(ⅱ) polyamorphism in MGs and (ⅲ) the atomic structure evolution of metallic melts. The main results are summarized as follows.(1) Starting from quaternary Zr46Cu37.64Ag8.36Al8alloy system, we developed a series of ZrCuAgAlBe BMGs with critical size over35mm by optimizing the Cu/Be ratio, among which the best glass former is Zr46Cu30.14Ag8.36Al8Be7.5with the critical size of73mm. Up to now it is still the largest BMG prepared by direct copper mould casting that has been reported. The enhanced GFA of this alloy system is due to that the Be addition not noly hinders the phase separation but also sustains the relatively low difference in Gibbs free-energy between the crystalline and the supercooled liquid phase.(2) By applying in situ high-pressure synchrotron x-ray diffraction techniques, we investigated high-pressure behavior of Ca-Al binary MGs, pressure-induced amorphous-to-amorphous configuration changes were observed in the Ca-rich part. This is for the first time that polyamorphism has ever been reported in a non-f-electron-containing MG system. The transfer of s and p electrons into d orbitals under pressure, reported for the pressure-induced phase transformations in pure polycrystalline Ca, is suggested to explain the observation of the polyamorphism in this Ca-Al MG system. We further studied the pressure-and temperature-dependent resistance behaviors of a Ca72.7Al27.3MG. One distict change from positive to negative coefficient of pressure-induced resistivity was detected during compression at ambient temperature. By comparing with the previous works reported in the literature and by theoretical simulations, we verify that the Ca72.7Al27.3amorphous alloy does change from a simple-metal-like amorphous alloy at lower pressures to a transition-metal-like amorphous alloy at high pressures during compression via a charge transfer mechanism.(3) Using synchrotron X-ray diffraction technique, we find a general trend that the average distance between a center atom and atoms in the first nearest-neighbor shell contracts for several metallic melts upon heating. By applying molecular dynamics simulations, we elucidate that this anomaly is caused by the redistribution of polyhedral clusters affected by temperature. In metallic melts, the high-coordinated polyhedra are inclined to evolve into low-coordinated ones with increasing temperature. As the coordination number decreases, the average atomic distance between a center atom and atoms in the first shell of polyhedral clusters is reduced. This phenomenon is a ubiquitous feature for metallic melts consisting of various-sized polyhedra.