Understanding the Unique Fatigue Behavior of Magnesium Alloys

工程中的大多数结构部件，特别是那些承受重复载荷并在高温下运行的部件，都是金属。镁合金是一些最轻的金属，表现出非常理想的高强度/重量比和刚度/重量比。此外，这些金属是无毒的，这也突显了它们在生物医学应用中的潜力。然而，阻碍镁合金广泛应用的一个主要障碍是对其疲劳行为的有限了解--材料在重复机械载荷下的行为，由于其复杂性，这在结构金属中是独一无二的。该奖项支持通过直接实验观察对镁合金疲劳行为的基础研究。这项研究将提供导致疲劳失效的变形机制的基本知识，并提供一个实验观察数据库，以开发和验证镁基部件工程设计所必需的材料模型。这一努力将促进轻质、高效合金的工程科学，促进国家健康、繁荣和福祉；并通过在汽车、航空航天、医疗保健和国防工业中的低成本应用，保障国防安全。这项研究将被整合到一个包括研究生和本科生的教育项目中，以促进对镁研究中复杂的科学概念的理解。例如，将为本科生和公众开发具有有趣的实验视觉和3D动画的教育视频；与镁合金相关的自导和开放式项目将被整合到现有的Capstone高级设计课程中。镁合金特有的疲劳行为是孪晶的存在、孪晶-去孪晶活动以及它们与滑移和晶界/孪晶边界相互作用的结果。这项研究的总体目标是了解孪晶和孪生-去孪晶活动分别如何导致镁合金的疲劳损伤。为了实现这一目标，将设计和进行三种类型的实验：1)不涉及双胞胎和双胞胎的疲劳，2)显著参与孪生-去孪生的疲劳，以及3)预先设计的双胞胎不涉及双胞胎-去孪生的疲劳。这些实验将使用具有强烈织构的镁合金来实现，并针对材料取向适当地设计应力状态和加载历史。将进行非原位实验和使用配套样品的现场实验，以便于进行微观表征。通过观察孪晶、孪晶-孪晶界以及孪晶和晶界的相互作用对疲劳微裂纹的影响，阐明了影响疲劳过程的主要微观机制。这项研究的结果将为开发基于机理的镁合金疲劳损伤模型奠定基础，并为设计具有更高抗疲劳性能的镁合金提供信息。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。

Most structural components in engineering, particularly those bearing repetitive loading and operating at an elevated temperature, are metals. Magnesium alloys are some of the lightest metals and exhibit the very desirable high strength-to-weight and stiffness-to-weight performance ratios. Moreover, these metals are nontoxic, which also underline their potential for biomedical applications. However, one major obstacle preventing magnesium alloys from a wider use is the limited understanding of their fatigue behavior--material behavior under repeated mechanical loads, which is unique among structural metals due to its complexity. This award supports a fundamental study of the fatigue behavior of magnesium alloys through direct experimental observations. The research will provide basic knowledge of the mechanisms of deformation that lead to failure due to fatigue, and a database of experimental observations to develop and validate material models essential for the engineering design of magnesium-based parts. The effort will promote the engineering science of light-weight, high-efficiency alloys; and advance the national health, prosperity, and welfare; and secure the national defense through cost-efficient applications in the automotive, aerospace, healthcare, and defense industries. The research will be integrated into an education program involving both graduate and undergraduate students to facilitate an understanding of sophisticated scientific concepts in magnesium research. For example, an educational video with interesting experimental visuals and 3D animations will be developed for undergraduate students and public; and self-directed and open-ended projects with relevance to magnesium alloys will be integrated into the existing capstone senior design classes.The characteristic fatigue behavior of magnesium alloys is the result of the existence of twins, twinning-detwinning activities, and their interactions with slips and grain/twin boundaries. The overall objective of this research is to understand how twins and twinning-detwinning activities, respectively, contribute to fatigue damage in magnesium alloys. To achieve this objective, three types of experiments will be designed and conducted: 1) fatigue with no involvement of twins and twinning, 2) fatigue with significant involvement of twinning-detwinning, and 3) fatigue with pre-designed twins without involving twinning-detwinning. These experiments will be realized using a magnesium alloy with a strong texture, and a proper design of the stress state and loading history with respect to the material orientation. Ex-situ experiments together with in-situ experiments using companion specimens will be conducted to facilitate microscopic characterizations. Fatigue microcracking with the influence of twins, twin-twin boundaries, and interactions of twining and grain boundaries will be observed to elucidate the major micromechanisms governing the fatigue processes. The results from the research will form a base to develop mechanism-based fatigue damage models for magnesium alloys and provide information for the design of magnesium alloys with improved fatigue resistance.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.