Mechanism for amorphization of boron carbide B<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>4</mml:mn></mml:msub></mml:math>C under uniaxial compression

Aryal, Sitaram; Rulis, Paul; Ching, W. Y.

doi:10.1103/physrevb.84.184112

Cited by 81 publications

(67 citation statements)

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“…To understand these issues, it is necessary to examine the structural and bonding character in boron carbide under stresses. Previous simulations studied only hydrostatic or uniaxial compression on arbitrary directions [8,25]. We find that the directional nature of amorphous band formation requires study of a variety of deformation paths.…”

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confidence: 87%

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

2014

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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...

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

2014

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“…For boron carbide, with a low-symmetry rhombohedral lattice (26), different structure models predict different values. Using ab initio simulation, Aryal et al (27) calculated values of C 33 = 553.1 GPa and C 13 = 76.8 GPa; ignoring the small difference between C 13 and C 23 , Eq. 1 simplifies to…”

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Directional amorphization of boron carbide subjected to laser shock compression

Zhao

Kad

Remington

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

109

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Solid-state shock-wave propagation is strongly nonequilibrium in nature and hence rate dependent. Using high-power pulsed-laserdriven shock compression, unprecedented high strain rates can be achieved; here we report the directional amorphization in boron carbide polycrystals. At a shock pressure of 45∼50 GPa, multiple planar faults, slightly deviated from maximum shear direction, occur a few hundred nanometers below the shock surface. High-resolution transmission electron microscopy reveals that these planar faults are precursors of directional amorphization. It is proposed that the shear stresses cause the amorphization and that pressure assists the process by ensuring the integrity of the specimen. Thermal energy conversion calculations including heat transfer suggest that amorphization is a solid-state process. Such a phenomenon has significant effect on the ballistic performance of B 4 C.oron carbide is one of the hardest materials on earth while extremely lightweight, making it excellent for ballistic protection applications such as body armor (1-6). Thus, its dynamic behavior under impact/shock loading has been the subject of intensive studies for decades (1,(5)(6)(7)(8)(9)(10)(11)(12)(13). It is known that boron carbide undergoes an abrupt shear strength drop at a critical shock pressure around 20∼23 GPa, suggesting a deteriorated penetration resistance (8). Based on similar observations in geological materials (5), Grady (7) hypothesized that this was caused by localized softening mechanisms such as shear localization and/or melting. Examining fragments collected from a ballistic test using transmission electron microscopy (TEM), Chen et al. (14) were the first to identify localized amorphization in boron carbide, which appeared to be aligned to certain crystallographic planes. However, because the loading history of these fragments is unknown, its effect on the observed microstructure is not understood. Additionally, although the more well-defined loading conditions associated with quasi-static diamond-anvil cell (15) and nanoindentation (16, 17) experiments have provided greater insight into amorphization of boron carbide, they do not address the regime of high strain rate.The laser shock experimental technique offers promise in bridging the gaps of the previous experiments by enabling boron carbide to be shock compressed under controlled and prescribed uniaxial strain loading conditions and then recovered for postshock characterization by TEM. To ensure the integrity of the specimen, the duration of the stress pulse should be smaller than the characteristic time for crack propagation which is typically on the microsecond scale [limited by Rayleigh wave speed (18)]. Traditional dynamic loading methods such as plate impact and split Hopkinson bar cannot deliver the strain rates required because the stress pulses of both techniques occur on microsecond time scales. Therefore, brittle solids such as B 4 C will fail catastrophically by crack nucleation, propagation, and coalescence (19). To solve this ch...

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“…These experimental observations demonstrated that the amorphous shear bands are the dominant deformation and failure modes of B 4 C at room temperature. On the basis of computer simulations and spectroscopic investigations, several controversial models have been proposed to explain the formation of amorphous shear bands in the super-hard material 1,13,14,[16][17][18][19][20][21] . Nevertheless, the formation mechanisms of the amorphous shear bands still remain intensely debated owing to the lack of direct experimental observation with a sufficient spatial resolution to reveal the detailed structure of the amorphous bands.…”

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Atomic structure of amorphous shear bands in boron carbide

et al. 2013

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Amorphous shear bands are the main deformation and failure mode of super-hard boron carbide subjected to shock loading and high pressures at room temperature. Nevertheless, the formation mechanisms of the amorphous shear bands remain a long-standing scientific curiosity mainly because of the lack of experimental structure information of the disordered shear bands, comprising light elements of carbon and boron only. Here we report the atomic structure of the amorphous shear bands in boron carbide characterized by state-of-the-art aberration-corrected transmission electron microscopy. Distorted icosahedra, displaced from the crystalline matrix, were observed in nano-sized amorphous bands that produce dislocation-like local shear strains. These experimental results provide direct experimental evidence that the formation of amorphous shear bands in boron carbide results from the disassembly of the icosahedra, driven by shear stresses.

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Mechanism for amorphization of boron carbide B4C under uniaxial compression

Cited by 81 publications

References 40 publications

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

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

Directional amorphization of boron carbide subjected to laser shock compression

Atomic structure of amorphous shear bands in boron carbide

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