Collaborative Research: Coordinated In-situ Dynamic Experiments and Atomistic Modeling of Surface Segregation in Alloys

非技术摘要：表面偏析现象--一种元素在表面相对于块体的浓缩--在材料科学的许多领域具有巨大的后果，因为大多数工程材料要么是为改善性能而设计的合金，要么是加工后残留的杂质，或者是来自周围的污染。通常，材料中局部发生的微小成分修改会导致性能的急剧变化，如耐腐蚀性、催化功能、断裂强度和界面粘附性。该项目旨在揭示表面偏析对可控参数的依赖，如主体成分、表面取向和形貌、外部刺激以及表面偏析过程的微观机制。这一基本见解将对广泛的材料体系、性质和反应具有相当重要的实际意义，因为偏析不仅改变了多组分材料的表面化学和组成，还改变了表面和亚表面区域的原子结构和应变状态。控制表面组成和结构演变的基本工艺原型的基础知识将为通过操纵合金的整体性能及其与周围环境的相互作用来设计具有所需表面性能的合金开辟新的视角。作为这项研究计划的一部分，研究生和本科生将学习新的显微镜、光谱学和计算技术，以及处于当前材料研究前沿的材料问题方面的工作。在更广泛的材料科学领域对学生进行培训，将造就更有能力解决社会面临的复杂能源和环境问题的未来领导者。该项目的成果还将被纳入本科和研究生水平的课程和高中推广计划，以促进与纳米材料相关的教育。技术摘要：尽管描述块体合金相/结构选择的热力学条件的相图已经很好地建立起来，但由于合金元素的表面偏析，合金表面的成分和结构可能与块体的显著不同。要从微观上理解多组分材料表面发生的许多物理和化学过程，首先需要对表面偏析引起的成分和结构演变进行原子尺度的了解。尽管这很重要，但在实际条件下，控制表面偏析的开始、促进和终止的原子过程在很大程度上是未知的。该项目采用原子信息的方法，通过获得合金表面成分和结构动力学的转换知识，从机理上理解表面偏析现象。这项研究基于原子实验和计算程序的结合，反馈回路紧密集成，包括使用互补的前沿技术在现实环境条件下动态测量合金的表面成分、结构和化学的定量原位计量学，以及从第一性原理计算到大规模分子动力学和蒙特卡罗模拟的密切协调的原子模拟。这一全面的理解将揭示许多多组分系统，因为它们共享一套管理表面分离的典型基本过程，包括热力学驱动力、化学有序和元素分离之间的相互作用以及原子交换的动力学障碍。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical abstract:Surface segregation phenomena - the enrichment of one element at the surface relative to the bulk - have enormous consequences in many areas of materials science because most engineering materials are alloys either by design for improving properties or by impurities remaining after processing or by contamination from the surrounding. Often, minor compositional modifications that occur locally in a material can lead to drastic changes in properties such as corrosion resistance, catalytic function, fracture strength, and interfacial adhesion. This project seeks to uncover the dependence of surface segregation on controllable parameters such as bulk composition, surface orientation and morphology, and external stimuli as well as the microscopic mechanism of the surface segregation process. The fundamental insight will have considerable practical importance for a wide range of material systems, properties, and reactions because segregation not only modifies surface chemistry and composition but also the atomic structure and strain state in the surface and subsurface regions in multicomponent materials. The fundamental knowledge of the prototypes of basic processes controlling surface composition and structure evolution will open up new perspectives of designing alloys with desired surface properties by manipulating bulk properties of the alloy and their interplay with the surroundings. As part of this research program, students at the graduate and undergraduate levels will learn about new microscopy, spectroscopy and computational techniques as well as work on materials issues that are at the forefront of current materials research. The training of students in the broader area of materials science will result in future leaders that are better equipped to solve the complex energy and environmental problems that face society. Results from this project will also be incorporated into undergraduate- and graduate-level courses and high school outreach programs to advance nanomaterials-related education.Technical abstract: Although phase diagrams delineating the thermodynamic conditions for phase/structure selection in bulk alloys are well established, the composition and structure of an alloy surface can be significantly different from those of the bulk due to the surface segregation of the alloying element. A microscopic understanding of many physical and chemical processes taking place at the surface of multicomponent materials requires as a prerequisite atomic-scale understanding of surface segregation induced compositional and structural evolution. Despite this importance, the atomic processes governing the onset, promotion, and termination of surface segregation under practical conditions are largely unknown. This project employs atomistically informed approaches toward a mechanistic understanding of surface segregation phenomena by gaining transformational knowledge of surface compositional and structural dynamics of alloys. The research is based on a combined atomistic experimental and computational program with tightly integrated feedback loops, including quantitative in-situ metrology that uses complementary forefront techniques for dynamically measuring surface composition, structure and chemistry of the alloys under realistic environment conditions and closely coordinated atomistic modeling ranging from first-principles calculations to large-scale molecular dynamics and Monte Carlo simulations. The comprehensive understanding will shed light on many multicomponent systems due to a shared set of prototypical basic processes governing surface segregation, including thermodynamic driving forces, interplay between chemical ordering and elemental segregation, and kinetic obstacles of atomic exchanges.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.