Computational and Experimental Characterization of Twin-twin Interactions in Hexagonal Metals

具有六方密排结构的镁和钛等六方金属由于其上级强度-密度比而具有提高燃料消耗效率的潜力。特别是，镁的密度低到足以使其比铝轻35%，比钢轻78%，这可能是这些轻质材料中最有吸引力的。在汽车应用中，用镁基材料取代钢结构材料将使燃油效率提高50%以上。然而，镁的使用仍然相当有限，因为在一个称为双胞胎相互作用的现象的知识差距。孪晶是六方晶系材料的特定特征，可以从纳米级到微米级发生，并负责所谓的变形孪晶，这是镁塑性的主要模式，并负责其低强度和差的变形性。本研究将侧重于通过在实验观察支持的多尺度下进行建模，了解孪生相互作用对镁变形机制的影响。其结果将是一个预测模型，使工程师和科学家能够优化六角形金属的加工路线，以满足特定的结构和环境应用。作为这项研究的一部分，学生将有机会在国家实验室接受培训。本研究的长期目标是建立一个定量表征和预测六方晶系材料孪晶诱导微结构和力学性能的研究框架。它将宏观塑性响应与导致它们的晶体学机制联系起来，同时也解释了微观结构的演变。为此，研究小组将通过原子模拟和透射电子显微镜在原子水平上表征孪-孪晶界的微观结构;通过原子模拟和扫描电子显微镜中的原位纳米力学测试在原子/微观尺度上确定孪-孪晶界主导的变形机制;结合原位力学测试和有限元分析，建立细观尺度下的孪晶-孪晶相互作用模型，并将细观尺度模型转化为宏观尺度下的有效介质多晶塑性模型。通过该项目开发的数值和实验工具最终将能够为特定的结构和环境应用设计制造工艺。

Hexagonal metals, such as magnesium and titanium with hexagonal close packed structure, have the potential for increasing the efficiency of fuel consumption because of their superior strength-to-density ratio. In particular, magnesium with a density low enough to make it 35 percent lighter than aluminum and 78 percent lighter than steel is perhaps the most appealing of these lightweight materials. Replacing steel structural materials with magnesium-based materials in automotive applications would boost fuel efficiency by more than 50 percent. However, magnesium's use remains fairly limited because of a gap in the knowledge of a phenomenon called twin-twin interactions. Twins are specific features in hexagonal materials that can occur from the nanoscale to microscale and responsible for what is called deformation twinning, which is a major mode of plasticity in magnesium and responsible for its low strength and poor deformability. This research will focus on understanding the effect of twin-twin interactions on the mechanisms of deformation in magnesium by performing modeling at multiple scales supported by experimental observations. The outcome will be a predictive model that would allow engineers and scientists to optimize the processing routes of hexagonal metals for specific structural and environmental applications. As part of this research, students will be provided an opportunity to be trained at a national laboratory. Undergraduate students from underrepresented groups will be hired as part of the research team, and K-12 students will be exposed to the research through outreach events.The long-term goal of this research is to establish a research framework for quantitatively characterizing and predicting the twinning induced microstructures and mechanical properties of hexagonal materials. It will link the macroscopic plastic responses to the crystallographic mechanisms causing them, while also accounting for microstructure evolution. To do so, the research team will characterize microstructures of twin-twin boundaries at the atomic level through atomistic simulations and transmission electron microscopes; identify twin-twin boundaries dominated deformation mechanisms at the atomic-/micro-scales by using atomistic simulations and in situ nanomechanical testing in scanning electron microscopies; establish twin-twin interaction models at the meso-scale by combining in situ mechanical testing and finite element analysis, and implement the meso-scale models into macro-scale effective-medium polycrystal plasticity models. The numerical and experimental tools developed through this project will eventually enable design of manufacturing processes for specific structural and environmental applications.