A. V. Korznikov

Structure and properties of ultrafine-grained materials produced by severe plastic deformation

Abstract Strain-heat methods of obtaining ultrafine-grained (UFG) metallic materials with grain sizes as small as 20 nm and peculiarities of their structure are considered. It is shown that intercrystalline boundaries are the main element of the structure of UFG materials and that they are typically in a non-equilibrium state. The formation of a special grain boundary phase, i.e. a thin near-boundary layer with high dynamic activity of atoms, has been found. This unusual structure leads to the manifestation of promising new elastic, strength, superplastic, damping and magnetic properties of UFG materials.

The mechanism of nanocrystalline structure formation in Ni3Al during severe plastic deformation

The microstructure evolution of the intermetallic compound Ni3Al during severe deformation by torsion under high quasi-hydrostatic pressure, which eventually results in the formation of a disordered nanocrystalline structure with a high level of internal stresses, was investigated as a function of the amount of shear strain. At the microstructural level, the crystals were first fragmented by the propagation of twins, then nanosized equiaxed crystallites with high misorientations were formed. At the macroscopic level, there is evidence that the cold-worked structure first formed at the sample surface and then propagated into the whole volume.

Deformation Induced Vacancies with Severe Plastic Deformation: Measurements and Modelling

In discussing hardening characteristics in terms of crystalline lattice defects, in most cases the properties and kinetics of dislocations and their arrangement have been considered. However, during plastic deformation also vacancies and/or vacancy type defects are produced in very high densities which are typically close to those of vacancies in thermal equilibrium at the melting point. The effect of high vacancy concentrations on the hardening characteristics is twofold: (i) direct effects by impeding the movement of dislocations (ii) indirect one by inducing climbing and annihilation of edge dislocations leading to softening or even absolute decreases in strength. This paper presents first measurements of deformation induced vacancies in SPD materials which have been achieved by combined evaluation of resistometry, calorimetry and X-ray diffraction. The density of vacancies during and after SPD deformation is found to be markedly higher than in cases of conventional deformation and/or coarse grained material which may be partly attributed to the particular conditions of SPD namely the enhanced hydrostatic pressure as well as the changes in deformation path. It is suggested to make this high vacancy concentration responsible for both dynamic and static recovery and/or recrystallisation processes recently found during and after SPD, being potential reasons for enhanced ductility and superplasticity which only occur with nanomaterials originating from SPD. Recent publications show that in alloys, SPD induced vacancies can also enable the existence of phases which do not appear in the equilibrium diagram.

Nanocrystalline structure and phase transformation of the intermetallic compound TiAl processed by severe plastic deformation

Abstract Bulk samples of nanocrystalline TiAl were produced by severe plastic deformation. Different amounts of cold work lead to different nanocrystalline structures and phase contents. Based on transmission electron microscopy and X-ray data an assumption has been made to explain the origin of the observed structural changes.

On the limiting minimum size of grains formed in metallic materials produced by high-pressure torsion

Abstract-Results are presented concerning the experimental studies of the limiting minimum grain size dmin in nanostructured metallic materials produced by high-pressure torsion in Bridgman anvils. It is shown that important factors that determine dmin in the process of severe plastic deformation are the critical stresses for dislocation shear and the mobility of point defects, which control the paths of these defects in stress fields, the characteristic scales of the microvolumes involved into reorientation processes, and therefore the sizes of grains and subgrains of submicroscopic and nanoscopic scales.

