Collaborative Research: In situ Diffraction and Cohesive-Zone Studies of the Fatigue-Crack-Growth Behavior in Mg Alloys

摘要：由于镁合金具有低密度、高强重比和高比刚度等特点，镁合金在汽车和航空航天领域的应用日益广泛。然而，镁合金的疲劳性能和相关的失效机制尚未得到很好的表征，这严重限制了这些轻量化合金的技术可行性。该奖项支持基础研究，以提供微观结构层面的失效过程的理解，最终控制镁合金的疲劳寿命。该研究将为智能化设计先进、轻量化、提高疲劳寿命的合金结构铺平道路。从更广泛的意义上说，这项研究将影响航空航天和汽车工业，并将帮助美国提高其制造业竞争力。独特的实验和建模工具将有助于丰富田纳西大学和伊利诺伊大学目前的力学和材料课程。一个示范工具包将向两所大学的高中生展示断裂和失效的概念，以及如何引入裂纹停止机制来提高疲劳寿命。技术摘要：本研究的目的是将镁合金的疲劳裂纹扩展研究与原位无损测量和微观力学建模研究相结合，从而建立微观破坏过程与宏观疲劳裂纹扩展特性之间的联系。主要目的是确定镁合金周围塑性和裂纹尖端加工区在抗疲劳裂纹扩展中的作用。在镁合金周围的塑性区，原位中子衍射测量和高能同步x射线衍射技术将为镁合金在任意应力多轴性下的塑性各向异性、双极性、流动非正态性和织构演化提供前所未有的信息。在无法进行实验测量的过程区域内，将使用一种新的非线性场投影格式，从衍射实验测量的周围变形场中唯一地反重构疲劳裂纹扩展的内聚区规律。通过将自上而下的应力分析与自下而上的颗粒间和颗粒内破坏机制联系起来，这项基础研究可以建立基于微观结构理解的预测模型，材料科学家可以利用这些模型来提高先进结构合金的疲劳寿命。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 Abstract: The accelerated adoption of magnesium alloys as structural components in the automobile and aerospace industry is driven by their unique properties of low density, high strength-to-weight ratio, and high specific stiffness. However, the fatigue properties of magnesium alloys and associated failure mechanisms have not been well-characterized, which severely limits the technological viability of these lightweight alloys. This award supports the fundamental research to provide the microstructural-level understanding of the failure processes, which ultimately govern the fatigue life of magnesium alloys. The research will pave the way towards the intelligent design of advanced, lightweight structural alloys with the improved fatigue life. In the broader sense, this research will impact the aerospace and automotive industries and would help U.S. improve its manufacturing competitiveness. The unique experimental and modeling tools will help enrich the current course curriculum on mechanics and materials at both the University of Tennessee and University of Illinois. A demonstrative toolkit will expose high school students at both universities to concepts of fracture and failure, and how crack stopping mechanisms can be introduced to improve fatigue life.Technical Abstract:The goal of this research is to couple fatigue-crack-growth studies of magnesium alloys, with in situ nondestructive measurements and micromechanical modeling investigations, which will establish the connection between microscopic failure processes and macroscopic fatigue-crack-growth properties. The primary objective is to identify the roles of the surrounding plasticity and crack-tip process zones in the resistance to fatigue-crack growth of magnesium alloys. Within the surrounding plastic zone, in situ neutron-diffraction measurements and high-energy synchrotron X-ray diffraction techniques will provide the unprecedented information on plastic anisotropy, twin polarity, flow non-normality, and texture evolution in magnesium alloys under the arbitrary stress multiaxiality. Within the process zone, which is inaccessible to experimental measurements, a novel nonlinear field projection scheme will be used to inversely reconstruct the cohesive zone laws for fatigue-crack growth uniquely from the surrounding deformation fields that are measured by the diffraction experiments. By linking top-down stress analyses with bottom-up failure mechanisms at inter- and intra-granular scales, this fundamental research can lead to predictive models based on microstructural understanding with which materials scientists can utilize to improve the fatigue life of advanced structural alloys.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.