5 research outputs found
Shear-Induced Brittle Failure along Grain Boundaries in Boron Carbide
The
role that grain boundaries (GBs) can play on mechanical properties
has been studied extensively for metals and alloys. However, for covalent
solids such as boron carbide (B<sub>4</sub>C), the role of GB on the
inelastic response to applied stresses is not well established. We
consider here the unusual ceramic, boron carbide (B<sub>4</sub>C),
which is very hard and lightweight but exhibits brittle impact behavior.
We used quantum mechanics (QM) simulations to examine the mechanical
response in atomistic structures that model GBs in B<sub>4</sub>C
under pure shear and also with biaxial shear deformation that mimics
indentation stress conditions. We carried out these studies for two
simple GB models including also the effect of adding Fe atoms (possible
sintering aid and/or impurity) to the GB. We found that the critical
shear stresses of these GB models are much lower than that for crystalline
and twinned B<sub>4</sub>C. The two GB models lead to different interfacial
energies. The higher interfacial energy at the GB only slightly decreases
the critical shear stress but dramatically increases the critical
failure strain. Doping the GB with Fe decreases the critical shear
stress of at the boundary by 14% under pure shear deformation. In
all GBs studied here, failure arises from deconstructing the icosahedra
within the GB region under shear deformation. We find that Fe dopant
interacts with icosahedra at the GB to facilitate this deconstruction
of icosahedra. These results provide significant insight into designing
polycrystalline B<sub>4</sub>C with improved strength and ductility
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Directional amorphization of boron carbide subjected to laser shock compression
Solid-state shock-wave propagation is strongly nonequilibrium in nature and hence rate dependent. Using high-power pulsed-laser-driven 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 B4C
Directional amorphization of boron carbide subjected to laser shock compression
Solid-state shock-wave propagation is strongly nonequilibrium in nature and hence rate dependent. Using high-power pulsed-laser-driven 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
Locating Si atoms in Si-doped boron carbide: A route to understand amorphization mitigation mechanism
he well-documented formation of amorphous bands in boron carbide (B4C) under contact loading has been identified in the literature as one of the possible mechanisms for its catastrophic failure. To mitigate amorphization, Si-doping was suggested by an earlier computational work, which was further substantiated by an experimental study. However, there have been discrepancies between theoretical and experimental studies, about Si replacing atom/s in B12 icosahedra or the C-B-C chain. Dense single phase Si-doped boron carbide was produced through a conventional scalable route. A powder mixture of SiB6, B4C, and amorphous boron was reactively sintered, yielding a dense single phase Si-doped boron carbide material. A combined analysis of Rietveld refinement on XRD pattern coupled with electron density difference Fourier maps and DFT simulations were performed in order to investigate the location of Si atoms in the boron carbide lattice. Si atoms occupy an interstitial position, between the icosahedra and the chain. These Si atoms are bonded to the chain end C atoms, which result in a kinked chain. Additionally, these Si atoms are also bonded to the neighboring equatorial B atom of the icosahedra, which is already bonded to the C atom of the chain, forming a bridge like structure. Owing to this bonding, Si is anticipated to stabilize the icosahedra through electron donation, which is expected to help in mitigating stress-induced amorphization. Possible supercell structures are suggested along with the most plausible structure for Si-doped boron carbide