A. V. Nokhrin

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.

Superhard nanodisperse tungsten heavy alloys obtained using the methods of mechanical activation and spark plasma sintering

We have studied the structure and mechanical properties of nanodisperse tungsten-based heavy alloys of the W-Ni-Fe system. The temperature dependence of the density of compacted alloys exhibits a nonmonotonic character with a maximum that corresponds to the optimum temperature of sintering. The effect of the regime of solid-state pulsed spark plasma sintering (SPS) on the structure and mechanical properties of mechanically activated W-Ni-Fe heavy alloys has been studied. It is established that, using preliminary mechanical activation in a planetary ball mill and the subsequent high-rate SPS, it is possible to obtain superhard tungsten-based heavy alloys with mechanical properties that substantially exceed those of the analogous standard alloys.

Dispersion Limit upon Equal-Channel Angular Pressing. Temperature Effect

Experimental and theoretical results concerning the deformation refining of grains upon intense plastic deformation are reported. Experimental results on deformation dispersion of pure metals and alloys based on magnesium and aluminum are summarized. A model is developed to calculate the minimum grain size that can be obtained by the method of equal-channel angular pressing (ECAP). Expressions describing the grain refining limit as a function of material properties and temperature of intense plastic deformation are obtained.

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.

The effect of the local chemical composition of grain boundaries on the corrosion resistance of a titanium alloy

The influence of the structural-phase state of grain boundaries in a Ti4Al2V (commercial PT3V grade) pseudo-alpha-titanium alloy on its susceptibility to hot-salt intergranular corrosion (IGC) has been studied. It is established that IGC-tested alloy samples exhibit corrosion-induced defects of two types. More extended defects of the first type occur at the V-rich boundaries of coarse grains, while short defects of the second type reside at the grain boundaries with composition close to that of the grain body. The existence of the two types of IGC defects is explained by the classical theory of galvanic microcouples (microcells), according to which the IGC intensity is proportional to the difference of corrosion-active impurity concentrations between the grain boundary and body.

The effect of grain boundaries state on the thermal stability of a submicrocrystalline titanium alloy structure

The thermal stability of the structure and the mechanical properties of submicrocrystalline (SMC) titanium alloy Ti-4Al-2V (industrial designation PT3V) are investigated. The alloy was produced by equal-channel angular pressing (ECAP). It is demonstrated that the enhanced thermal stability of the SMC alloy structure is associated with a change in the concentration of aluminum at the grain boundaries during ECAP.