Atomistic Explanation of Shear-Induced Amorphous Band Formation in Boron Carbide

Abstract

Boron carbide (B 4 C) is very hard, but its applications are hindered by stress-induced amorphous band formation. To explain this behavior, we used density function theory (Perdew-Burke-Ernzerhof flavor) to examine the response to shear along 11 plausible slip systems. We found that the ð0111Þ=h1101i slip system has the lowest shear strength (consistent with previous experimental studies) and that this slip leads to a unique plastic deformation before failure in which a boron-carbon bond between neighboring icosahedral clusters breaks to form a carbon lone pair (Lewis base) on the C within the icosahedron. Further shear then leads this Lewis base C to form a new bond with the Lewis acidic B in the middle of a CBC chain. This then initiates destruction of this icosahedron. The result is the amorphous structure observed experimentally. We suggest how this insight could be used to strengthen B 4 C. DOI: 10.1103/PhysRevLett.113.095501 PACS numbers: 61.50.Ks, 62.20.M-, 64.70.-p, 82.40.Fp Boron carbide (B 4 C) exhibits such novel properties as high melting temperature, high thermal stability, high hardness, low density, and high abrasion resistance [1][2][3][4][5][6][7][8][9][10][11]. The combination of these properties makes it widely useful in refractory applications, as abrasive powders, in body armor, and as a neutron radiation absorbent [1][2][3][4]. However, the brittle failure under impact exhibited by B 4 C prevents the wide insertion of boron carbide into extended engineering applications. Although B 4 C has a high Hugoniot elastic limit (HEL) of 17-20 GPa, approximately twice that of normal ceramics, it fractures easily just above the HEL at high impact velocities and pressures [12][13][14][15]. Chen et al. reported the observation of shock-induced local amorphization bands that might be responsible for the low fracture toughness of B 4 C [6]. In addition, amorphization bands had been observed in nanoindentation and scratch experiments, where the loading rate is much lower than for dynamical shock loading [16][17][18][19]. Particularly, recent nanoindentation experiments revealed amorphous shear bands along the (0111) plane [19]. Indeed, the same amorphous shear band has been observed experimentally in boron suboxide (B 6 O) [20]. An attempt to understand this through Gibbs free-energy calculations based on density functional theory suggested that the ðB 12 ÞCCC structure provides a possible source of failure of boron carbide just above the HEL [7]. However, despite these extensive experimental and theoretical efforts, the atomistic mechanism underlying pressure-induced amorphization and phase transitions of boron carbide is still not known [3].Stressed materials normally dissipate the accumulating elastic energy through plastic deformation that is mediated by dislocation slip and deformation twinning in metallic systems. Unlike these general deformation mechanisms in conventional metallic materials, we find that boron carbide deforms by atomic scale amorphous band formation, a particular mode of deforma...

Structure and Properties of Boron-Very-Rich Boron Carbides: B12 Icosahedra Linked through Bent CBB Chains

“…36 This is consistent with the experimental data for boron rich boron carbide, 20 indicating that the higher boron content leads to lower elastic moduli. The Pugh's ductility indexes B/G for linear and bent chain structures are 1.18 and 1.12, suggesting that both structures are brittle.…”

“…The Pugh's ductility indexes B/G for linear and bent chain structures are 1.18 and 1.12, suggesting that both structures are brittle. To investigate the brittle failure processes of (B 12 )CBB structures, we applied pure shear deformation on the linear chain structure along the most plausible slip system (001)/<100> in B 4 C. 36 We find that the high energy linear chain structure transforms easily to the bent chain 12 structure (discussed below), again indicating an unstable structure. In addition, in real materials the bent CBB chain can randomly bend in various planes with respect to nearby icosahedra, making the structures more complex.…”

“…16,20,[30][31][32][33] The (B 11 C) icosahedron in B 4 C satisfies Wade's rule 34,35 (26 e within icosahedron) by transferring one electron from the C-B-C chain to form a formal C-(B+)-C closed shell chain. 36 This seems unlikely for boron rich boron carbide. Thus, (B 13 C 2 ) with the structure (B 12 )(CBC) would only have 25e within the icosahedron, which might reduce symmetry or cause distortion of the structure.…”

Structure and Failure Mechanism of the Thermoelectric CoSb3/TiCoSb Interface

“…This relaxation method has been proven to be an effective tool to calculate the ideal strength, and sheds light on the intrinsic failure mechanism at the atomistic scale. [30][31][32] The tensile stress ij σ was calculated by, 33 …”

Особенности структуры карбида бора после приложения сдвиговых деформаций под давлением до 55 GPa

“…При исследовании дефектности карбида бора установ-лено, что он деформируется на атомарном уровне с образованием аморфной полосы [23]. В [8] наблюдалось образование внутризеренных аморфных полос шириной 2−3 nm.…”