Temperature-Dependent Structures and Atomic Mixing Behaviors in High-Entropy Alloys

The goal of the present dissertation is to advance the fundamental understanding of the atomic mixing behavior in a new class of alloys, known as high-entropy alloys (HEAs), and provide new methods to develop HEAs for practical applications. The HEA design strategy is based upon the influence of configurational entropy of mixing, which serves as a driving force for the formation of disordered solid-solution structures in certain alloys. In particular, alloys containing multiple elements have a tendency to form stable, disordered structures, sometimes with exceptional engineering properties. Despite the tendency toward structural disorder, HEAs usually have some degree of structural order. To date, rigorous descriptions and quantifications of the structural order are lacking. In the literature, the HEA structures are usually categorized in an all-or-nothing “ordered” versus “disordered” fashion. Furthermore, the HEA development efforts are mainly aimed at achieving disordered solid-solution structures. The present Ph.D. study develops new experimental and theoretical techniques for understanding and controlling structural order in HEAs. These new techniques are actually extensions of well-established approaches, such as the Bragg-Williams modeling of binary alloys, Monte Carlo simulations of atomic mixing, complementary neutron and X-ray scattering studies, and microscopy studies. The results reveal a persistent entropy effect, even when the HEAs undergo ordering transitions. which are quite complex and counterintuitive compared to the more familiar ordering behaviors in binary alloys. The conclusion is he HEA design strategy may be understood and applied in a broader fashion, with the goal of producing new engineering alloys, suitable for applications where high strength, toughness, and corrosion resistance at elevated temperatures are crucial.