Defect Structure and Mechanical Behavior of High Entropy Alloys

TECHNICAL SUMMARY: Convention has dictated that multi-component alloy systems either form intermetallic phases, brittle microstructures, or metallic glasses during alloying. Recently it has been found that some alloys with five components or more, all with approximately equiatomic compositions, form simple structures with unique physical properties. These materials may exhibit high mechanical hardness and strength. A fundamental understanding of the mechanical properties and the degree of the stability of these desirable structures has not been reached. The proposed work investigates the nature of defects as well as the mechanical properties of a series of fcc solid solution alloys that contain two to seven components of equiatomic composition. Alloy samples will be created experimentally by either casting or ball milling methods. EDS and SEM will be used to perform chemical analysis, TEM and XRD will be used to measure stable stacking fault energies, and DSC will be used to study structural stability. Mechanical properties will be measured through both tensile tests and nanoindentation. First-principles density functional theory based methods will be used to investigate the physical properties of the alloy system. Elastic constants will be calculated and used to help interpret the stacking fault energies measured experimentally with XRD. Additionally, the elastic constants will also be used to directly evaluate the mechanical stability of the alloy. Stable and unstable stacking fault energies will be directly calculated through calculation of generalized stacking fault energy curves. These quantities will be used to predict and interpret the plastic mechanical response. Finally, low-angle grain boundary energies and any preferential atomic segregation to grain boundaries will be directly studied from first principles.

NON-TECHNICAL SUMMARY: A fundamental understanding of the nature of defects and the mechanical characteristics of high entropy alloys is expected to provide a transformative assessment of the potential capabilities of these novel alloys. The properties of these materials will be systematically explored as a function of number of components (i.e. atom types) for alloys with the same underlying atomic structure. This study will provide critical insight into how defects in the same lattice change as a function of number of components. This derived understanding could then provide a foundation for how to better design multi-component alloys with simple metallic crystal structures. The resulting fundamental understanding of the mechanical properties and the nature and degree of the stability of these desirable structures could provide transformative capabilities in the design of future novel alloys and enable their use in a variety of applications. An equally important goal of this project is the development of intellectual resources in the form of M.S. and Ph.D. research students. The PIs are committed to this education effort, especially for groups underrepresented in science.