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Using the CASTEP software based on density functional theory, the crystal structure model of Mg8 Sn4-xMx (M = AI, Cu; x =0, 1 or 2) was constructed, and its lattice constant, structural stability and elastic constant were calculated by first-principles, as well as the electronic properties, elastic properties and intrinsic hardness of the crystal structure model with different amounts M atoms solid solution in Mg2Sn phase were analyzed. The calculation results show that the solid solution of the M atoms in Mg2Sn can generate spontaneously, and the obtained Mg8 Sn4-xMx(x = 1 or 2) crystal structure can exist stably; the crystal structure converts from cubic system to tetragonal system when two M atoms dissolve in Mg2Sn. The analysis results of density of states indicate that the atoms in the system exist obvious orbital hybridization by M atoms solid solution, then show strong covalent bonds, and increasing the number of M atoms will not affect the contribution of each atom to the density of states, but will increase the contribution of electrons in the atom to the density of states. The analysis results of elastic properties and intrinsic hardness indicate that the mechanical properties of the system are still stable after dissolving M atoms in Mg2Sn, and the hardness of the system decreases gradually and the plasticity increases continuously with the increase of M atoms solid solubility, i. e., the toughness and plasticity of the system can be improved by increasing M atoms solid solubility. [ABSTRACT FROM AUTHOR]

In this work, a machine learning (ML) model was created to predict intrinsic hardness of various compounds using their crystal chemistry. For this purpose, an initial dataset, containing the hardness values of 270 compounds and counterpart applied loads, was employed in the learning process. Based on various features generated using crystal information, an ML model, with a high accuracy (R2 = 0.942), was built using extreme gradient boosting (XGB) algorithm. Experimental validations conducted by hardness measurements of various compounds, including MSi2 (M = Nb, Ce, V, and Ta), Al2O3, and FeB4, showed that the XGB model was able to reproduce load-dependent hardness behaviors of these compounds. In addition, this model was also used to predict the behavior based on prototype crystal structures that are randomly substituted with elements.

(Co, Cr)23C6 type carbide is a typical metallic compound in many cobalt bearing alloys, and it acts as the strengthening phase in the form of bulk eutectic carbides or precipitated carbides. In this work, first-principles calculations were carried out to investigate the electronic structure, phase stability, mechanical and magnetic properties of (Co, Cr)23C6 with different cobalt occupation. Some of the calculated values are compared with previous studies and, they are found to be in a good agreement. The method considering curvature radius is firstly used to describe the degree of anisotropy. The hardness calculated through elastic constants presents an approximate downtrend with the cobalt concentration. Analysis of the density of states (DOS), overlapped population and electron density maps, indicates that the bonds in (Co, Cr)23C6 are the mixture of covalent, ionic and metallic bonds, the interactions of d-d orbits between metallic atoms contribute most to the hybridization mode. According to the population analysis, the reduction in hardness can attribute to the increase of metallicity and iconicity of the interacted metallic atoms. In addition, the formation of a large quantity of antibonding also plays a negative role in intrinsic hardness of (Co, Cr)23C6 when massive substitution of cobalt atom. • The mechanical and magnetic properties are calculated for (Co, Cr)23C6 system. • A new method is introduced to more intuitively describe the anisotropy properties. • Density of state, population and electronic density are combinedly analyzed to explain the tendency of hardness. [ABSTRACT FROM AUTHOR]

Abstract A deep knowledge of micromechanical properties of each constitutive phase of cemented carbides is crucial to improve their performance on the basis of microstructural design optimization. In the present work, a systematic experimental procedure has been followed to determine the intrinsic hardness of individual WC and (W,Ti,Ta,Nb)C particles, as well as that of the metallic binder, within a WC-Co composite containing a solid solution of mixed carbide as a third phase. In doing so, massive nanoindentation and statistical analysis of the gathered data are combined. Results showed that (W,Ti,Ta,Nb)C particles are significantly harder than WC ones, independent of the hardness anisotropy exhibited by the latter. Hardness assessment for the metallic binder required further analysis, including data deconvolution using thin film models and consideration of substrate effects. The attained hardness values are then used for estimation of effective flow stress of the metallic phase by means of Tabor's equation, yielding values ranging between 1.3 and 2.0 GPa. These high constraining-related values are finally validated by experimental assessment of stress levels at which strain bursts are identified in stress-strain curves obtained by uniaxial compression of micropillars. Graphical abstract Image 1 Highlights • Hardness for cubic mixed carbide in a cemented carbide composite is isotropic. • Surrounding carbides strongly affect hardness value for the constraint binder. • Constraint degree of binder is elevated and affects its flow stress. • Flow stress for constrained binder is validated by compression of micropillars. [ABSTRACT FROM AUTHOR]

