General quantification of fatigue damage with provision for microstructure: A review

Influence of induction hardening residual stresses on rolling contact fatigue lifetime

Boron carbide is super-strong and has many important engineering applications such as body armor and cutting tools. However, the extended applications of boron carbide have been limited by its low fracture toughness arising from anomalous brittle failure when subjected to hypervelocity impact or under high pressure. This abnormal brittle failure is directly related to the formation of a tiny amorphous shear band of 2–3 nm in width and several hundred nm in length. In this Perspective, we discuss mitigating the amorphous shear bands in boron carbide from various strategies including microalloying, grain boundary engineering, stoichiometry control, and the addition of a second phase. Combined with recent theoretical and experimental studies, we discuss strategies that can be applied in synthesizing and producing boron carbide-based materials with improved ductility by suppressing the formation of the amorphous shear band.

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Mitigating the formation of amorphous shear band in boron carbide

In this work, we focus on the understanding gained from the investigation of the physical properties of boron carbides with theoretical methods based on density functional theory (DFT). Together with the examination of the DFT total energies of various atomic configurations in the unit cell, comparison with the experiments of the theoretical vibrational or NMR spectra has led to the determination of the atomic structure of B4C as C-B-C chains linking mostly B11C icosahedra, and a few percents of B10C2 icosahedra. In the icosahedron, the carbon atom is found to be in the polar site (B4Cp). When there are two carbon atoms, they are found to be in antipodal polar positions. At carbon concentrations other than 20%, we find that only four structural models have a negative formation energy with respect to a formation from alpha-boron + diamond. Moreover, they all have a positive formation energy with respect to B4Cp, showing a tendency to decompose into B4Cp + alpha-boron or B4Cp + diamond. This metastability explains actual difficulties in the synthesis of clean samples, in particular for B13C2. Finally, the idea of combining high hardness and superconductivity in the same material by doping boron-rich solids has emerged. We show results on the strength of the electron-phonon coupling constant obtained with DFT-based methods in B13C2.

http://absimage.aps.org/image/MAR09/MWS_MAR09-2008-005515.pdf

Boron carbides from first principles

Homogenous distribution of whiskers in the ceramic matrix is difficult to be achieved. To solve this problem, B4C-SiCw powder mixtures were freeze dried from a slurry dispersed by cellulose nanofibrils (CellNF) in this work. Dense B4C ceramics reinforced with various amounts of SiCw up to 12 wt% were consolidated by spark plasma sintering (SPS) at 1800 °C for 10 min under 50 MPa. During this process, CellNF was converted into carbon nanostructures. As iron impurities exist in the starting B4C and SiCw powders, both thermodynamic calculations and microstructure observations suggest the dissolution and precipitation of SiCw in the liquids composed of Fe-Si-B-C occurred during sintering. Although not all the SiCw grains were kept in the final ceramics, B4C-9 wt% SiCw ceramics sintered at 1800 °C still exhibit excellent Vickers hardness (35.5 ± 0.8 GPa), flexural strength (560 ± 9 MPa) and fracture toughness (5.1 ± 0.2 MPa·m1/2), possibly contributed by the high-density stacking faults and twins in their SiC grains, no matter in whisker or particulate forms. 

Processing and mechanical properties of B4C-SiCw ceramics densified by spark plasma sintering

Accounting for microstructure sensitivity and plasticity in life prediction of heavily loaded contacts under rolling contact fatigue

Intrinsic hardness of boron carbide: Influence of polymorphism and stoichiometry

Shock response of single-crystal boron carbide along orientations with the highest and lowest elastic moduli

Bond resistance, bond strength and electronegativity models have been widely utilized for predicting intrinsic hardness of novel covalent materials. Although these models are generally perceived to be distinct, this manuscript recognizes them to be drawing input parameters from the same basis set comprising of bond density, bond length, electronegativity, and coordination number, while primarily differing in the exponents assigned to them. A numerical bound on the feasible space of these exponents is derived, followed by a search operation of this space to determine optimal values. The resulting model yields hardness estimates in good conformance with experimental data for numerous binary and ternary ceramic compounds. The study also provides a plausible explanation for the diversity of parameter–exponent combinations in the existing intrinsic hardness models, that is derived from the variability exhibited by these exponents in capturing a given experimental dataset at a constant fitting error.

Amith Adoor Cheenady

Papers

Accounting for microstructure sensitivity and plasticity in life prediction of heavily loaded contacts under rolling contact fatigue

Intrinsic hardness of boron carbide: Influence of polymorphism and stoichiometry

Shock response of single-crystal boron carbide along orientations with the highest and lowest elastic moduli

Intrinsic hardness of covalent crystals: a unified multiparametric framework

Stress wave propagation through a 180° bend junction in a square cross-sectional bar