G. P. Grabovetskaya

Grain Boundary Diffusion and Mechanisms of Creep of Nanostructured Metals

Comparative investigations of diffusion in coarse-grained (d = 20 μm), nanocrystalline (d = 0.04 μm) and nanostructured nickel (d = 0.3 μm) have been carried out in a temperature interval of 0.2–0.3 melting temperature. The reasons for difference of parameters of copper grain-boundary diffusion in the above materials are discussed. The effect exerted by grain boundary state and grain boundary diffusion fluxes of impurity on creep mechanisms of nanostructured nickel and copper in the temperature interval of 373–473 K have been studied. Significant change in the apparent creep activation energy under copper grain boundary diffusion fluxes is described as a consequence of different contribution of grain boundary sliding to overall deformation.

Microstructures and mechanical properties of ultrafine-grained Ti foil processed by equal-channel angular pressing and cold rolling

We processed coarse-grained Ti and equal-channel angular pressing (ECAP) processed ultrafine-grained (UFG) Ti into 20-μm-thick Ti foils by cold rolling and intermediate annealing. The foils produced from rolling the UFG Ti exhibit a homogeneous nanostructure, while foils produced from rolling the coarse-grained Ti exhibit heterogeneous structures with a mixture of nanostructured regions and coarse-grained regions. The former foils also have higher strength and ductility and exhibit uniform deformation over a larger strain range at room temperature than the latter ones. This work demonstrated the advantage and viability of producing nanostructured Ti foil by rolling ECAP-processed UFG Ti stock.

The structure and microhardness evolution in submicrocrystalline molybdenum processed by severe plastic deformation followed by annealing

Abstract A possibility to form submicrocrystalline structure in molybdenum using severe plastic deformation treatment by torsion under high pressure (HPT) at elevated temperatures has been studied. Quantitative parameters of grain–subgrain structure have been obtained by optical microscopy and transmission electron microscopy methods. Thermal stability of microstructure and mechanical properties have been studied. It is established that HPT results in the formation of a submicrocrystalline structure in Mo and in disappearance of residual porosity. The average grain size of HPT-Mo is 0.2 μm. The formation of submicrocrystalline structure enhances significantly (by 2.4 times) the microhardness of Mo relative to that in the as-received (before HPT treatment) state. It is found that grain growth begins at 1173 K and develops intensively at T ⩾1273 K.

Diffusion — Induced creep of polycrystalline and nanostructured metals

Abstract Kinetic mechanisms of phenomena to be induced by the diffusion on the grain boundaries in polycrystalline and nanostructured (NS) materials have been analyzed. By way of example of nickel and copper nanostructured (grain size 100÷300 nm) it was shown that the effect of diffusion induced loss of strength by creep was much observed at temperatures low than 400 K in comparison with polycrystalline condition with grain size 20 μm. It was supposed that the physical reason for the decrease in the temperature for the effect to manifest itself is much higher value of diffusion coefficients of impurities in nanostructures.

Creep features of nanostructured materials produced by severe plastic deformation

Abstract Using nanostructured copper, nickel and CuAl 2 O 3 composite as an example, characteristic features of creep of nanostructured materials produced by severe plastic deformation was studied at temperatures T melt (T melt is the melting point). The role of grain boundary state in the development of plastic deformation during creep of the above materials is analyzed. The differences in the creep activation energies in nanostructured and coarse-grained state is explained by the different contributions of grain boundary sliding to the overall deformation.

Structure and properties of grain boundaries in submicrocrystalline molybdenum prepared by high-pressure torsion

Transmission electron microscopy was used to study submicrocrystalline structure and thermal stability of molybdenum prepared by severe plastic deformation at 400°C by high-pressure (6 GPa) torsion using 5 revolutions of anvils. Emission Mössbauer spectroscopy was used to study the state of grain boundaries in submicrocrystalline Mo annealed at temperatures of 350–800°C.

Structure and mechanical and electrochemical properties of ultrafine-grained Ti

A study is conducted into the microstructure and physico-mechanical properties of ultrafine-grained titanium produced by severe plastic deformation using the method of equichannel angular pressing. The effects of thermal and mechanical treatment on these characteristics are investigated. The possibility of forming mechanical properties in titanium that compare well with those of highly doped titanium alloys is shown.

Grain-boundary diffusion of nickel in submicrocrystalline molybdenum processed by severe plastic deformation

The depth-concentration profiles of nickel upon diffusion in submicrocrystalline (SMC) molybdenum processed by severe plastic deformation (SPD) have been studied by Auger electron spectroscopy. The coefficients (Db) and activation energies of the grain-boundary diffusion of nickel in SMC molybdenum were determined in a 973–1123 K temperature interval. The results indicate that a difference between the Db values in SMC and coarse-grained molybdenum is related to a nonequilibrium state of grain boundaries in the SPD-processed metal.

Diffusion and Plasticity of Submicrocrystalline Metals and Alloys

Comparative investigations of the diffusion in submicrocrystalline a d coarse-grained metals and alloys by means of direct measurements and indirect evaluations on the basis of experimentally obtained parameters of creep and superplastic flow were carried out. The diffusivity determination by secondary ion mass spectroscopy in pure metals ( Ni, Ti, Cu) reveals the enhancement of a grain boundary diffusion coefficient in submicrocrysta lline materials produced by severe plastic deformation by factor of 1 to 5 comparison to that i n coarse-grained materials. The diffusivity evaluation using parameters of diffusion controlled process es during creep of pure metals or superplastic flow of Al-based alloys also demonstrate an increase of the grain boundary diffusion coefficient by 1-2 orders of magnitude in submicrocrystalli ne materials. The physical reasons for the enhanced grain boundary diffusivity in submicrocrysta lline state, relative to that in coarse-grained metals and alloys, are discussed.

Structure and mechanical properties of ultrafine-grained Ti-6Al-4V alloy made by applying reversible hydrogen alloying

The special features of the ultrafine-grained structure forming in a Ti-6Al-4V alloy with the use of a method combining severe plastic deformation and reversible hydrogen treatment are studied by electron microscopy and X-ray diffraction analysis. Martensite and the α ↔ β phase transformations due to hydrogen presence are found to promote structure refining during severe plastic deformation of the alloy. This makes it possible to decrease the degree of deformation required for obtaining the ultrafine-grained structure with the size of elements of 0.1–0.5 μm. Formation of the ultrafine-grained structure in a Ti-6Al-4V alloy is shown to lead to growth of its strength properties by approximately a factor of 1.5, an increase in the hydrogen embrittlement resistance, and a decrease in the temperature transition to the superplastic state by 200–300 K.