M. Yu. Gryaznov

Nanostructured crystals of Sr 1− x R x F 2+ x fluorite phases and their ordering: 6. Microindentation analysis of crystals

Hardness, crack resistance, brittleness, and effective fracture energy have been studied for crystals of 24 fluorite phases Sr1 − xRxF2 + x (R are 14 rare earth elements (REEs); 0 < x ≤ 0.5) and SrF2 grown by the Bridgman method from a melt. These characteristics change nonlinearly with an increase in the REE content for Sr1 − xRxF2 + x (0 < x ≤ 0.5) with R = La, Nd, Sm, Gd, and Lu; it is maximum in the range x < 0.1 for all REEs. The changes in a number of REEs have been traced for an isoconcentration series of Sr0.90R0.10F2.10 crystals (R = La, Nd, Sm, Gd, Ho, Er-Lu, or Y) and crystals (similar in composition) with R = Tb and Dy. The hardness of Sr1 − xRxF2 + x crystals is higher by a factor of ∼2–3 than that of SrF2. The effect of decrease in microstresses in SrF2 crystals is confirmed by the isomorphic introduction of R3+ ions into this crystalline matrix.

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.

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.

Simultaneous increase in the strength, plasticity, and corrosion resistance of an ultrafine-grained Ti–4Al–2V pseudo-alpha-titanium alloy

The influence of severe plastic deformation on the structural-phase state of grain boundaries in a Ti–4Al–2V (commercial PT3V grade) pseudo-alpha-titanium alloy has been studied. It is established that increase in the strength, plasticity, and corrosion resistance of this alloy is related to the formation of an ultrafine- grained structure. In particular, it is shown that an increase in the resistance to hot-salt intergranular corrosion is due to diffusion-controlled redistribution of aluminum and vanadium atoms at the grain boundaries of titanium formed during thermal severe plastic deformation.

Effect of grain boundary diffusion acceleration during structural superplasticity of nano- and microcrystalline materials

It is known that the superplasticity phenomenon isassociated with the development of grainboundarysliding (GBS). This is a special deformation mode,which acts at enhanced temperatures and especiallyeffectively operates in finegrained materials. It wasshown in [1–5] that the effective accommodation ofsliding in the grain joints and a special nonequilibriumstate of grain boundaries are necessary for GBS development. The expression for strain rate in superplasticmaterials has the form [2](1)where

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

The effect of an increase in the coefficient of the grain-boundary diffusion upon recrystallization and superplastic deformation of submicrocrystalline (SMC) materials prepared by severe plastic deformation has been studied. It is shown that the coefficient of the grain-boundary diffusion of the SMC materials is dependent on the intensity of the lattice dislocation flow whose value is proportional to the rate of the grain boundary migration upon annealing of SMC metals or the rate of the intragrain deformation under conditions of superplastic deformation of SMC alloys. It is found that, at a high rate of grain boundary migrations and high rates of superplastic deformation, the intensity of the lattice dislocation flow bombarding grain boundaries of SMC materials is higher than the intensity of their diffusion accommodation, which leads to an increase in the coefficient of the grain-boundary diffusion and a decrease in the activation energy. The results of the numerical calculations agree well with the experimental data.

Superplasticity of an aluminum-lithium 1420 alloy in various structural states

The superplasticity of Al-Li-Mg alloy 1420 with a grain size of 0.3–20 μm has been studied in various structural states. A nonlinear dependence of the superplastic elongation on the grain size is revealed. The optimum temperatures and strain rates are determined for nanocrystalline and submicrocrystalline alloys: they have large elongations (1200–1500%), high strain-rate sensitivity coefficients (higher than 0.45), and low activation energies (60–70 kJ/mol). The existence of an optimal grain size for reaching the maximum strain under structural superplasticity conditions is explained using a theoretical model proposed in this work.