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整体的富集——在材料科学的许多领域都有巨大的影响，因为大多数工程材料是合金，要么是为了提高性能而设计的，要么是加工后残留的杂质，要么是周围环境的污染。通常情况下，材料局部发生的微小成分变化会导致材料性能的剧烈变化，如耐腐蚀性、催化功能、断裂强度和界面附着力。本项目旨在揭示表面偏析对体组成、表面取向和形貌、外部刺激等可控参数的依赖，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系统、性质和反应具有相当大的实际意义，因为偏析不仅改变表面化学和组成，而且改变多组分材料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过程原型的基本知识将通过操纵合金的整体特性及其与周围环境的相互作用，为设计具有所需表面性能的合金开辟新的视角。作为该研究计划的一部分，研究生和本科生将学习新的显微镜、光谱学和计算技术，以及研究当前材料研究前沿的材料问题。在更广泛的材料科学领域对学生的培训将导致未来的领导者能够更好地解决社会面临的复杂能源和环境问题。该项目的成果也将纳入本科和研究生水平的课程以及高中推广计划，以推进纳米材料相关的教育。技术摘要：虽然描述大块合金相/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大块合金的成分和结构有很大的不同。对发生在多组分材料表面的许多物理和化学过程的微观理解，需要作为对表面偏析引起的成分和结构演化的原子尺度理解的先决条件。尽管这很重要，但实际条件下控制表面偏析开始、促进和终止的原子过程在很大程度上是未知的。该项目通过获得合金表面成分和结构动力学的转化知识，采用原子信息方法对表面偏析现象进行机械理解。该研究基于原子实验和计算程序相结合的紧密集成反馈回路，包括定量原位计量，使用互补的前沿技术动态测量现实环境条件下合金的表面组成、结构和化学，以及密切协调的原子建模，从第一性原理计算到大规模分子动力学和蒙特卡罗模拟。由于一套共同的控制表面偏析的原型基本过程，包括热力学驱动力、化学有序和元素偏析之间的相互作用以及原子交换的动力学障碍，这种全面的理解将揭示许多多组分系统。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.