Yu. G. Lopatin

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.

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.

Effect of Recovery and Recrystallization on the Hall–Petch Relation Parameters in Submicrocrystalline Metals: I. Experimental Studies

Yield strength σy, macroelastic limit σ0, and effective grain-boundary hardening coefficient Keff in the Hall–Petch relation (

Changes in the diffusion properties of nonequilibrium grain boundaries upon recrystallization and superplastic deformation of submicrocrystalline metals and alloys

{\sigma _y} = {\sigma _0} + {K_{eff}}/\sqrt d

Preparation and investigation of ultrafine-grained tungsten carbide with high hardness and fracture toughness

σy=σ0+Keff/d) in the submicrocrystalline (SMC) materials produced by equalchannel angular pressing are experimentally studied. It is shown that, as compared to parameter σ0 and K in the Hall–Petch relation for coarse-grained metals, the SMC metals are characterized by higher values of σ0 and lower values of Keff. The critical grain size (d1) at which Keff in the σy–d–1/2 relations of SMC materials changes falls in the range 0.2–0.5 μm. The dependences of macroelastic limit σ0 and coefficient Keff on the annealing temperature are found to be determined by recrystallization. If abnormal grain growth develops in annealing of SMC metals, anomalous hardening is detected and a nonmonotonic temperature dependence of coefficient Keff takes place. In the case of conventional recrystallization at a high annealing temperature, SMC metals exhibit a smooth decrease in σ0 and an increase in Keff to the values of K characteristic of coarsegrained materials.