【作者】 楼鸿波；

【导师】 蒋建中；

【作者基本信息】 浙江大学 ， 材料学， 2013， 博士

【摘要】 金属玻璃是材料家族中相对较年轻的成员,由于其不同于传统晶态合金材料的结构,自从发现以来,一直成为人们研究的热点问题。强玻璃形成能力的合金.体系的开发以及强玻璃形成能力的原因一直是金属玻璃研究的核心内容之一；金属玻璃独特无方向性金属键“无序”密堆结构为研究非平衡态中的诸如非晶态转变、玻璃结构、玻璃转变以及熔体过冷行为等一系列重大基础问题提供了独特的研究机会；另外,金属玻璃一般由金属熔体快速冷却制备,所以金属玻璃的结构和性能也取决于熔体的结构,研究金属熔体的结构及其在变温过程中的变化也一直是材料科学家和凝聚态物理领域的热门课题。本论文的主要研究工作是围绕强玻璃形成能力合金体系开发,金属玻璃非晶态相变以及金属熔体结构变化这三个基础科学问题展开,获得以下主要结果：(1)本文通过在ZrCuAgAl合金基础上添加Be成分,并通过成分优化,成功的开发出了一系列大尺寸的金属玻璃材料,其中Zr46Cu30.14Ag8.36Al8Be7.5成分的最大临界尺寸达到73mm,是目前世界上最大的通过铜模直接浇铸方法得到的金.属玻璃。通过对其结构、热力学等方面的研究发现Be含量的添加,抑制了原ZCuAgAl四元合金的相分离,使得新的五元合金更加稳定,同时小原子Be的添加使熔体结构更加紧密,粘度增大,抑制了原子的长程迁移,降低形核率,从而提高玻璃形成能力,另外晶相和过冷液相之间相对较小的吉布斯自由能差也是其强玻璃形成能力的原因。同时对该合金体系进行了详细的力学、物理和化学性能的研究。(2)通过原位高压同步辐射技术研究了Ca-Al金属玻璃在高压下的结构变化,首次发现了在不含f电子的金属玻璃中存在压力诱导的非晶-非晶结构变化,通过实验与理论相结合,提出s、p电子向d轨道转移是该金属玻璃体系在高压下发生相变的原因。进一步对Ca72.7Al27.3金属玻璃高压下的电阻测量结果发现,在压力下该合金确实发生了一个电子结构转变。并且通过与前人工作进行比较,·以及理论计算表明Ca72.7Al=金属玻璃的输运性能在压力下经历了从简单金属玻璃到类过渡元素金属玻璃的转变,进一步证实了Ca-Al金属玻璃中s、p电子向d轨道转移的现象。(3)金属熔体的结构因为其在物质的熔化、凝固、玻璃转变等过程中扮演的重要角色而一直是材料领域、凝聚态物理领域研究的重点问题之一。本文通过先进的原位高温同步辐射技术,研究了A1?In、Sn、Zn四种单质金属熔体的结构变化,发现随着温度的升高,液体中的第一近邻原子间距不是通常意义上的膨胀,反而是收缩的。通过系统的实验和理论模拟研究,提出如果金属液体结构是由多种大小不一的团簇组成的话,在温度升高时,大团簇由于体积较大,能量较高,其金属键更容易被打破,从而形成小团簇。从整体上看,随着温度的升高,大团簇的含量越来越少,小团簇的含量越来越多,而小团簇的平均原子间距比大团簇的小,从而使得熔体中第一近邻的平均原子距离缩短。更多还原

【Abstract】 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.更多还原