Mechanics of Fatigue in High to Medium Entropy Alloys

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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.