CAREER: Extending the lattice stability framework in ab initio alloy thermodynamics

技术总结材料研究司和网络基础设施办公室为这一职业奖项提供资金。该奖项支持旨在提高我们基于第一原理量子力学计算预测固态合金相图的能力的计算和理论研究和教育，而不依赖于实验输入。这个项目提供了一个坚实的概念框架和相关的算法来模拟在技术相关的合金中常见的现象，但目前合金理论领域缺乏处理这些现象的能力。这项研究有两个方面：1)晶格不稳定性。成熟的CALPHAD和团簇展开形式基本上是基于这样的假设，即一组定义良好的晶格(例如，bcc，fcc，hcp)在所有组成下至少保持亚稳定。然而，在许多合金系统中(例如，Ti-Al，Cu-Fe等)，一些晶格甚至不是机械稳定的，因此阻碍了计算适当的自由能的任何尝试。这个项目通过适当地限制传统上用来计算基于严格几何划分技术的自由能的积分区域，而不需要完全的非谐处理来处理这一基本的概念问题。虽然存在强大的方法(如团簇展开)来模拟公共底层晶格上的合金有序化现象，但本项目设计了适用于违反这一假设的补充方法(特别是在Lave相、sigma相等)。这个想法是将合金系统的能量分解为原子团簇贡献的总和。然后，该算法试图利用组合技术对最低能量的团簇进行重组，以寻找低能量的晶体结构。所提出的方法将在Pi的合金理论自动化工具包(ATAT)中实现，这是一个已经拥有用户基础的软件包。这项研究活动的教育部分包括通过一个配套的教育网站增强ATAT，该网站将提供侧重于热力学和相图的教程和社交网络工具，以及在本科生和研究生课堂上使用ATAT，让学生亲身体验未来的材料设计工具。为了促进多样性，这个项目将建立在加州理工大学MRSEC从加州州立大学洛杉矶分校招收少数族裔本科生的努力基础上。非技术性总结材料研究部和网络基础设施办公室为这一职业奖项提供资金。该奖项支持旨在提高我们预测“相图”能力的计算和理论研究。相图通常被称为“材料科学路线图”，因为它们表明当不同的化学元素合金化时，哪些化合物是温度和压力的函数，从而为材料设计提供关键的指导。这个项目的重点是基于量子力学计算的预测方法，而不需要实验输入，这一能力对于发现真正的新材料是必不可少的。该项目提供了一个坚实的概念框架和相关的算法来模拟在技术相关的合金中常见的现象，但目前合金理论领域缺乏处理这些现象的能力。建议的方法将在圆周率合金理论自动化工具包(ATAT)中实施，这是一个已经建立了用户基础的软件包。这项研究活动的教育部分包括通过一个配套的教育网站增强ATAT，该网站将提供侧重于热力学和相图的教程和社交网络工具，以及在本科生和研究生课堂上使用ATAT，让学生亲身体验未来的材料设计工具。为了促进多样性，该项目将建立在加州理工大学材料研究科学与工程中心招收加州州立大学洛杉矶分校少数族裔本科生的努力基础上。

TECHNICAL SUMMARYThe Division of Materials Research and the Office of Cyberinfrastructure contribute funding to this CAREER award. This award supports computational and theoretical research and education aimed at improving our ability to predict phase diagrams of solid state alloys, based upon first-principles quantum mechanical calculations, without relying on experimental input. This project provides a firm conceptual framework and associated algorithms to model phenomena that commonly occur in technologically-relevant alloys, but that the field of alloy theory is currently poorly equipped to handle. The research has two thrusts:1) Lattice instabilities. The well-established CALPHAD and cluster expansion formalisms are fundamentally based on the assumption that a set of well-defined lattices (e.g. bcc, fcc, hcp) remain at least metastable at all compositions. However, in numerous alloy systems (e.g. Ti-Al, Cu-Fe, etc.), some lattices are not even mechanically stable, thus impeding any attempts at calculating a proper free energy. This project handles this fundamental conceptual issue by suitably constraining the domain of integration traditionally employed to calculate free energies based on rigorous geometrical partitioning techniques, but without necessitating a full anharmonic treatment.2) Novel compound prediction. While powerful methods (e.g. the cluster expansion) exist to model alloy ordering phenomena on a common underlying lattice, this project devises complementary methods that are applicable when this assumption is violated (notably, in Lave phases, sigma phases, etc.). The idea is to decompose the energy of an alloy system as a sum of atom cluster contributions. The proposed algorithm then attempts to re-assemble the lowest energy clusters, using combinatorial techniques, in search of low-energy crystal structures.The proposed methods will be implemented in the PI's Alloy Theoretic Automated Toolkit (ATAT), a software package that already has an established user base. The educational components of this research activity include augmenting ATAT by a companion educational web site, which will provide tutorials and social networking tools focusing on thermodynamics and phase diagrams, as well as using ATAT in undergraduate and graduate classes to give students hands-on experience with materials design tools of the future. To promote diversity, this project will build upon the efforts of the MRSEC at Caltech in recruiting minority undergraduates from California State University, Los Angeles.NON-TECHNICAL SUMMARYThe Division of Materials Research and the Office of Cyberinfrastructure contribute funding to this CAREER award. This award supports computational and theoretical research aimed at improving our ability to predict "phase diagrams". Phase diagrams are often called the "road maps of materials science", as they indicate which compounds form as a function of temperature and pressure, when different chemical elements are alloyed, thus providing crucial guidance in material design. This project focuses on prediction methods based on quantum mechanical calculations, without necessitating experimental input, a capability that is essential to enable the discovery of truly novel materials. The project provides a firm conceptual framework and associated algorithms to model phenomena that commonly occur in technologically-relevant alloys, but that the field of alloy theory is currently poorly equipped to handle. The proposed methods will be implemented in the PI's Alloy Theoretic Automated Toolkit (ATAT), a software package that already has an established user base. The educational components of this research activity include augmenting ATAT by a companion educational web site, which will provide tutorials and social networking tools focusing on thermodynamics and phase diagrams, as well as using ATAT in undergraduate and graduate classes to give students hands-on experience with materials design tools of the future. To promote diversity, this project will build upon the efforts of the Materials Research Science and Engineering Center at Caltech in recruiting minority undergraduates from California State University, Los Angeles.