Mechanics of Fatigue in High to Medium Entropy Alloys

结构材料构成了一个国家基础设施的支柱，决定了核、航空航天、汽车和民用部门的重大设计进步。为了迈向节能的未来，人们对更高的材料强度和抗疲劳性的需求越来越大。直到最近，材料设计中的所有创新都依赖于“合金化”的概念，即理想的固体-固体金属溶液来自单一的主要基础元素。然而，近年来出现了一种范式转变，发现了含有多个元素(5个或更多)的等量高熵合金，而不是单一的主元素，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观机制的研究，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”，另一种称为“孪生边界”，它们之间的相互作用非常复杂，表现出对结构性能产生关键影响的各种结果。在这种机制的支配下，材料强化和抗疲劳的预测理论还没有出现，并将成为支持研究的重点。这项研究还将培训不同的研究生和本科生群体，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。该项目将提出一种新的方法，从第一性原理分析高到中等熵合金的固有强度、应变硬化和抗疲劳性。对这些性能特征的最新理解受累于对位错核心结构的不恰当处理。发展了位错芯宽度的预测框架，该框架既能捕捉连续应变能，又能捕捉原子错配的断层能。这一框架被进一步丰富，以捕捉在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、滑移和孪生的层错能以及滑移-孪晶相互作用的残余Burgers矢量，从而从头计算预测了屈服时的流动应力和应变硬化过程中的应力升高。结合这些预测，将开发一个新的疲劳门槛模型，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，带来一种严格的科学方法。科学方法的优点是通过确定关键的微观结构参数来实现的，这些参数将作为开发高到中等熵合金的研究机构的优化目标。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。

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.