Atomic-Level Understanding of “Asymmetric Twins” in Boron Carbide

Abstract

Recent observations of planar defects in boron carbide have been shown to deviate from perfect mirror symmetry and are referred to as "asymmetric twins." Here, we demonstrate that these asymmetric twins are really phase boundaries that form in stoichiometric B 4 C (i.e., B 12 C 3 ) but not in B 13 C 2 . TEM observations and ab initio simulations have been coupled to show that these planar defects result from an interplay of stoichiometry, atomic positioning, icosahedral twinning, and structural hierarchy. The composition of icosahedra in B 4 C is B 11 C and translation of the carbon atom from a polar to equatorial site leads to a shift in bonding and a slight distortion of the lattice. No such distortion is observed in boron-rich B 13 C 2 because the icosahedra do not contain carbon. Implications for tailoring boron carbide with stoichiometry and extrapolations to other hierarchical crystalline materials are discussed. DOI: 10.1103/PhysRevLett.115.175501 PACS numbers: 61.72.Nn, 61.50.Ah, 61.50.Nw In crystalline materials the formation of twin boundaries, which separate adjacent crystallographic regions whose lattices are related by mirror symmetry, have been associated with both crystal growth and deformation processes. Because of their inherent symmetry, twin boundaries are usually coherent, have low interfacial energy, and are relatively stable as compared to general grain boundaries of random misorientation [1]. The formation of twins and the presence of twin boundaries can significantly affect the plasticity and strength of materials. The latter is demonstrated by the development of twinning-induced steels [2], recent observations that nanotwinned Cu is ten times stronger than coarse-grained Cu [3], and by reports that nanotwinned cubic BN is harder than diamond [4,5]. In this light, understanding how twins are formed and developing effective strategies for incorporating twin boundaries into polycrystalline microstructures offer an attractive approach for enhancing the mechanical response of metals and ceramics.Twin boundaries in relatively simple systems, such as fcc, bcc, and hcp, can be easily identified with the unambiguous twin planes and misorientation angles. However, as the crystal structure becomes more complicated and exhibits secondary and tertiary structural hierarchy (e.g., boron carbide [6]), the matrix-twin relationship can be complex. Recently, Fujita et al. discovered a new type of planar defect in boron carbide and characterized it with spherical-aberration-corrected scanning transmission electron microscopy (STEM) [7]. At first glance their highresolution STEM images suggest that the planar defects are conventional twin boundaries, but closer investigation reveals that the lattices do not mirror each other exactly, the angle between the (100) and (010) planes differs by ∼2°o n either side of the boundary. Upon realizing the loss of mirror symmetry, the authors named these planar defects "asymmetric twins" and stated that their formation mechanisms were not fully understood. At this poi...

New Ground-State Crystal Structure of Elemental Boron

“…The predicted density of τ-B 106 is 2.326 g=cm 3 , which is similar to predicted densities of 2.328 g=cm 3 for β-B 106 , 2.476 g=cm 3 for α-B 12 , and 2.566 g=cm 3 for γ-B 28 . The bonding in β-B is complex making it difficult to understand why τ-B 106 is more stable than β-B 106 but the electron counting rules proved valuable in understanding other phases of boron [43,46] should apply equally to τ-B.…”

“…However, the quantum mechanics studies [15,16] predict that the α-B 12 structure is more stable than β-B 105 by 25.3 meV=atom, leading to a long debate of which phase is the ground state structure for elemental boron [6,[15][16][17]24]. Recent quantum mechanics studies have suggested that particular choices for the partial occupation sites in β-B 105 and including zero point motion might lead to an energy for β-B 105 that is more stable than α-B 12 structure at ambient conditions [16,25].Twinned structures have been observed in β-B 105 [26,27] and boron related materials such as B 4 C [28]. Although the growth conditions to form these twinned structures are not clear [27,29], the twinned structure might dramatically change material properties such as charge capacitance [30].…”

“…Twinned structures have been observed in β-B 105 [26,27] and boron related materials such as B 4 C [28]. Although the growth conditions to form these twinned structures are not clear [27,29], the twinned structure might dramatically change material properties such as charge capacitance [30].…”

Directional amorphization of boron carbide subjected to laser shock compression

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Competition between densification and microstructure development during spark plasma sintering of B4C–Eu2O3

“…The noted difference in inclination angle (~1.8°) between the planes at either side of the boundary signifies asymmetric nature of twin. Asymmetric twin boundaries are reported to be influenced by B 4 C stoichiometry and its atomic configurations . In the present study, during the course of reaction sintering, the formation of structural vacancies and the diffusion of constituent elements would alter the local atomic arrangements and thereby cause deviation from the stoichiometry of B 4 C phase.…”