: In situ observation of atomic scale twinning Process in HCP Crystals

: 原位观察 HCP 晶体原子级孪生过程

• 批准号: 1808046
• 负责人: Guofeng Wang
• 金额: $ 43.27万
• 依托单位: University of Pittsburgh
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-07-01 至 2023-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1808046&HistoricalAwards=false
• 关键词: situ; observation; atomic; scale; twinning

NON-TECHNICAL SUMMARYPlastic deformation plays a crucial role in mechanical behaviors of crystals. Particularly, where the atoms are arranged in the pattern of hexagons, called hexagonal close packed metals and alloys such as magnesium or titanium-based alloys, twinning (two separate crystals having the same structure in a symmetrical manner) is an important type of plastic deformation, which critically influences the mechanical behaviors such as ductility, strength, work hardening, and fracture. As such, twinning has to be understood and controlled for designing and processing the hexagonal close packed metals and alloys. However, this has been impeded by the elusive understanding of atomic scaled mechanisms of twinning processes in the metals. Despite tremendous research efforts, for decades, how atom movements influence the mechanism of twinning remains poorly understood. The proposed research will employ high resolution transmission electron microscopy to investigate atomic-scale twinning processes in the materials, providing in-depth understanding on the role of atom movement in twinning of complex crystal structures. The project will provide important guidance for twinning-based alloy design and processing for achieving superior mechanical properties. Thereby, it will advance the application of light metal-based structures. The program will integrate research and education through training graduate/undergraduate students with diverse demographic backgrounds (particularly, female and minority) and their participation in national laboratories as well as outreach to elementary school through Pittsburgh Carnegie Science Museum.TECHNICAL SUMMARYTwinning plays a crucial role in mechanical behaviors of crystals. Particularly, in hexagonal close packed (HCP) metals and alloys, twinning, in addition to dislocation slip, can be profusely activated and critically influences their ductility, strength, work hardening, texture formation and fracture, primarily because twinning can carry deformation along the c axis of the HCP crystal where dislocation plasticity is limited. As such, twinning has to be controlled for designing and processing HCP alloys with improved mechanical properties. However, this has been impeded by the elusive understanding of atomic scaled mechanisms of twinning nucleation and growth in HCP crystals. In twinning, a part of the parent lattice is reoriented and the product lattice is mirrored by the parent about the twinning plane. Classically, such a lattice reorientation is achieved by a homogeneous simple shear which carries all or a fraction of the lattice points to the twin. The shear is mediated by coordinated movement of twinning dislocations on the twinning plane. The classical description of deformation twinning has been validated extensively in cubic structures. A significant difference in twinning of double-lattice structures, such as HCP, is that a twinning shear cannot carry all the parent lattice points to the twin positions. As a result, additional atomic movements, called shuffles, are required to accomplish twinning. Despite tremendous research efforts, for decades, how atom shuffles influence the mechanism of twinning remains poorly understood. Atomically-resolved direct experimental investigation are necessary for exploring the actual atomic shuffle and shear during twinning nucleation and growth, and hence obtaining a fundamental understanding on twinning mechanisms in HCP crystals. The proposed research will employ state-of-the-art in situ high resolution transmission electron microscopy (HRTEM) to investigate atomic-scale twinning processes in HCP crystals, such as twinning nucleation, growth and pertinent transformations as well as the orientation-dependent competition between dislocation plasticity and twinning at atomic resolution.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.

非技术摘要塑性变形在晶体的机械行为中起着至关重要的作用。特别是，当原子以六边形图案排列时，称为六方密堆积金属和合金，例如镁或钛基合金，孪晶（两个单独的晶体以对称方式具有相同的结构）是一种重要的塑性变形类型，它严重影响延展性、强度、加工硬化和断裂等机械行为。因此，在设计和加工六方密堆积金属和合金时必须理解和控制孪晶。然而，对金属孪生过程的原子尺度机制的难以理解阻碍了这一进程。尽管进行了大量的研究工作，但几十年来，人们对原子运动如何影响孪生机制仍然知之甚少。拟议的研究将采用高分辨率透射电子显微镜来研究材料中的原子级孪生过程，深入了解原子运动在复杂晶体结构孪生中的作用。该项目将为孪晶合金设计和加工提供重要指导，以实现卓越的机械性能。从而，将推动轻金属基结构的应用。该计划将通过培训具有不同人口背景（特别是女性和少数族裔）的研究生/本科生、让他们参与国家实验室以及通过匹兹堡卡内基科学博物馆向小学进行推广，将研究和教育结合起来。技术摘要结对在以下方面发挥着至关重要的作用：晶体的力学行为。特别是，在六方密排 (HCP) 金属和合金中，除了位错滑移之外，孪生也可以被大量激活，并严重影响其延展性、强度、加工硬化、织构形成和断裂，主要是因为孪生可以沿 c 方向进行变形。位错塑性受到限制的 HCP 晶体轴。因此，必须控制孪晶以设计和加工具有改进机械性能的 HCP 合金。然而，由于对 HCP 晶体中孪晶成核和生长的原子尺度机制的难以理解，这一点受到了阻碍。在孪晶中，母体晶格的一部分被重新定向，并且产物晶格被母体围绕孪晶平面镜像。传统上，这种晶格重新取向是通过均匀简单剪切来实现的，该剪切将全部或部分晶格点带到孪晶。剪切是通过孪生平面上孪生位错的协调运动来调节的。变形孪生的经典描述已在立方结构中得到广泛验证。双晶格结构（例如 HCP）孪生的一个显着差异是孪生剪切不能将所有母晶格点带到孪生位置。因此，需要额外的原子运动（称为洗牌）来完成孪生。尽管进行了大量的研究工作，但几十年来，人们对原子洗牌如何影响孪生机制仍然知之甚少。原子分辨的直接实验研究对于探索孪生成核和生长过程中实际的原子洗牌和剪切是必要的，从而获得对 HCP 晶体孪生机制的基本了解。拟议的研究将采用最先进的原位高分辨率透射电子显微镜（HRTEM）来研究 HCP 晶体中的原子级孪生过程，例如孪生成核、生长和相关转变以及取向相关的竞争位错可塑性和原子分辨率孪晶之间的关系。该奖项反映了 NSF 的法定使命，并且通过使用基金会的智力优点和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Direct observation of dual-step twinning nucleation in hexagonal close-packed crystals

• DOI: 10.1038/s41467-020-16351-0
• 发表时间: 2020-05-18
• 期刊: NATURE COMMUNICATIONS
• 影响因子: 16.6
• 作者: He, Yang;Li, Bin;Mao, Scott X.
• 通讯作者: Mao, Scott X.

Revealing shear-coupled migration mechanism of a mixed tilt-twist grain boundary at atomic scale

• DOI: 10.1016/j.actamat.2023.119237
• 发表时间: 2023-08
• 期刊: Acta Materialia
• 影响因子: 9.4
• 作者: Zheng Fang;Boyang Li;Susheng Tan;S. Mao;Guofeng Wang