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

塑性变形在晶体力学行为中起着至关重要的作用。特别是，当原子以六边形的模式排列时，称为六边形紧密排列的金属和合金，如镁或钛基合金，孪晶（两个独立的晶体以对称的方式具有相同的结构）是一种重要的塑性变形类型，它对塑性、强度、加工硬化和断裂等机械行为产生重要影响。因此，在设计和加工六边形密排金属和合金时，必须理解和控制孪晶。然而，由于对金属中孪生过程的原子尺度机制的难以捉摸的理解，这一直受到阻碍。尽管进行了大量的研究，但几十年来，原子运动如何影响双胞胎的机制仍然知之甚少。本研究将采用高分辨率透射电子显微镜研究材料的原子尺度孪晶过程，深入了解原子运动在复杂晶体结构孪晶中的作用。该项目将为孪生基合金的设计和加工提供重要的指导，以获得优异的力学性能。从而促进轻金属基结构的应用。该项目将通过培训具有不同人口背景（特别是女性和少数民族）的研究生/本科生，以及他们在国家实验室的参与，以及通过匹兹堡卡内基科学博物馆向小学推广，将研究和教育结合起来。技术摘要孪生在晶体的力学行为中起着至关重要的作用。特别是，在六角形紧密堆积（HCP）金属和合金中，除了位错滑移外，孪晶还会被大量激活，并严重影响它们的延展性、强度、加工硬化、织构形成和断裂，这主要是因为孪晶可以沿着位错塑性有限的HCP晶体的c轴进行变形。因此，在设计和加工具有改进机械性能的HCP合金时必须控制孪晶。然而，对HCP晶体中孪生成核和生长的原子尺度机制的难以捉摸的理解阻碍了这一点。在孪生中，父晶格的一部分被重新定向，产物晶格被父晶格在孪生平面上镜像。经典地，这样的晶格重定向是通过一个均匀的简单剪切来实现的，它将所有或部分晶格点带到孪晶上。剪切是由孪晶位错在孪晶平面上的协调运动介导的。变形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（如HCP）孪晶的一个显著区别是，孪晶剪切不能将所有母晶格点带到孪晶位置。因此，需要额外的原子运动，称为洗牌，来完成成对。尽管付出了巨大的研究努力，几十年来，原子洗牌如何影响双胞胎的机制仍然知之甚少。原子分辨的直接实验研究对于探索孪晶成核和生长过程中原子的实际洗牌和剪切是必要的，从而对HCP晶体的孪晶机制有一个基本的认识。本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晶体的原子尺度孪晶过程，如孪晶成核、生长和相关转变，以及原子分辨率下位错塑性和孪晶之间的取向依赖竞争。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 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.