Mechanics of Fatigue in High to Medium Entropy Alloys

结构材料是一个国家基础设施的支柱，它决定着核能、航空航天、汽车和民用领域的主要设计进步。为了实现节能的未来，对更高强度和抗疲劳材料的需求不断增加。直到最近，所有材料设计的创新都依赖于“合金化”的概念，即理想的固-固金属溶液来自单一的主要基本元素。然而，近年来见证了范式的转变，发现高熵合金含有多种元素（五种或更多）以等比例而不是一个主要元素，实现了前所未有的强度和抗疲劳水平。该奖项将支持对赋予这种前所未有的性能的主要微尺度机制的研究，这本质上是材料晶体结构中两种缺陷之间的相互作用。一种缺陷被称为“位错”，而另一种缺陷被称为“孪生边界”，它们的相互作用非常复杂，表现出丰富多样的结果，严重影响结构性能。在这种机制的治理下，材料增强和抗疲劳的预测理论尚未出现，将成为支持研究的重点。这项研究还将培养一批多样化的研究生和本科生，使他们能够与该领域的其他研究人员互动，并为未来的劳动力做好准备。该项目将提出一种从第一性原理分析高中熵合金固有强度、应变硬化和抗疲劳性能的新方法。目前对这些性能特征的理解受到核心结构位错处理不当的影响。提出了一种同时包含连续应变能和失配原子断层能的位错核宽度预测框架。该框架进一步丰富，以捕捉这些合金中丰富的滑移孪晶相互作用中位错的核心演化。这两个框架综合考虑了弹性各向异性、滑移和孪晶的层错能以及滑移-孪晶相互作用的残余Burgers向量，从而从头开始预测屈服时的流动应力和应变硬化过程中的应力升高。一个新的疲劳阈值模型将与这些预测相结合，以量化疲劳裂纹扩展的阻力。这项工作将消除一切经验主义，带来一种严谨的科学方法。科学方法的优点是通过确定关键的显微组织参数来实现，这些参数将成为开发高至中等熵合金的研究机构的可优化目标。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。

Structural materials form the backbone of a country’s infrastructure dictating major design advancements in nuclear, aerospace, automotive, and civil sectors. There is an ever-increasing need for higher material strength and fatigue-resistance to advance toward an energy-efficient future. Until recently, all innovations in materials-design relied on the concept of “alloying” where desirable solid-solid metallic solutions were derived from a single predominant base element. However, recent years have witnessed a paradigm shift with the discovery of high entropy alloys containing multiple elements (five or more) in equal proportions instead of one principal element, realizing unprecedented levels of strength and fatigue-resistance. This award will support research into the primary micro-scale mechanism that imparts such unprecedented properties, which is essentially an interaction between two kinds of defects within the material’s crystalline structure. One defect is known as a “dislocation” while the other a “twin boundary” and their interaction is highly complex, exhibiting a rich variety of outcomes critically affecting structural performance. A predictive theory for material strengthening and fatigue-resistance under governance of such a mechanism has not emerged and will be the focus of supported research. The research will also train a diverse group of graduate and undergraduate students, allowing them to interact with other researchers in the field, and preparing them for the future workforce. The project will forward a novel methodology for analyzing intrinsic strength, strain-hardening and fatigue-resistance of high-to-medium entropy alloys from first principles. State of the art understanding of these performance characteristics suffer from an improper treatment of core structure of dislocations. A predictive framework for dislocation core-width capturing both continuum strain-energies and atomistic fault-energies of misfit is developed. This framework is further enriched to capture core-evolution of dislocations in slip-twin interactions abundant in these alloys. Both frameworks comprehensively account for elastic anisotropy, stacking fault energies of slip and twinning and residual Burgers vectors of slip-twin interactions resulting in ab initio predictions of flow stress at yield and stress-elevations during strain-hardening. A novel model for the fatigue-threshold will be developed coupling with these predictions to quantify resistance to fatigue crack extension. The work will remove all empiricism and bring about a rigorous scientific approach. The merits of the scientific approach are realized by the identification of key microstructural parameters which will serve as optimizable targets for the body of research on the development of high-to-medium entropy 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.