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

非技术总和塑性变形在晶体的力学行为中起着至关重要的作用。特别是，当原子以六边形的方式排列时，称为六方密排金属和合金，如镁或钛合金，孪生(两个对称地具有相同结构的单独晶体)是一种重要的塑性变形类型，它严重影响着塑性、强度、加工硬化和断裂等力学行为。因此，在设计和加工六方密排金属和合金时，必须了解和控制孪生现象。然而，由于对金属中孪生过程的原子尺度机制的难以理解，这一点一直受到阻碍。尽管做出了巨大的研究努力，但几十年来，原子运动如何影响孪生机制仍然知之甚少。这项拟议的研究将使用高分辨率透射电子显微镜来研究材料中的原子尺度孪生过程，从而深入了解原子运动在复杂晶体结构孪生中的作用。该项目将为孪生合金的设计和加工提供重要的指导，以实现优异的机械性能。从而推动轻质金属基结构的应用。该计划将通过培训具有不同人口统计背景的研究生/本科生(特别是女性和少数族裔)和他们参与国家实验室以及通过匹兹堡卡内基科学博物馆延伸到小学来整合研究和教育。特别是，在六方密排(HCP)金属和合金中，孪生除了位错滑移外，还可以被大量激活，并严重影响其塑性、强度、加工硬化、织构形成和断裂，这主要是因为孪生可以沿着位错塑性有限的HCP晶体的c轴进行变形。因此，在设计和加工具有更高机械性能的HCP合金时，必须控制孪生。然而，由于对HCP晶体中孪生成核和生长的原子尺度机制的难以理解，这一点一直受到阻碍。在孪生中，父晶格的一部分被重新定向，并且乘积晶格被父晶格围绕孪生平面镜像。经典地说，这样的晶格重新定向是通过均匀的简单剪切来实现的，该剪切将所有或部分晶格点带到孪晶。剪切是通过孪生位错在孪生面上的协调运动来实现的。变形孪生的经典描述在立方体结构中得到了广泛的验证。双晶格结构(如HCP)孪生的一个显著区别是孪生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，也就是所谓的改组，才能完成孪生。尽管付出了巨大的研究努力，但几十年来，人们对原子洗牌如何影响孪生的机制仍知之甚少。原子分辨的直接实验研究对于探索孪晶形核和生长过程中原子的实际洗牌和剪切是必要的，从而对孪晶机制有一个基本的了解。这项拟议的研究将使用最先进的原位高分辨率电子显微镜(HRTEM)来研究HCP晶体中原子尺度的孪生过程，如孪晶成核、生长和相关转变，以及原子分辨率下位错可塑性和孪晶之间的取向竞争。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.