Stability and mechanical, thermodynamic and optical properties of WC polytypes and the TiC-WC solid solutions: A first-principles study

First-principles calculations and molecular dynamics (FPMD) simulations in the constant number of particles - pressure - temperature (NPT) ensemble were used to investigate the phase stability, phase diagram, electronic and phonon structures, chemical bonding, mechanical, thermodynamical and optical properties of various tungsten carbide polytypes as well as the hexagonal (alpha) and cubic (beta) Ti1-xWxCy solid solutions. The known alpha-WC (P-6m2) and beta-WCy (Fm-3m) polytypes and eight hypothetical ones were studied. Gibbs free energy calculations predicted the alpha-WC -> beta-WCy and alpha-WC -> tP4-129 (P4/nmm) phase transitions. The P-6m2 -> Fm-3m -> I41md, Fm-3m -> P4/nmm and P-6m2 -> Pm-3m phase transformations were revealed during FPMD simulations of: alpha-WC at T = 2500 K, beta-WC at T = 100 K, and alpha-WC at T = 2500 K and P = 500 GPa, respectively. It is found that the known alpha-polytype and new tI8-109 (I41md), oP12-25 (Pmm2), hP4-194 (P63/mmc), hP8-194 (P63/mmc), cP2-221 polytypes are ultraincompressible and ultrastiff materials, and exhibit the highest hardness (29-38 GPa), fracture toughness (4.7-7.2 MPa m1/2) and Debye temperature (615-663 GPa). Other characteristics of the WC polytypes, namely, the stress-shear strain curves, dielectric function, reflectivity spectra, heat capacity and spatial distribution of the elastic moduli, were calculated and discussed. The stability, mechanical properties and lattice parameter of beta-WCy and Ti1-xWxCy as functions of composition were studied. The binodal and spinodal for Ti1-xWxC were calculated. The calculated characteristics are compared with available experimental data and used for their interpretation.