Synthesis of Dual-Phase High-Entropy Ultrahigh Temperature Ceramics and Reactive Sintering of Boride-Based High-Entropy Ceramics

The everlasting demands of new materials to fulfill certain functional or structural applications stimulate the exploration and innovation of materials science. As a brand new field in material development, high-entropy materials have received great amounts of research effort in the past decade. In the community of metallurgy, high-entropy alloys are widely reported to possess excellent physical and mechanical properties beyond each of their individual components. As ceramics counterparts to high-entropy alloys, high-entropy ceramics present another big family in high-entropy materials and have attracted increased attention since their inception in 2015. Over the past few years, numerous amounts of high-entropy ceramics of different kinds have been synthesized and characterized; and various fabrication routes have been developed and analyzed. Despite all these achievements, high-entropy ceramics is still a fledging topic with plenty of opportunities and challenges. In this dissertation, bulk synthesis and densification of dual-phase high-entropy ultrahigh temperature ceramics and four classes of high-entropy borides of different structures are investigated. First, a new series dual-phase high-entropy boride and carbide ultrahigh temperature ceramics are fabricated to full density in bulk pellets. Binary borides and carbides are utilized as the precursors. Extra addition of graphite and prolong holdings at elevated temperatures during sintering facilitate the removal of intrinsic oxides. The sintered dual-phase specimens are demonstrated to possess tunable microstructures and properties, as well as higher hardness than the average of single-phase high-entropy components. Then, three classes of refractory metal high-entropy borides, each with prototype of Ta3B4, CrB, or AlB2, are synthesized via in-situ reactive spark plasma sintering of ball-milled elemental powders; and all sintered specimens are close to fully dense without measurable oxides. W-containing high-entropy diborides from elemental precursors are fabricated to be single-phase, which is not alternatively achievable from binary boride precursors; and somewhat unexpectedly, these W-containing high-entropy diborides are measured to possess higher hardness, although WB2 are predicted to have lower hardness than other refractory metal diborides. On the other hand, high-entropy monoborides represent a promising series towards superhard materials, with composition (V0.2Cr0.2Nb0.2Ta0.2W0.2)B demonstrated to be harder than the reported superhard ternary monoboride solid solutions. The final part of the dissertation focuses on the rare earth high-entropy borides; and a novel class of high-entropy tetraborides with UB4-typed tetragonal structure has been successfully developed. The homogenous elemental distributions in the sintered specimen have been verified by both micro-scale SEM and nano-scale TEM characterizations. These high-entropy tetraboride represent the first equimolar high-entropy ceramics in tetragonal structure. The production of these novel high-entropy materials further expands the scope of high-entropy ceramics; and the fabrication route of reactive sintering from elemental precursors facilitates the synthesis of other high-entropy materials at different cation-anion ratios. The successful synthesis of these new materials provides another platform for composition adjustment and property tailoring