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

塑性变形在晶体的力学行为中起着至关重要的作用。特别地，在原子以六边形图案排列的情况下，称为六方密堆积金属和合金，例如镁或钛基合金，孪晶（以对称方式具有相同结构的两个单独晶体）是一种重要类型的塑性变形，其严重影响机械行为，例如延展性、强度、加工硬化和断裂。因此，必须理解和控制孪晶，以设计和加工六方密排金属和合金。然而，这一直阻碍了难以捉摸的理解原子尺度的机制，孪生过程中的金属。尽管付出了巨大的研究努力，几十年来，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原子尺度的孪生过程，深入了解原子运动在复杂晶体结构孪生中的作用。该项目将为基于孪生的合金设计和加工提供重要指导，以实现上级机械性能。从而推动了轻金属基结构的应用。该计划将通过培养具有不同人口背景（特别是女性和少数民族）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推广到小学，将研究和教育结合起来。技术总结孪生在晶体的力学行为中起着至关重要的作用。特别地，在六方密排（HCP）金属和合金中，除了位错滑移之外，孪生还可以被大量激活并且严重影响它们的延展性、强度、加工硬化、织构形成和断裂，主要是因为孪生可以沿着HCP晶体的c轴进行变形，其中位错塑性受到限制。因此，为了设计和加工具有改进的机械性能的HCP合金，必须控制孪生。然而，这一直阻碍了难以捉摸的理解的原子尺度机制的孪生成核和生长的HCP晶体。在孪生中，父晶格的一部分被重定向，并且父晶格关于孪生平面镜像乘积晶格。传统上，这种晶格重新取向是通过均匀的简单剪切实现的，该剪切将所有或一部分晶格点带到孪晶。剪切是通过孪晶位错在孪晶面上的协调运动来介导的。形变孪晶的经典描述在立方结构中得到了广泛的验证。双晶格结构（例如HCP）的孪生中的显著差异在于孪生剪切不能将所有母晶格点携带到孪生位置。因此，需要额外的原子运动，称为洗牌，以实现孪生。尽管付出了巨大的研究努力，几十年来，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是必要的，以探索实际的原子洗牌和剪切过程中的孪生成核和生长，从而获得一个基本的理解在HCP晶体中的孪生机制。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晶体中的原子尺度孪生过程，如孪生成核，增长和相关的转变以及方向-原子分辨率下位错塑性和孪晶之间的依赖竞争。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的学术价值和更广泛的影响审查标准。

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.