Vol 41, No 5 (2019)

The Mo–Ni–B alloys with a boron content ca.41 at % were prepared by arc melting, annealed at subsolidus temperatures and studied by X-Ray diffraction method, differential thermal analysis and SEM/EPMA microscopy, and the incipient melting temperatures were measured by the Pirani-Alterthum technique. Based on the authors’ experimental data and in view of the data published elsewhere, the partial solidus surface projection has been constructed for the first time for the Mo–Ni–B system in the region Mo–MoB_1.0–NiB_∼0.8–Ni.

The pressure effects on the structural, elastic and thermodynamic properties of tantalum mononitride in WC-type phase (i.e. WC–TaN) are investigated by the first-principles plane wave pseudo-potential density functional theory method and the quasi-harmonic Debye model. The obtained equilibrium structure parameters and ground state properties are in excellent agreement with the experimental and other theoretical results. The calculations of the phonon dispersion curve and the density of phonon states verify that the WC–TaN is dynamically stable. A full elastic tensor and anisotropies behavior of the WC–TaN is also evaluated and discussed in the wide pressure range. The results show that WC–TaN is elastic anisotropy and mechanically stable up to 100 GPa, and the compression along c-axis direction is more difficult than along a-axis. The obtained superior mechanical properties show that WC–TaN is a promising candidate structure to be one of the ultra-incompressible and hard materials. Finally, by using the quasi-harmonic approximation model, we predicted the thermodynamic properties of WC–TaN under pressure and temperature. The heat capacity C_V, Debye temperature θ, the thermal expansion α and the Grüneisen constant γ are obtained successfully in the ranges of 0–100 GPa and 0–2000 K.

The electrochemical behavior of ultrafine WC/μ-WC-Co alloy with different grain sizes of WC particles was investigated in 1M HCl and 1M NaOH solutions using potentiodynamic polarization and surface analytical techniques. Good linear grain size dependence existed for the corrosion current density (I_corr). Alloys with smaller WC exhibited better corrosion resistance. Addition of μ-size WC particles can weaken the corrosion resistance of ultrafine WC/μ-WC-Co cemented carbide. The corrosion mechanism is discussed based on the SEM and EDS analysis. The predominant dissolution of the Co particles as the binder phase in HCl solution as well as WC matrix remained at the tangent surface. The benefits of Co(OH)₂ protective film can only be utilized in alkaline solution.

To determine the dependence of the yield of detonation nanodiamonds (DNDs) on the power of explosives used, a new concept is introduced—the specific power of explosives (to which the detonation velocity of explosives and the pressure of gases in the Chapman-Jouguet plane are related), which is given by the ratio of the heat of explosion to the unit mass and time. The DNDs yield has been found to depend on the detonation velocity and pressure in the Chapman-Jouguet plane. The optimum yield of DNDs (> 5 wt %) is achieved when the specific power of explosives is 30000 to 60000 kJ/(kg·µs), the detonation velocity is 7250 to 8000 m/s, and the pressure in the Chapman-Jouguet plane is 21 to 28 GPa.

It is shown for the first time that the presence of porous space in cutting-tool ceramics makes it possible to influence, through water absorption, the performance of diamond machining of such ceramics and, first of all, improve their machinability. The effective duration of water pre-saturation of oxide-carbide ceramics has been found to be 48 h. For lower-porosity ceramic materials the saturation period is longer, e.g., it is 96 h for oxide ceramics and 192 h for cermets.

Residual thermal stresses in a three-phase cermet composite with a layer-structured refractory component (WC-coated SiC grains) are studied by analytical methods. The layer of residual thermal stresses in such alloys is found to significantly depend on the composition and volume content of the layers and binder phases. The data obtained for the alloys with a large amount of the binder phase are in good agreement with the results of calculation by a different numerical model.