Future energy must be sustainable, with minimum pollutants and reduced carbon footprint, and more efficiently generated. For efficient energy conversion, the operating temperature of a heat engine should be as high as possible and materials used for the engine components must be able to withstand the high operating temperature. Traditional design of alloys is to select the major component based on a specific property requirement, and further to use alloying additions to confer secondary properties without sacrificing the primary property. High-entropy alloys are multi-components materials containing at least five components in approximately equiatomic proportions, in contrast to traditional alloys, which are primarily based on one major component except with some minor alloying additions. Recent experiments show that these alloys can be promising as high-temperature materials. This discovery opens a new avenue for the development of high-temperature alloys. The current project is expected to advance the research and engineering education activities in the area of advanced materials at the University of Tennessee. It also incorporates several programs to recruit and enhance women and minority participation in science and engineering projects. Programs include National Consortium for Graduate Education for the Minorities and UT College of Engineering's Pipeline Engineering Diversity Program, to mentor undergraduate students and foster their interests in the research. In addition, the the principal investigator will participate in outreach to local high schools to promote student awareness of science and technologies.
High-entropy alloys are a new class of materials, which contain typically over 5 components in approximately equiatomic proportions, in contrast to traditional alloys, which are primarily based on one major component alloyed with some additional minor elements. As a result of high configuration entropy, these new alloys generally exhibit significant and surprising degree of mutual solubility in a single face-centered cubic or body-centered cubic phase. Also due to partitioning of several different atoms, these alloys generally show sluggish diffusion, thus are promising materials for high-temperature creep resistance. Some of these alloys were reported to be hardenable upon aging and exhibited significant strengthening, similar to conventional Al, steels, and Ni based alloys. Recent experiments from this group demonstrated that large volume fraction of coherent nanoprecipitates with a lattice mismatch strain as small as 0.58% were formed in some high-entropy alloys, in a fashion similar to that observed in Ni superalloys (mismatch strain ~0.5%). Based upon these promising observations, this project aims at developing an understanding of precipitating coherent nanoparticles in high-entropy alloys and also the resulting improvement on deformation resistance, in particular, at elevated temperatures, in this new class of materials. The research includes tasks of a basic study of the precipitation kinetics, factors determining the thermal stability of nanoprecipitates, and the underpinned mechanisms of the physical/chemical interactions between dislocations and nanoprecipitates. Specifically, the project will study and improve the thermal stability of precipitates in high-entropy alloys through deliberate control of the precipitate-matrix interface energy. High-resolution electron microscopy, atom probe and high-energy x-ray will be employed to characterize the composition, morphology and distribution of nanoprecipitates with special attention on the structure and chemistry of the nanoprecipitate-matrix interface. Factors determining the interface thermal stability will be identified. Mechanical interactions between dislocations and nanoprecipitates during deformation at both room and high temperatures will be examined and the strengthening efficacy will be evaluated. The initial study of L12 precipitate-strengthened face-centered cubic high-entropy alloys, such as the CoCrFeMnNi-based, will be conducted and, eventually, extended to body-centered cubic alloy systems. On the theory side, preliminary simulations will be conducted on interface solute segregation and its effect on the interface energy in simple binary or even tertiary systems. Calculations will be expanded to more complex alloy systems only after practical tools are available. The completion of this project will offer useful technical information to the development of structural nanomaterials. The evaluation of precipitation-hardened high-entropy alloys can also aid in the design and improved application of high-temperature structural components or functional devices, for example, diffusion barriers for microelectronic interconnects. Moreover, research on high-entropy alloys will stimulate the scientific interest on the study and understanding of multicomponent systems, in particular, near the center of the phase diagram, which is virtually unexplored.