This article attempts to critically review a rather extended field of ab initio calculations of mechanical properties of materials. After a brief description of the density functional theory and other approximations utilized in a majority of ab initio calculations, methods for predictions of elastic constants and moduli are presented. A relatively large space is devoted to computations of theoretical strength under various loading conditions. First we focus on results for perfect crystals and make an overview of advanced approaches to crystal stability. As case studies, elastic stability conditions defined according to both the adopted definition of elastic coefficients and the kind of applied loading are shown for isotropic tensile loading of molybdenum crystal and a model of microscopic deformation is illustrated for a soft phonon found in the dynamic stability analysis of isotropic loading of platinum crystal. Collected values of ideal strength under uniaxial/isotropic tension and simple shear for selected metallic and covalent crystals are discussed in terms of their comparison with available experimental data. Further attention is paid to results of studies on interfaces and grain boundaries. Applications of computed values of the moduli and the theoretical strength to prediction of intrinsic hardness and brittle/ductile behavior of crystalline materials and simulation of pop-in effect in nanoindentation tests are also included. Finally, remarks about possible topics for future ab initio studies and challenges for further development of computational methods are attached. [ABSTRACT FROM AUTHOR]

In this work, new ternary cubic spinel structures are designed by the substitutional method. The structures, elasticity properties, intrinsic hardness and Debye temperature of the cubic ternary spinel nitrides are studied by first principles based on the density-functional theory. The results show that γ-CSn2N4, γ-SiC2N4, γ-GeC2N4and γ-SnC2N4are not mechanically stable. The elastic constantsCijof these cubic spinel structures are obtained using the stress–strain method. Derived elastic constants, such as bulk modulus, shear modulus, Young's modulus, Poisson coefficient and brittle/ductile behaviour are estimated using Voigt–Reuss–Hill theories. TheB/Gvalue, the Poisson's ratio and anisotropic factor are calculated for eight ternary stable crystals. Based on the microscopic hardness model, we further estimate the Vickers hardness of all the stable crystals. From the calculated hardness of the stable group IVA ternary spinel nitrides by Gao's and Jiang's methods, it is observed that the stable group IVA ternary spinel nitrides are not superhard materials except for γ-CSi2N4. Furthermore, the Debye temperature for the eight stable crystals is also estimated. [ABSTRACT FROM AUTHOR]

Abstract: In this work, the new ternary nitrides (CSi2N4 and SiC2N4) are designed by the substitution method. The structures, elastic properties, intrinsic hardness and Debye temperature of the new ternary nitrides (CSi2N4 and SiC2N4) are studied by first-principles calculations based on the density-functional theory. The elastic constants of these new ternary nitrides are obtained using the stress–strain method. Derived elastic constants, such as bulk modulus, shear modulus, Young''s modulus, Poisson coefficient and brittle/ductile behavior are estimated using Voigt–Reuss–Hill theories. The results indicate that γ-CSi2N4, p-CSi2N4 and p-SiC2N4 are mechanically stable. Calculated values and Poisson''s ratio for γ-CSi2N4, p-CSi2N4 and p-SiC2N4 indicate that these materials are brittle. The calculated anisotropy parameters indicate that γ-CSi2N4 shows weak anisotropy and p-SiC2N4 and p-CSi2N4 have larger anisotropy. Based on the microscopic hardness model, p-CSi2N4, p-SiC2N4 and γ-CSi2N4 should be viewed as superhard materials with some peculiar mechanical properties. [Copyright &y& Elsevier]

Tungsten tetraboride (WB4) is an interesting candidate as a less expensive member of the growing group of superhard transition metal borides. WB4 was successfully synthesized by arc melting from the elements. Characterization using powder X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) indicates that the as-synthesized material is phase pure. The zero-pressure bulk modulus, as measured by high-pressure X-ray diffraction for WB4, is 339 GPa. Mechanical testing using microindentation gives a Vickers hardness of 43.3 ± 2.9 GPa under an applied load of 0.49 N. Various ratios of rhenium were added to WB4 in an attempt to increase hardness. With the addition of 1 at.% Re, the Vickers hardness increased to approximately 50 GPa at 0.49 N. Powders of tungsten tetraboride with and without 1 at.% Re addition are thermally stable up to approximately 400 °C in air as measured by thermal gravimetric analysis. [ABSTRACT FROM AUTHOR]