Ordering and Phase Transitions in Supercooled Metallic Liquids and Glasses

技术摘要：块状金属玻璃 (BMG) 可以在与常见的硅酸盐玻璃类似的加工条件下形成，其技术应用越来越多。为什么少数金属液体会形成 BMG，而大多数金属液体不会形成 BMG，目前尚不清楚。 我们和其他人之前的研究表明，金属玻璃包含中程和短程有序结构，其中二十面体短程有序（ISRO）最为常见。 我们已经证明，这种结构有序化起源于远高于玻璃化转变温度的高温液体，并且通常伴随着化学有序化。由于ISRO与晶体周期性不相容，因此它提高了晶相的成核势垒，从而在某些情况下增强了玻璃的成形性。分子动力学研究表明，这种排序影响粘性行为背后的原子运动，这在玻璃形成中也起着关键作用。此外，在许多玻璃中都观察到了细尺度的相分离；这种情况是否会发生在过冷液体中（它也会影响玻璃的形成），此前尚未进行过研究。 对这些问题的研究需要在无容器的环境中处理液体，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度过冷的金属和半导体液体进行 X 射线结构研究。 在这笔资助下，这些研究将通过进行广角和小角度同步加速器 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来扩展。从这项研究中获得的见解将有助于改善受控失透过程中对玻璃形成和纳米结构发展的控制。非技术摘要：硅酸盐玻璃是常见的材料，数千年来一直用于生产装饰性和实用性物品。 金属眼镜不太为人所知。 它们于 1960 年首次被发现，通常需要以接近每秒 100 万度的速度冷却形成它们的液体，这极大地限制了它们的技术实用性。现在已知一些金属玻璃可以使用比用于生产硅酸盐玻璃的冷却速度慢得多的冷却速度来制造。 这些新颖材料通常比钢更坚固、极其坚硬且耐腐蚀。 此外，它们可以模制成传统金属无法获得的复杂形状。 这些玻璃现在用于压力传感器、微型齿轮电机、变压器铁芯、生物医学部件和运动器材。然而，目前尚不清楚为什么少数金属液体会形成这些玻璃，而大多数金属液体却不会。 在这笔赠款的支持下将进行的研究将解决这个关键问题。 该小组率先使用静电悬浮进行液体结构的无容器研究，并证明金属玻璃形成液体会产生显着的拓扑短程和中程有序。 将测量来自这些液体的 X 射线和中子的散射，以确定化学排序并研究纳米尺度上可能的液相分离，这可能伴随着结构排序。 这些数据将与对玻璃形成重要的关键物理特性的测量相关，包括液体密度、粘度和抗结晶性。除了与提高基本认识相关之外，这些研究的结果还将带来金属玻璃形成以及玻璃和液晶结晶过程中纳米结构发展控制的改进方法。 该研究将为研究生和本科生提供科学培训。 受这项研究启发，简单实用的材料科学实验和演示将构成一系列由PI和他的学生领导的中学教师讲习班，帮助教师向高中生传达科学原理和科学职业的兴奋感。

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.