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首页> 外文学位 >Undercooling, local structure and phase transition in icosahedral quasicrystals forming titanium-zirconium-nickel alloy liquids.
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Undercooling, local structure and phase transition in icosahedral quasicrystals forming titanium-zirconium-nickel alloy liquids.

机译：二十面体准晶体形成钛-锆-镍合金液体的过冷，局部结构和相变。

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摘要

While lacking the long-range translationally periodic order of crystalline solids, liquids contain a significant amount of short-range order (SRO), which distinguishes them from gases. The local structure of a liquid is important to understand its chemical and physical properties. One of the remarkable phenomena related to the short-range order of liquids is undercooling. That is, liquids can be retained below their melting temperature for a long time without crystallization. Turnbull first demonstrated this for metallic liquids in 1952. To explain the surprising results, Frank hypothesized in 1952 that the local structure of liquid metals is icosahedral. This structure is quite different from those of crystal phases giving a large nucleation barrier and making the undercooling of liquids possible. However, a complete verification of Frank's hypothesis has not been possible thus far. In this dissertation, this goal has been achieved by demonstrating a direct connection between the nucleation barrier and the icosahedral SRO (ISRO) in Ti-Zr-Ni alloy liquids. Containerless environments and in-situ x-ray scattering experiments, essential for such studies, were possible because of the development of electrostatic levitation (ESL) and Beamline-ESL techniques. In addition, distorted icosahedral SRO in liquids will be shown, which has been expected but never observed.;The other important topic related to the undercooled liquids is a liquid-liquid (L-L) transition. Since the undercooled liquids are essentially metastable, the L-L transition could be expected. However, L-L transitions in undercooled liquids have not been observed experimentally, although elemental liquids of P, C, and Si have shown first order transitions above the liquidus temperatures under high pressure. From specific heat measurements of a series of Ti-Zr-Ni alloys by the ESL technique, a maximum in the specific heat at constant pressure was observed in a few quasicrystal forming alloy compositions in the undercooled state, indicating a L-L transition. Surprisingly, the configurational specific heat for the quasicrystal-forming liquid rapidly approaches zero below the maximum, indicating the attainment of a state of constant entropy. Assuming that this is a configurational ground state (at least a constrained equilibrium one), a modified two-level model fits the maximum specific heat data well.
机译：虽然缺少结晶固体的远距离平移周期性顺序，但液体包含大量的短距离有序（SRO），可将它们与气体区分开。液体的局部结构对于了解其化学和物理性质很重要。与液体的短程有序相关的显着现象之一是过冷。即，液体可以长时间保持低于其熔融温度而不结晶。特恩布尔在1952年首次对金属液体进行了证明。为解释令人惊讶的结果，弗兰克在1952年假设液体金属的局部结构为二十面体。这种结构与赋予较大形核屏障并使得液体过冷成为可能的结晶相完全不同。但是，到目前为止，尚不可能对弗兰克的假设进行完整的验证。本文通过证明成核屏障与Ti-Zr-Ni合金液体中的二十面体SRO（ISRO）之间的直接连接来实现这一目标。由于静电悬浮（ESL）和Beamline-ESL技术的发展，无容器环境和原位X射线散射实验对于此类研究至关重要。此外，还会显示液体中的二十面体SRO变形，这是预期的，但从未观察到。;与过冷液体有关的另一个重要主题是液-液（L-L）过渡。由于过冷的液体基本上是亚稳态的，因此可以预期到L-L转变。然而，尽管P，C和Si元素液体在高压下高于液相线温度已显示出一阶跃迁，但从实验上未观察到过冷液体中的L-L跃迁。通过ESL技术从一系列Ti-Zr-Ni合金的比热测量中，在过冷状态下的几种准晶体形成合金组合物中，在恒定压力下观察到比热的最大值，表明存在L-L转变。令人惊讶的是，准晶体形成液体的构型比热迅速接近最大值以下的零，表明达到了恒定的熵态。假设这是一个构型基态（至少是一个受约束的平衡态），则经过修改的两级模型可以很好地拟合最大比热数据。

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  • 作者

    Lee, Geun Woo.;

  • 作者单位

    Washington University in St. Louis.;

  • 授予单位 Washington University in St. Louis.;
  • 学科 Condensed matter physics.
  • 学位 Ph.D.
  • 年度 2004
  • 页码 150 p.
  • 总页数 150
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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