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_4C), the role of GB on the inelastic response to applied stresses is not well established. We consider here the unusual ceramic, boron carbide (B_4C), 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_4C 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 B4C. 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 B4C with improved strength and ductility

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_4C), the role of GB on the inelastic response to applied stresses is not well established. We consider here the unusual ceramic, boron carbide (B_4C), 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_4C 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 B4C. 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 B4C with improved strength and ductility

Locating Si atoms in Si-Doped Boron Carbide: a Route to Understand Amorphization Mitigation Mechanism

The 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 B_(12) 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 SiB_6, B_4C, 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

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 (B₄C), the role of GB on the inelastic response to applied stresses is not well established. We consider here the unusual ceramic, boron carbide (B₄C), 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 B₄C 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 B₄C. 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 B₄C with improved strength and ductility

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

Shock-induced Amorphization in Covalently Bonded Solids

Deposition of powerful pulsed laser energy onto a material, ablates its surface and drives a compressive shock wave propagating through it. Using this technique, unprecedented states of matter with extremely high pressures, temperatures, and strain rates can be experimentally studied. Here we report on laser-shock induced amorphization in four covalently bonded solids, namely silicon (Si), germanium (Ge), boron carbide (B4C) and silicon carbide (SiC). Post shock transmission electron microscopy reveals that the newly formed amorphous materials exhibit a shear band alike morphology, suggesting that shear stress play a dominant role in this process. The density of these amorphous band decreases as a function of the distance to the surface and eventually disappeared at certain depth, which is coincident with the decay of the shock wave and indicates that there might be a critical stress for the onset of amorphization. Synchrotron XRay tomography of a recovered silicon target shows that large amounts of cracks are formed within the materials and the density also decrease with depth. Unlike amorphous bands, these cracks can propagate through the target, albeit without shattering the entire material. It is proposed that shock-induced amorphization is a new deformation mechanism of matter under extremely high rate deformation

Shock-induced Amorphization in Covalently Bonded Solids

Deposition of powerful pulsed laser energy onto a material, ablates its surface and drives a compressive shock wave propagating through it. Using this technique, unprecedented states of matter with extremely high pressures, temperatures, and strain rates can be experimentally studied. Here we report on laser-shock induced amorphization in four covalently bonded solids, namely silicon (Si), germanium (Ge), boron carbide (B4C) and silicon carbide (SiC). Post shock transmission electron microscopy reveals that the newly formed amorphous materials exhibit a shear band alike morphology, suggesting that shear stress play a dominant role in this process. The density of these amorphous band decreases as a function of the distance to the surface and eventually disappeared at certain depth, which is coincident with the decay of the shock wave and indicates that there might be a critical stress for the onset of amorphization. Synchrotron XRay tomography of a recovered silicon target shows that large amounts of cracks are formed within the materials and the density also decrease with depth. Unlike amorphous bands, these cracks can propagate through the target, albeit without shattering the entire material. It is proposed that shock-induced amorphization is a new deformation mechanism of matter under extremely high rate deformation