K. V. Ivanov

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.

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.

Microplastic deformation of polycrystalline and submicrocrystalline titanium during static and cyclic loading

A study is made of the laws governing the accumulation of microplastic strain during the static and cyclic loading of polycrystalline and submicrocrystalline titanium. It is shown that a change from the polycrystalline structure to the submicrocrystalline structure does not change the character of development of microplastic strain for either type of loading, but it does increase fatigue strength and fatigue limit. A correlation between the fatigue strength based on 106 cycles and the macroscopic elastic limit was found to exist for both types of loading.

Investigation of possibility to get superplastic state of nanostructured copper

Abstract The possibility of realisation of superplastic state of nanostructured (NS) copper (grain size 0.1–0.3 μm) produced by equal-channel angular pressing has been studied during creep and tensile tests in vacuum and under the influence of aluminium grain boundary diffusion fluxes from external source (coating). The creep acceleration effect for NS copper under the influence of aluminium grain boundary diffusion fluxes is found. The effect takes place in a lower temperature range as compared with fine-grained copper. It has been established that at 473K strong increase of strain rate sensitivity (m = ∂ lgσ ∂ lg ϵ) takes place during deformation of copper under influence of aluminium diffusion fluxes (m = 0.5) in comparison with pure copper (m = 0.19)

Bulk nanostructuring intermetallic composite material

The article states the results of a study of the impact rendered by the plastic strain occurring in a high-temperature synthesis product during the thermal explosion of a nickel-aluminum powder mixture on the grain structure, strength and ductility of the Ni3Al synthesized intermetallic compound.

Creep and diffusion parameters in submicrocrystalline metals

We have studied the creep of nickel and copper in a submicrocrystalline (SMC) state in a vacuum and in the presence of a diffusion contact with an impurity (Cu and Al, respectively). It is shown that a reduction of the resistance in the presence of a diffusion contact with an impurity is observed in the SMC materials in the temperature range 398 to 473 K. This range is 200 to 400 K lower than the corresponding range for coarse-grained material. It is shown that in this temperature interval the coefficients of grain boundary diffusion for copper in SMC nickel are 5 to 6 orders of magnitude larger than in the coarse-grained material. We propose that the reduction in the temperature for the manifestation of a creep activation effect in the presence of a diffusion contact with an impurity in SMC materials is caused by the increase in the diffusion permeability of the submicrocrystalline grain boundaries.

Structure Evolution and Deformation Mechanisms in Ultrafine-Grained Aluminum under Tension at Room Temperature

Deformation mechanisms occurring by tension of ultrafine-grained aluminum processed by equal-channel angular pressing at room temperature are investigated using comparative study of the microstructure before and after tensile testing as well as deformation relief on the pre-polished surface of the sample tested. Deformation behavior and structure evolution during tension suggest development of grain boundary sliding in addition to intragrain dislocation slip. Contribution grain boundary sliding to the overall deformation calculated using the magnitude of shift of grains relative to each other is found to be ~40%.

Influence of Grain Boundary Diffusion Fluxes of Aluminum on Strength Properties and Creep of Copper and Cu-0.9%Vol Al2O3 Nanocomposite

The practical application of nanostructured materials obtained by severe plastic deformation involves two main problems at least. Firstly, there is the instability of non-equilibrium structure connected with high energy accumulated by severe plastic deformation [1]. The instability is the reason for intense softening at relatively low temperature. This temperature is close to room temperature for several nanostructured materials, for example, copper [2]. Secondly, there is the increased diffusivity of nanostructured material state. This leads to high sensitivity to the influence of surrounding. One of the most important type of this action is grain boundary fluxes from an external source, for example, coating [3, 4].