Effect of severe plastic deformation on the microstructure and tribological properties of a babbit B83

The effect of severe plastic deformation carried out at room temperature by the methods of equal-channel angular (ECA) pressing and surface friction treatment (SFT) on the microstructure, rate of wear, and friction coefficient of a babbit B83 (11.5% Sb, 5.5% Cu, Sn for balance) has been investigated. It has been shown that severe plastic deformation that leads to a drop in the grain size of the babbit to 100–300 nm and to a strong refinement of particles of intermetallic phases (SnSb, Cu3Sn) causes a considerable (twofold-fourfold) reduction in the rate of wear and a decrease in the friction coefficient of a steel-babbit pair under test conditions with lubrication at small (0.07 m/s) and enhanced (4.5 m/s) sliding velocities. As was shown by structural investigations performed with the use of scanning electron microscopy, this positive influence of severe plastic deformation on the tribological properties of the babbit is connected with the formation on the deformed-babbit surface of a developed porosity, which improves conditions for lubrication of the babbit-steel friction pair due to the action of the self-lubrication effect and thereby favors the retention of a stable regime of boundary friction of this pair. The formation of porosity is a result of the accelerated spalling of hard brittle intermetallic particles of SnSb and Cu3Sn from the friction surface of the deformed babbit, which is caused by weakening and loss of the bonding of these particles with the plastic matrix (α solid solution based on tin) in the course of severe plastic deformation of the babbit. At the same time, under the conditions of dry sliding friction of the babbit-steel 45 pair, when a fatigue mechanism of wear of the alloy under consideration predominantly develops, this plastic deformation yields an approximately 1.6-fold increase in the rate of wear of the babbit. This increase is mainly due to numerous defects (microcracks) that are introduced into the babbit structure upon its severe plastic deformation and reduce the resistance of the surface layer of this material to the fatigue mechanism of wear.

Evolution of the defect substructure in V-4Ti-4Cr alloy under severe plastic deformation

The evolution of the defect substructure in V-4Ti-4Cr alloy under its severe plastic deformation by torsion in Bridgman anvils is studied by transmission electron microscopy. Nanoband structural states with a dipole or multipole character of misorientations and a crystallite (or nanoband) size varying from several to several tens of nanometers form in the true logarithmic strain range e ≈ 3.0−6.6. Such crystallites form inside 100-nm submicrocrystallites or coalesce (at e ≥ 6) to yield mesobands with a pronounced vortex character of their propagation. The formation of these states is related to the activation (by the flows of nonequilibrium point defects in stress fields) of quasi-viscous deformation and lattice reorientation mechanisms, which provide the generation and propagation of partial disclination nanodipoles followed by the development of collective effects in a disclination substructure. These effects lead to the group motion of nanodipoles inside the mesobands.

Evolution of Structural and Phase States at Large Plastic Deformations of an Austenitic Steel 17Cr-14Ni-2Mo

We present the results of the investigation of the evolution of the defect substructure and phase transformations in the process of deformation by rolling and high-pressure torsion of a chromium-nickel steel 17Cr-14Ni-2Mo. Experimental evidences have been found in favor of the occurrence of γ → α(α′) → γ transformations as one of the mechanisms of plastic deformation and reorientation of the crystal lattice at large plastic deformations of the steel under study. It has been shown that the nanostructured states are formed in the process of interaction of localized deformation bands with microbanded twin structures. Mechanisms of deformation and reorientation of the crystal lattice upon the formation of the above-indicated states and possible mechanisms of phase transformations in the process of large plastic deformations of this steel have been discussed.

Microstructural evolution of nickel under high-pressure torsion

The evolution peculiarities of grain and defect structures in nickel under high-pressure torsion were studied by transmission electron microscopy and X-ray diffraction analysis. Lattice reorientation mechanisms characteristic of different stages of plastic deformation were disclosed. The conditions and features of cooperative realization of various structure formation mechanisms under severe deformation were discussed.

On the nature of low-temperature brittleness of BCC steels

The study demonstrates the possibility to suppress the ductile-brittle transition in bcc-structured steels at low strain temperatures on the example of pipe steel subjected to severe plastic deformation. The suppression of the ductile-brittle transition in the material is associated with structural changes in its planar subsystem (surface layers and grain boundaries in polycrystals) and substructure formation in its 3D crystalline subsystem.