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High-Entropy Boride-Carbide Two-Phase Ultrahigh Temperature Ceramics Fabricated by Reactive Spark Plasma Sintering | | Mingde Qin
; Joshua Gild
; Chongze Hu
; Haoren Wang
; Md Shafkat Bin Hoque
; Jeffrey L. Braun
; Tyler J. Harrington
; Patrick E. Hopkins
; Kenneth S. Vecchio
; Jian Luo
; | | Date: |
22 Feb 2020 | | Abstract: | A series of high-entropy boride-carbide two-phase ultrahigh temperature
ceramics (UHTCs) are fabricated via a novel reactive spark plasma sintering
(SPS) based route, each from five individual commercial powders, including N
metal diborides and (5-N) metal carbides, of equimolar amounts. Greater than
~99% of the theoretical densities have been achieved with virtually no native
oxides. These two-phase UHTCs consist of a hexagonal high-entropy boride (HEB)
phase and a cubic high-entropy carbide (HEC) phase, where a chemical
equilibrium between the two phases that drive them away from equimolar
compositions. A thermodynamic relation that governs the compositions of the HEB
and HEC phases in equilibrium is discovered and a thermodynamic model is
proposed. A new observation is that the high-entropy two-phase UHTCs have
higher hardness than the weighted linear average of the two single-phase HEB
and HEC, which are already harder than the rule-of-mixture (RoM) averages of
individual UHTCs. Several intriguing observations about the moduli are revealed
with accurate acoustic waves measurements. On the one hand, the Young’s moduli
of high-entropy two-phase UHTCs are always higher than the theoretical RoM
averages. On the other hand, the shear modulus of HEC is higher, but that of
HEB is lower, than the theoretical RoM average. The latter unexpected
observation is explained by the unique layered structure of the HEB, where an
expansion in lattice parameter c is confirmed by X-ray diffraction. This study
extends the state of the art by introducing two-phase high-entropy ceramics,
which provide a new platform to tailor various properties via changing the
phase fraction and microstructure. | | Source: | arXiv, 2002.9756 | | Services: | Forum | Review | PDF | Favorites | |
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