N. V. Sakharov

Study of the Structure and Mechanical Properties of Nano and Ultradispersed Mechanically Activated Heavy Tungsten Alloys

Mechanisms of sintering and the structure and mechanical properties of nano- and ultradispersed W-Ni-Fe (WNF) and W-Ni-Fe-Co (WNFC) heavy tungsten alloys are investigated. The effect of tungsten particle sizes on the optimal sintering temperature is studied. The size of particles has been changed by the mechanical activation (MA) of the source W-Ni-Fe coarse-grained (CG) charge and by adding ultradispersed particles obtained using plasmochemical synthesis. Nanodispersed powders and ultradispersed powders (UDPs) have been sintered using the techniques of free sintering and pulse plasma sintering (PPS). It has been revealed that the dependence of the alloy density on heating temperature is nonmonotonic, with the maximum corresponding to the optimum sintering temperature. It has been shown that an increase in the time of MA and acceleration of grinding bodies in the process of MA accompanied by a decrease in the size of alloy particles and formation of nonequilibrium solid solutions lead to a reduction in the optimal sintering temperature. It has been shown that, using planetary high-energy milling methods and high-rate spark plasma sintering, it is possible to obtain ultrastrong tungsten alloys whose mechanical properties (macroelasticity stress and yield stress) substantially exceed analogous properties of commercial alloys.

Fabrication of NaZr 2 (PO 4 ) 3 -type ceramic materials by spark plasma sintering

Ceramic materials based on Ca0.5Zr2(PO4)3 and NaFeNb(PO4)3, structural analogs of NaZr2(PO4)3 (NZP), were prepared by spark plasma sintering. At sintering temperatures of 1100–1200 and 880°C and sintering times of 12 and 3 min, the relative densities reached were 99.1 and 99.9%, respectively. According to X-ray diffraction data, the sintering process caused no changes in phase composition. The ceramics had a dense, homogeneous microstructure and ranged in grain size from 0.5 to 2.5 μm.

Phosphorus-containing cesium compounds of pollucite structure. Preparation of high-density ceramic and its radiation tests

Multicomponent oxides of pollucite structure, containing Cs and Ba, were synthesized as powders and ceramics. Their chemical compositions, Cs[MgAl0.5P1.5O6] and Cs0.875Ba0.125[Li0.125Zn0.875Al0.5P1.5O6], were modeled on the basis of the known structural features, taking into account the principles of iso- and heterovalent isomorphism of cations. From powdered samples synthesized using sol-gel process, a ceramic was prepared by spark plasma sintering (SPS). The sintering time was 3–4 min in the temperature interval 600–850°C. The relative densities were 97 and 99%. To evaluate the radiation resistance of the ceramics, the samples were irradiated with 132Xe26+ ions (E = 167 MeV) in the fluence interval from 6 × 1010 to 1 × 1013 cm−2 (ion flux density ∼109 s−1 cm−2). The amorphization took place at fluences of (1.2–1.3) × 1012 cm−2. This fact suggests the decisive role of the ion energy loss for ionization in the generation of radiation defects. Conditions were found for the transition of the metamict form into the crystalline form on heating.

Influence of the grain size and structural state of grain boundaries on the parameter of low-temperature and high-rate superplasticity of nanocrystalline and microcrystalline alloys

A model has been proposed for calculating the grain size optimum for the deformation of nanocrystalline and microcrystalline materials under superplasticity conditions. The model is based on the concepts of the theory of nonequilibrium grain boundaries in metals. It has been demonstrated that the optimum grain size dopt can be calculated as the size at which a high level of nonequilibrium of grain boundaries is combined with a high intensity of the accommodation of grain boundary sliding. The dependences of the quantity dopt on the rate and temperature of the strain and the thermodynamic parameters of the material have been derived. The results obtained have been compared with the experimental data on the superplasticity of nanocrystalline and microcrystalline aluminum and magnesium alloys.

