An official website of the United States government
Twin boundaries (TBs) play an essential role in enhancing the mechanical, electronic and transport properties of polycrystalline materials. However, the mechanisms are not well understood. In particular, we considered that they may play an important role in boron rich boron carbide (B vr BC), which exhibits promising properties such as low density, super hardness, high abrasion resistance, and excellent neutron absorption. Here, we apply first-principles-based simulations to identify the atomic structures of TBs in B vr BC and their roles for the inelastic response to applied stresses. In addition to symmetric TBs in B vr BC, we identified a new type of asymmetric twin that constitutes the phase boundaries between boron rich boron carbide (B 13 C 2 ) and B vr BC (B 14 C). The predicted mechanical response of these asymmetric twins indicates a significant reduction of the ideal shear strength compared to single crystals B 13 C 2 and B 14 C, suggesting that the asymmetric twins facilitate the disintegration of icosahedral clusters under applied stress, which in turn leads to amorphous band formation and brittle failure. These results provide a mechanistic basis towards understating the roles of TBs in B vr BC and related superhard ceramics.
more »
« less
Strengthening boron carbide through lithium dopant
Abstract The high strength of boron carbide (B4C) is essential in its engineering applications such as wear‐resistance and body armors. Here, by employing density functional theory simulations, we demonstrated that the strength of B4C can be enhanced by doping lithium to boron‐rich boron carbide (B13C2) to form r‐LiB13C2. The bonding analysis on r‐LiB13C2indicates that the electron counting rule (or Wade's rule) is satisfied in r‐LiB13C2whose formula can be written as r‐Li+(B12)2‐(CB+C). The shear deformation on r‐LiB13C2indicates that its ideal shear strength is larger than that of B4C because of the existing of Li dopant. The failure process of r‐LiB13C2under ideal shear deformation initiates from breaking the icosahedral‐icosahedral B‐B bonds. Then these B atoms react with the middle B in the C‐B‐C chain, resulting in the disintegration of icosahedral clusters and brittle failure. More interesting, the nanotwinned r‐LiB13C2is even stronger than r‐LiB13C2because of the directional nature of covalent bonding at the twin boundaries. This suggests that the nanotwinned r‐LiB13C2has a significant enhanced strength compared to B4C. Our simulation results illustrate the deformation mechanism of Li‐doped boron carbide and its nanotwinned microstructure. We proposed to improve the strength of boron carbide by doping Li into B13C2and increasing its twin densities.
more »
« less
- Award ID(s):
- 1727428
- PAR ID:
- 10458769
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 103
- Issue:
- 3
- ISSN:
- 0002-7820
- Format(s):
- Medium: X Size: p. 2012-2023
- Size(s):
- p. 2012-2023
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The low fracture toughness of strong covalent solids prevents them from wide engineering applications. Microalloying metal elements into covalent solids may lead to a significant improvement on mechanical properties and drastical changes on the chemical bonding. To illustrate these effects we employed density functional theory (DFT) to examine the bonding characteristic and mechanical failure of recently synthesized magnesium boride carbide (Mg3B50C8) that is formed by adding Mg into boron carbide (B4C). We found that Mg3B50C8has more metallic bonding charterer than B4C, but the atomic structure still satisfies Wade's rules. The metallic bonding significantly affects the failure mechanisms of Mg3B50C8compared with B4C. In Mg3B50C8, the B12icosahedral clusters are rotated in order to accommodate to the extensive shear strain without deconstruction. In addition, the critical failure strength of Mg3B50C8is slightly higher than that of B4C under indentation stress conditions. Our results suggested that the ductility of Mg3B50C8is drastically enhanced compared with B4C while the hardness is slightly higher than B4C.more » « less
-
As an electron-deficient element, boron possesses fascinating three-dimensional structures and unconventional chemical bonds. Nanoclusters of boron have also been found to exhibit intriguing structural properties, observed to have predominantly planar structures, in stark contrast to bulk boron allotropes, which are composed of the ubiquitous B12icosahedral building blocks. Here, we report observation of the 2D-to-3D transition and bulk-like structural features in the size-selected boron clusters, as revealed by photoelectron spectroscopy, chemisorption experiments, and first-principles calculations. In the small to medium cluster size range, planar boron cluster anions are found to be unreactive and only B46–and B56–are observed to chemisorb C2H4and CO under ambient conditions, suggesting major structural transitions at these cluster sizes. Notably, B56–is also found to be able to chemisorb and activate CO2. The global minimum of B46–is found to adopt a core-shell structure (B2@B44–), consisting of a B2core within a B44shell, reminiscent of the interstitial B2dumbbells in the high-pressureγ-B28form of bulk boron. More remarkably, both the global minimum and the second most stable isomer of B56–exhibit nest-like configurations, featuring the iconic B12icosahedral core surrounded by a B44half-shell (B12@h-B44–), signifying the onset of bulk-like structural characteristics in boron nanoclusters.more » « less
-
Abstract The structure of liquid lithium pyroborate, Li4B2O5(J= Li/B = 2), has been measured over a wide temperature range by high‐energy X‐ray diffraction, and compared to that of its glass and borate liquids of other compositions. The results indicate a gradual increase in tetrahedral boron fraction from 3(1)% to 6(1)% during cooling fromT= 1271(15) to 721(8) K, consistent with the largerN4 = 10(1)% found for the glass, and literature11B nuclear magnetic resonance measurements. van't Hoff analysis based on a simple boron isomerization reaction BØ3O2–⇌ BØO22–yields ΔH= 13(1) kJ mol–1and ΔS= 40(1) J mol–1 K–1for the boron coordination change from 4 to 3, which are, respectively, smaller and larger than found for singly charged isomers forJ ≤ 1. With these, we extend our model forN4(J,T), nonbridging oxygen fractionfnbr(J,T), configurational heat capacity , and entropySconf(J,T) contributions up toJ= 3. A maximum is revealed in atJ= 1, and shown semi‐quantitatively to lead to a corresponding maximum in fragility contribution, akin to that observed in the total fragilities by temperature‐modulated differential scanning calorimetry. Lithium is bound to 4.6(2) oxygen in the pyroborate liquid, with 2.7(1) bonds centered around 1.946(8) Å and 1.9(1) around 2.42(1) Å. In the glass,nLiO= 5.4(4), the increase being due to an increase in the number of short Li–O bonds.more » « less
-
Abstract Grain boundaries, ubiquitous in real materials, play an important role in the mechanical properties of ceramics. Using boron carbide as a typical superhard but brittle material under hypervelocity impact, we report atomistic reactive molecular dynamics simulations using the ReaxFF reactive force field fitted to quantum mechanics to examine grain‐boundary engineering strategies aimed at improving the mechanical properties. In particular, we examine the dynamical mechanical response of two grain‐boundary models with or without doped Si as a function of finite shear deformation. Our simulations show that doping Si into the grain boundary significantly increases the shear strength and stress threshold for amorphization and failure for both grain‐boundary structures. These results provide validation of our suggestions that Si doping provides a promising approach to mitigate amorphous band formation and failure in superhard boron carbide.more » « less
