Relations between Structure, Phase Formation and Phase Transitions in Supercooled Metallic Liquids and Glasses

技术概述：块体金属玻璃的冷却速度与普通硅酸盐玻璃的冷却速度相似，具有重要的基础和实用价值。形成这些玻璃的液体含有短程有序，当冷却到平衡熔化温度(过冷)以下时，这种有序通常会显著增加。这种演化顺序可以耦合到晶相的成核势垒上，帮助稳定过冷液体的结晶，使玻璃更容易形成。在过渡金属液体中，二十面体短程有序通常占主导地位，在某些锆基玻璃中，二十面体短程有序是玻璃转变的基础。然而，很明显，不同类型的短程有序在其他金属液体中占主导地位，这就提出了一个问题，即在这些情况下，玻璃形成和玻璃转变的基础是什么。此外，在许多金属玻璃形成液体的比热中也出现了极大值，在某些情况下，过冷度的密度也发生了突变。这些现象的起源尚不清楚。它们可能表示模式耦合的开始，指示液/液相转变，或对应于从脆弱到强烈的转变。液体中的这种反常现象是否是所有形成块状金属玻璃的液体的共同特征尚不清楚。为了解决这些问题和相关问题，将通过高Q X射线衍射、涨落电子显微镜和3D原子探针研究，在多个长度尺度上对选定的大块金属玻璃进行结构测量。此外，还将使用在圣路易斯华盛顿大学建造的新的静电悬浮设施，对形成这些玻璃的液体进行高Q衍射和热物理性质测量。这些数据将与玻璃形成和晶化动力学以及基于电子显微镜的随时间变化的形核率的研究结果相关联。这些研究的结果将有助于更深入地理解金属液体和玻璃的结构与相变之间的关系，包括晶体形核和玻璃相变。所获得的洞察力将导致改进玻璃析晶过程中玻璃形成和微结构发展的控制方法。非技术摘要：在晶体中，原子排列成规则的图案，在很长的距离内重复，而在液体和玻璃中，只有相邻原子之间的局部有序。传统玻璃，如窗户玻璃，含有硅和氧等原子。最近，人们发现了完全由金属原子制成的玻璃，这种玻璃可以像传统玻璃一样形成并吹制成复杂的形状。这些金属玻璃比广泛使用的晶体金属更坚固，更耐腐蚀。然而，金属玻璃的形成原因和玻璃的原子结构却知之甚少。为了解决这些问题，我们将对选定的金属液体和玻璃的结构和物理性质进行测量。这些数据将与玻璃的形成相关联。还将研究加热过程中的玻璃析晶，这通常会产生性能更优越的复合材料。这项研究将加深对液体和玻璃之间的关系的了解，并将导致改进玻璃形成和晶化控制的方法。这项研究将为研究生和本科生提供有价值的培训。首席研究员将把这项研究纳入给当地学校的演讲中，并将为高中生提供参观和在他的实验室工作的机会，让这些学生接触到材料科学职业生涯中令人兴奋的可能性。

TECHNICAL SUMMARY:Bulk metallic glasses, which can be formed at cooling rates similar to those used for common silicate glasses, are of significant basic and practical interest. The liquids that form these glasses contain short-range order that generally increases significantly upon cooling below the equilibrium melting temperature (supercooling). This evolving order can couple to the nucleation barrier for the crystal phase, helping to stabilize the supercooled liquid against crystallization, making glass formation easier. Icosahedral short-range order is frequently dominant in transition metal liquids and underlies the glass transition in some Zr-based glasses. However, it is clear that different types of short-range order are dominant in other metallic liquids, raising the question of what underlies glass formation and the glass transition in those cases. Also, there are suggestions of maxima in the specific heat of many metallic-glass-forming liquids, and in some cases sudden changes in density have also been noted on supercooling. The origin of these phenomena is unclear. They may signal the onset of mode coupling, indicate a liquid/liquid phase transition, or correspond to a fragile-to-strong transition. Whether such anomalies in the liquid are a common feature of all bulk-metallic-glass-forming liquids is unknown. To address these and related questions, structural measurements will be made at multiple length scales in select bulk metallic glasses by high-q x-ray diffraction, fluctuation electron microscopy and 3d atom probe studies. Also, high-q diffraction and thermophysical property measurements will be made on liquids that form these glasses, using a new electrostatic levitation facility that has been constructed at Washington University in St. Louis. These data will be correlated with glass formation and crystallization kinetics and the results of TEM-based studies of the time-dependent nucleation rates. The results of these studies will lead to a deeper understanding of the relations between the structures of metallic liquids and glasses and phase transitions, including crystal nucleation and the glass transition. The insight gained will lead to methods for improved control of glass formation and microstructure development during glass crystallization. NON-TECHNICAL SUMMARY:In crystals the atoms are arranged in regular patterns that repeat over long distances, while only local ordering among neighboring atoms occurs in liquids and glasses. Traditional glasses, such as window glass, contain atoms such as silicon and oxygen. Recently, glasses made entirely of metal atoms, which can be formed and blown into intricate shapes like traditional glasses, have been discovered. These metallic glasses are stronger and more corrosion resistant than the widely used crystalline metals. However, the reasons for metallic glass formation and the atomic structures of the glasses are poorly understood. To address these questions, we will make measurements of the structures and physical properties of select metallic liquids and glasses. These data will be correlated with glass formation. Glass crystallization during heating, which frequently produces composite materials with even superior properties, will also be studied. This investigation will give a deeper understanding of the relations between the liquids and glasses, and will lead to improved methods for glass formation and crystallization control. The research will provide valuable training for graduate and undergraduate students. The principal investigator will incorporate this research into presentations given to local schools, and will offer opportunities for high school students to visit and work in his lab, exposing those students to the exciting possibilities of a career in the science of materials.