Exploring the high-pressure behavior of superhard tungsten tetraboride

Abstract

Abstract:In this work, we examine the high pressure behavior of superhard material candidate WB 4 using high-pressure synchrotron X-ray diffraction in a diamond anvil cell up to 58.4 GPa. The zero-pressure bulk modulus, K 0 , obtained from fitting the pressure-volume data using the second-order Birch-Murnaghan equation of state is 326 ± 3 GPa. A reversible, discontinuous change in slope in the c/a ratio is further observed at ~42 GPa, suggesting that lattice softening occurs in the c direction above this pressure. This softening is not observed in other superhard transition metal borides such as ReB 2 compressed to similar pressures. Speculation on the possible relationship between this softening and the orientation of boronboron bonds in the c direction in the WB 4 structure is included. Finally, the shear and Young's modulus values are calculated using an isotropic model based on the measured bulk modulus and an estimated Poisson's ratio for WB 4 .

Indentation strength of ultraincompressible rhenium boride, carbide, and nitride from first-principles calculations

“…If we set Φ = 0 (so σ zz = 0) in the calculation, it is equivalent to require that all the five stress components (except σ xz ) become negligible during the structural relaxation, which is the relaxation procedure used in the previous calculations of pure ideal shear stresses [28][29][30][31][32][33][34][35][36][37][38][41][42][43][44][45][46][47][48] that neglect the effects of the normal compressive pressure and geometry (indenter angles) of the indenter. Our test calculations for the lattice vectors of the equilibrium structure of Re, Re 3 N, Re 2 N, Re 2 C, Re 2 B and ReB 2 , their bulk moduli and Poisson's ratios are given in Table I, which agree well with the previous calculation results 19,[21][22][23][24]54,55 . The bulk moduli are obtained by fitting the energy-volume curves near the equilibrium structures to the Murnaghan equation, and the Poisson's ratios are evaluated from the calculated elastic constants.…”

“…It is difficult to compress their volumes by hydrostatic pressure, which results in their high bulk moduli. However, in indentation hardness tests the uniaxial normal compressive pressure beneath the indenter can cause a large lateral volume expansion of the valence electrons in these structures as indicated by their large Poisson's ratios [19][20][21][22][23][24] . We examine how this lateral expansion affects the mechanical strength under indentation of these compounds with different chemical elements and compositions.…”

Interstitial-boron solution strengthened WB3+x

“…The more striking fact is that none of them agrees with the theoretically derived pressure-dependent c/a ratio [31] of hP 16-WB 3 . Therefore, this tungsten boride still needs further clarification.Within this context, by combining first-principles calculations [32,33] (unless otherwise mentioned, all calculations have been performed with the Perdew-BurkeErnzerh generalized gradient approximation (GGA-type PBE) [34]), variable-composition evolutionary algorithm search as implemented recently in USPEX [35,36] and the aberration-corrected images of high resolution transmission electron microscopy (Ac-HRTEM) (method details refers to Supporting information), we have confirmed the existence of WB 3+x (x < 0.5) (including the stoichiometric hP 16-WB 3 ) and denied the formation of the extensively debated boride of hP 20-WB 4 [17,27,28]. The previously experimentally attributed WB 4 is indeed WB 3+x .…”

Unexpectedly hard and highly stable WB3 with a noncompact structure

“…First-principles calculations suggested that hP6-WB 2 was energetically more stable than the experimentally reported hP3-WB 2 [12] and could have the high hardness with a value of 43.8-47.4 GPa [13,14]. It was once accepted that the claimed hP20-WB 4 [15] was an inexpensive superhard material (Vickers hardness 43.3-46.2 GPa under the load of 0.49 N) [16][17][18][19]. However, this long-assumed hP20-WB 4 has recently identified as hP16-WB 3 from the thermodynamic stability [20], mechanical properties [21] and XRD spectra comparisons [22,23].…”