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

技术概述：大块金属玻璃可以在与普通硅酸盐玻璃相似的冷却速率下形成，具有重要的基础和实用价值。形成这些玻璃的液体含有短时序，通常在冷却到低于平衡熔化温度（过冷）时显著增加。这种进化顺序可以与晶体相的成核屏障耦合，有助于稳定过冷液体，防止结晶，使玻璃更容易形成。二十面体近程序在过渡金属液体中占主导地位，是某些锆基玻璃转变的基础。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，这就提出了在这些情况下玻璃形成和玻璃转变的基础问题。此外，许多金属玻璃形成液体的比热有最大值，在某些情况下，密度的突然变化也在过冷时被注意到。这些现象的起源尚不清楚。它们可能标志着模式耦合的开始，表明液/液相变，或对应于脆弱到强的转变。液体中的这种异常是否是所有大块金属玻璃形成液体的共同特征尚不清楚。为了解决这些问题和相关的问题，将通过高q x射线衍射、涨落电子显微镜和三维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，高q衍射和热物理性质的测量将在液体形成这些玻璃，使用一个新的静电悬浮设施，已建成在圣路易斯华盛顿大学。这些数据将与玻璃形成和结晶动力学以及基于tem的随时间成核速率的研究结果相关联。这些研究结果将使我们对金属液体和玻璃的结构与相变（包括晶体成核和玻璃化转变）之间的关系有更深入的了解。所获得的见解将导致在玻璃结晶过程中改进控制玻璃形成和微观结构发展的方法。非技术总结：在晶体中，原子按规则的模式排列，并在很远的距离上重复排列，而在液体和玻璃中，邻近原子之间只有局部有序。传统的玻璃，如窗玻璃，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和吹成复杂的形状。这些金属玻璃比广泛使用的结晶金属更坚固，更耐腐蚀。然而，金属玻璃形成的原因和玻璃的原子结构却知之甚少。为了解决这些问题，我们将测量选定的金属液体和玻璃的结构和物理性质。这些数据将与玻璃的形成相关联。玻璃在加热过程中的结晶，经常产生复合材料甚至更优越的性能，也将进行研究。这项研究将使人们更深入地了解液体和玻璃之间的关系，并将导致改进玻璃形成和结晶控制的方法。这项研究将为研究生和本科生提供有价值的训练。首席研究员将把这项研究纳入当地学校的报告中，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领域令人兴奋的职业可能性。

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.