Sparking plasma sintering of tungsten carbide nanopowders

Spark Plasma Sintering studies of the high-speed consolidation of pure tungsten carbide WC nanopowders have been carried out. The influence of the initial size of the WC nanoparticles and modes of their receiption the density, structural parameters, and mechanical properties of tungsten carbide are studied. Samples of high-density nanostructured tungsten carbide with high hardness (up to 31–34 GPa) and an increased crack resistance (4.3–5.2 MPa m1/2) are obtained. It is found that the effect of accelerating tungsten carbide nanopowder sintering under conditions of high-speed heating is associated with the acceleration of diffusion along grain boundaries in the sintered material. It is shown that the nonmonotonic dependence of the optimal sintering temperature on the initial grain size is caused by a change in grain-boundary diffusion coefficient in conditions of abnormal grain growth. It is found that the size of abnormally large grains in spark plasma sintering depends on the volume fraction of particles of the nonstoichiometric phase.

Praseodymium and neodymium phosphates Ca9Ln(PO4)7 of whitlockite structure. Preparation of a ceramic with a high relative density

Praseodymium and neodymium phosphates Ca9Ln(PO4)7 of the whitlockite structure were synthesized in the form of powders via solid-phase reactions at high temperatures and in the form of ceramics using the two-step pressing + sintering process and one-step high-rate spark plasma sputtering (SPS) procedure. According to X-ray diffraction data, the phosphates were structural analogs of calcium phosphate, namely, of its low-temperature modification β-Ca3(PO4)2, space group R3c. The particle size in the powders was 80–110 nm. The relative densities of the ceramics reached 99% when using SPS. The optimum conditions were found for obtaining high-density ceramics containing Pr and Nd. Their mechanical characteristics (microhardness, cracking resistance) were determined, and the microstructure was characterized.

Methods of compacting nanostructured tungsten–cobalt alloys from Nanopowders obtained by plasma chemical synthesis

The paper summarizes the experience of using traditional and modern methods of sintering WC–Co nanopowders obtained by plasma chemical synthesis. A comparative analysis of structure formation processes occurring at sintering WC–Co nanopowders in a quasi-steady and high-speed heating is carried out. It is shown that the basic regularities of structure evolution during sintering are rather general in nature and are similar both to conventional vacuum sintering and to high-energy compaction methods. It was established that, under high-speed heating conditions, a significant contribution to the acceleration of the sintering of nanopowder materials at low temperatures is made not only by a small grain size but also the process of grainboundary diffusion. It is shown that the samples of hard alloys sintered from tungsten carbide nanopowders produced by plasma chemical synthesis have significantly higher hardness and fracture toughness than those produced using conventional synthesis techniques and compacting. Using the technology of plasma chemical synthesis and spark plasma sintering, nanostructured WC and WC–Co samples with significantly better mechanical properties (hardness, fracture toughness) than those of conventional fine materials were obtained.

Spark Plasma Sintering of high-strength ultrafine-grained tungsten carbide

The paper dwells on the research conducted into high-rate consolidation of pure tungsten carbide nanopowders using the Spark Plasma Sintering. Studies included the effect that the original size of WC nanoparticles and their preparation modes have on density, structure parameters, and mechanical properties of tungsten carbide. It has been found that materials that show abnormal grain growth during sintering have lower values of sintering activation energy as compared to materials the structure of which is more stable during high-rate heating. A qualitative model is proposed that explains this effect through the dependence of the grain boundary diffusion coefficient on the grain boundary migration rate.

Influence of high-energy ball milling on the solid-phase sintering kinetics of ultrafine-grained heavy tungsten alloy

The mechanisms for the sintering of ultrafine-grained 95%W–3.5%Ni–1.5%Fe heavy tungsten alloy powders have been investigated. It has been established that a decrease in the activation energy of grain boundary diffusion and the formation of a nonequilibrium solid solution of nickel and iron in the surface layer of tungsten particles upon high-energy ball milling are responsible for the decrease in optimal sintering temperature.

High-strength ultrafine-grained tungsten-carbide-based materials obtained by spark plasma sintering

Ultrafine-grained (UFG) tungsten carbide (WC) samples with high hardness (up to 34 GPa) and increased cracking resistance have been obtained by the method of spark plasma sintering (SPS). Initial powders have been prepared by two-stage plasmachemical synthesis. The influence of the initial size of WC nanoparticles on the density, structural parameters, and mechanical properties of UFG tungsten carbide obtained by SPS has been studied. It is established that the phenomenon of accelerated sintering of WC powder is related to enhanced grain-boundary diffusion.