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。人们对更高的材料强度和耐疲劳性的需求不断增加，以实现节能的未来。直到最近，材料设计的所有创新都依赖于“合金化”的概念，其中理想的固-固金属溶液来自于单一的主要基础元素。然而，近年来，随着高熵合金的发现，合金中含有等比例的多种元素（五种或更多种），而不是一种主要元素，实现了前所未有的强度和抗疲劳性。该奖项将支持对赋予这种前所未有的特性的主要微观机制的研究，这种机制本质上是材料晶体结构中两种缺陷之间的相互作用。一个缺陷被称为“位错”，而另一个是“孪生边界”，它们的相互作用非常复杂，表现出各种各样的结果，严重影响结构性能。在这种机制的治理下，材料强化和抗疲劳的预测理论还没有出现，将是支持研究的重点。该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。该项目将提出一种新的方法，从第一性原理分析高到中熵合金的本征强度，应变硬化和疲劳抗力。对这些性能特征的现有技术的理解受到对位错的核心结构的不适当处理的影响。一个预测框架的位错核心宽度捕获连续应变能和原子的错配能。该框架进一步丰富，以捕捉核心的位错在这些合金中丰富的滑移孪晶相互作用的演变。这两个框架全面考虑弹性各向异性，堆垛层错能的滑移和孪晶和剩余的伯格斯矢量的滑移-孪晶相互作用，从而导致在屈服和应力升高的应变硬化过程中的流动应力的从头算预测。将开发一种新的疲劳门槛值模型与这些预测耦合，以量化疲劳裂纹扩展阻力。这项工作将消除所有经验主义，并带来严格的科学方法。科学方法的优点是通过识别关键的微观结构参数来实现的，这些参数将作为高到中熵合金开发研究的优化目标。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.