Ordering and Phase Transitions in Supercooled Metallic Liquids and Glasses

大块金属玻璃(BMG)是一种在与常见硅酸盐玻璃相似的工艺条件下形成的玻璃，具有越来越广泛的技术应用前景。为什么少数金属液体形成BMG，而大多数金属液体不形成，目前尚不清楚。我们和其他人的研究表明，金属玻璃包含中程和短程结构有序，其中二十面体短程有序(ISRO)是最常见的。我们已经证明，这种结构有序起源于远高于玻璃化转变温度的高温液体，并经常伴随着化学有序。由于ISRO与晶体的周期性不相容，它提高了晶相的成核势垒，从而在某些情况下提高了玻璃的成形性。分子动力学研究表明，这种有序性影响粘性行为下的原子运动，这在玻璃的形成中也起着关键作用。此外，在许多玻璃中都观察到了微尺度的相分离；这种情况是否发生在过冷液体中，也会影响玻璃的形成，以前还没有进行过研究。对这些问题的研究要求在无容器环境中处理液体，以消除容器壁上的异质成核并增加过冷量。该小组率先使用静电悬浮技术对深过冷金属和半导体液体进行X射线结构研究。根据这笔赠款，这些研究将扩大，进行广角和小角同步X射线和弹性中子散射研究，并将这些研究与金属玻璃形成液体的密度、粘度和结晶的测量相关联。从这项研究中获得的见解将有助于在受控去玻璃化过程中改善对玻璃形成和纳米结构发展的控制。非技术摘要：硅酸盐玻璃是人们熟悉的材料，数千年来一直用于生产装饰性和实用性物品。金属玻璃就不那么常见了。它们于1960年首次被发现，通常要求形成它们的液体以接近每秒100万度的速度冷却，这大大限制了它们的技术用途。现在已知的一些金属玻璃可以用慢得多的冷却速度来制造，与用于生产硅酸盐玻璃的冷却速度相当。这些新材料通常比钢更坚固，极其坚硬，耐腐蚀。此外，它们可以被模制成传统金属无法获得的复杂形状。这些玻璃现在被用于压力传感器、微齿轮电机、变压器铁心、生物医学部件和运动器材。然而，目前还不清楚为什么少数金属液体会形成这些玻璃，而大多数不会。在这笔赠款的支持下进行的研究将解决这一关键问题。该小组率先将静电悬浮用于液体结构的无容器研究，并证明了金属玻璃形成的液体具有显著的拓扑短程和中程有序性。将测量这些液体对X射线和中子的散射，以确定化学有序性，并研究可能伴随结构有序性的纳米尺度上可能的液体相分离。这些数据将与对玻璃形成至关重要的关键物理性质的测量相关联，包括液体密度、粘度和抗结晶性能。除了与改进的基本认识相关外，这些研究的结果还将导致改进金属玻璃形成的方法，并控制玻璃和液体结晶过程中纳米结构的发展。这项研究将为研究生和本科生提供科学培训。在本研究的启发下，简单实用的材料科学实验和演示，将构成一系列由少年派和他的学生领导的中学教师工作坊，帮助教师向高中生传达科学原理和科学事业的兴奋。

TECHNICAL SUMMARY:Bulk metallic glasses (BMGs), which can be formed under similar processing conditions as the familiar silicate glasses, are finding an increasing number of technological applications. Why a few metallic liquids form BMGs, while the majority of metallic liquids do not, remains unclear. Previous studies by us and by others have demonstrated that metallic glasses contain medium-range and short-range structural order, with icosahedral short-range ordering (ISRO) being most common. We have demonstrated that this structural ordering originates in the high temperature liquid far above the glass transition temperature, and is often accompanied by chemical ordering. Since ISRO is incompatible with crystal periodicity, it raises the nucleation barrier for the crystal phases, thereby enhancing glass formability in some cases. Molecular dynamics studies indicate that this ordering influences the atomic motion underlying viscous behavior, which also plays a key role in glass formation. Additionally, fine-scale phase separation has been observed in many glasses; whether this occurs in supercooled liquids, where it would also impact glass formation, has not been investigated previously. Investigations of these issues require that the liquids be processed in a containerless environment, eliminating heterogeneous nucleation on container walls and increasing the amount of supercooling. This group has pioneered the use of electrostatic levitation techniques to make X-ray structural studies of deeply supercooled metallic and semiconductor liquids. Under this grant, those studies will be extended by making wide and small angle synchrotron X-ray and elastic neutron scattering studies, and correlating those with measurements of the densities, viscosities and crystallization of metallic glass forming liquids. The insights gained from this research will lead to improved control of glass formation and nanostructure development during controlled devitrification.NON-TECHNICAL SUMMARY:Silicate glasses are familiar materials, used for thousands of years to produce decorative and practical objects. Metallic glasses are much less familiar. First discovered in 1960, they typically require that the liquids from which they form be cooled at rates approaching one million degrees per second, significantly limiting their technological usefulness. A few metallic glasses are now known that can be fabricated using much slower cooling rates, comparable to those used to produce silicate glasses. These novel materials are often stronger than steel, extremely hard and resistant to corrosion. Further, they can be molded into complex shapes that cannot be obtained with conventional metals. These glasses are now used in pressure sensors, micro-geared motors, transformer cores, biomedical components and sports equipment. However, it remains unclear why a few metallic liquids will form these glasses, while most will not. The research that will be carried out under the support of this grant will address this key question. This group has pioneered the use of electrostatic levitation for containerless studies of liquid structures, and has demonstrated that metallic-glass-forming liquids develop significant topological short and medium-range order. The scattering of X-rays and neutrons from these liquids will be measured to determine the chemical ordering and to investigate possible liquid phase separation on the nanometer scale, which may accompany the structural ordering. These data will be correlated with measurements of key physical properties important for glass formation, including the liquid density, viscosity, and resistance to crystallization. In addition to their relevance to improved basic understanding, the results of these studies will lead to refined methods for metallic glass formation and for the control of nanostructure development during glass and liquid crystallization. The research will provide scientific training to both graduate and undergraduate students. Simple and practical materials science experiments and demonstrations inspired by this research, will constitute a series of workshops for secondary high-school teachers, led by the PI and his students, to help teachers convey scientific principles and the excitement of a scientific career to high school students.