title: High-Entropy Metal Diborides: A New Class of High-Entropy Materials and a New Type of Ultrahigh Temperature Ceramics.
abstract: Seven equimolar, five-component, metal diborides were fabricated via high-energy ball milling and spark plasma sintering. Six of them, including (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2, (Hf0.2Zr0.2Mo0.2Nb0.2Ti0.2)B2, (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2, (Mo0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, and (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2, possess virtually one solid-solution boride phase of the hexagonal AlB2structure. Revised Hume-Rothery size-difference factors are used to rationalize the formation of high-entropy solid solutions in these metal diborides. Greater than 92% of the theoretical densities have been generally achieved with largely uniform compositions from nanoscale to microscale. Aberration-corrected scanning transmission electron microscopy (AC STEM), with high-angle annular dark-field and annular bright-field (HAADF and ABF) imaging and nanoscale compositional mapping, has been conducted to confirm the formation of 2-D high-entropy metal layers, separated by rigid 2-D boron nets, without any detectable layered segregation along the c-axis. These materials represent a new type of ultra-high temperature ceramics (UHTCs) as well as a new class of high-entropy materials, which not only exemplify the first high-entropy non-oxide ceramics (borides) fabricated but also possess a unique non-cubic (hexagonal) and layered (quasi-2D) high-entropy crystal structure that markedly differs from all those reported in prior studies. Initial property assessments show that both the hardness and the oxidation resistance of these high-entropy metal diborides are generally higher/better than the average performances of five individual metal diborides made by identical fabrication processing.
authors.affiliation: Program of Materials Science and Engineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Joshua
authors.initials: J
authors.lastname: Gild
authors.affiliation: Program of Materials Science and Engineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Yuanyao
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Program of Materials Science and Engineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Tyler
authors.initials: T
authors.lastname: Harrington
authors.affiliation: Program of Materials Science and Engineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Sicong
authors.initials: S
authors.lastname: Jiang
authors.affiliation: Department of NanoEngineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Tao
authors.initials: T
authors.lastname: Hu
authors.affiliation: Department of NanoEngineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Quinn
authors.affiliation: Department of NanoEngineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: William M
authors.initials: WM
authors.lastname: Mellor
authors.affiliation: Program of Materials Science and Engineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Naixie
authors.initials: N
authors.lastname: Zhou
authors.affiliation: Program of Materials Science and Engineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.|Department of NanoEngineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Kenneth
authors.initials: K
authors.lastname: Vecchio
authors.affiliation: Program of Materials Science and Engineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.|Department of NanoEngineering, University of California, San Diego, La Jolla, CA 92093-0448, USA.
authors.forename: Jian
authors.initials: J
authors.lastname: Luo