N. A. Koneva

Thermodynamics of substructure transformations under plastic deformation of metals and alloys

The paper is devoted to behavior of the energy stored in deformed metals and alloys. The study is based on the well-known theoretical approaches and on the electron microscopy data obtained by the authors during investigation of the deformed Cu-Al and Cu-Mn f.c.c. alloys. Influence of the solid solution hardening value on the mechanisms of dislocation substructure evolution was studied. The accumulated energy for the substructure being formed at plastic deformation was evaluated. Ways of improvement of such evaluations were considered.

Features of Work Hardening of Polycrystals with Nanograins

The present work is devoted to the investigation of the influence of the grain size on the main mechanical characteristics of nanopolycrystals of different metals. The Hall-Petch parameter behaviour for Al, Cu, Ni, Ti and Fe was examined in the wide grain size interval. The stages of plastic deformation and the parameters of work hardening for nanocrystalline copper were analysed in detail. The deformation mechanisms and critical grain sizes accounting for the transition from the dislocation slip to the grain boundary sliding were described.

Internal field sources, their screening and the flow stress

Abstract The experimental investigation of the internal elastic stress fields in polycrystals of deformed Cu–Al and Cu–Mn alloys was carried out. The basic sources of the internal stress field were identified and the decrease of the field with increasing distance from the sources was determined. The average values of the internal stresses as a function of strain were measured. On this basis, the separation of sequences of dislocation substructure transformations into low-energy and the high-energy configurations was carried out.

Structure of triple junctions of grains, nanoparticles in them and bending -torsion in metal nanopolycrystals

The paper presents the results of the transmission electron microscopic (TEM) investigation of the structure and phase composition of nanocrystalline copper obtained by the severe plastic deformation using the high pressure torsion (HPT) method. Special attention is paid to the triple junctions of grain boundaries. It was established that the triple junctions contained the partial disclinations and particles of the secondary phases. The dependences of such junction fractions on the deformation were measured of the formation at nanocrystalline copper. The phase analyses of the secondary phase structure were carried out, the sizes of the phase particles and their volume fractions were determined. The bending – torsion of the crystal lattice arising near the triple junctions was measured. The problem of the long – range stress field screening was considered.

Contact and barrier dislocation resistance and their effect on characteristics of slip and work hardening

Abstract The basic mechanisms of substructure hardening of network and cell dislocation structures were considered in terms of the quantitative data of the TEM investigation of foils and replicas. The comparative analysis of the barrier and contact dislocation resistance was carried out. These types of the dislocation resistance make the main contribution to the work hardening. Sharp distinction of the dislocation ensembles properties where the work hardening is determined by the relations τ∼ρ 1/2 and τ∼D − 1 was shown. The attention was paid on presence of the softening fluctuation contribution into the flow stress, which is increased with the increase of inhomogeneity of the dislocation distribution.

Evolution of the dislocation structure and the hardening mechanisms of multiple slip oriented Ni3Ge single crystals

We studied the evolution of the dislocation structure of Ni3Ge single crystals deformed by compression at room temperature. It is shown that the distribution of dislocations is spatially uniform in the studied alloy. The uniformity in the dislocation distribution is produced by relatively high amounts of the frictional stresses of the dislocations. On the basis of the obtained values of the dislocation structure parameters, the contributions of the various mechanisms in the dislocation drag are determined. It is shown that the resistance to deformation is determined primarily by overcoming reactive and unreactive “forest” dislocations, the total contribution of which is 0.9 of the applied stress.

Band structure and packet-martensite structure. Comparison of evolution paths

We consider the similarities and differences between the band substructure and the packet-martensite structure in Ni3Fe alloyed with 38KhN3MFA steel. The variation of their quantitative characteristics with strain and with initial polycrystal grain size is analyzed. The main factors governing the morphological features and the behavior of the considered structures are found to be the laws governing the evolution of the dislocation-disclination ensemble.

Effect of thermal treatment and re-doping on the volume fraction of the γ′ phase in complex-doped Ni-Al-Cr-based superalloy

Phase transformations of complex-redoped Ni-Al-Cr-based superalloys under different thermal treatment are investigated by means of diffraction electron microscopy. Alloys are obtained via directional crystallization. The factors affecting the formation of the long-range atomic order of the γ′ phase (an ordered multicomponent phase based on superstructure L12) are established from the experimental results. It is shown that the degree of long-range order is directly dependent on the strengthening of the superalloy.

Role of the third component in the high-temperature hardening of Ni3Al phase

The high-temperature solid solution hardening of the ternary Ni3Al + Me phase (Me is a metal) is analyzed. The hardening resulting from the alloying by refractory elements (Nb, Ta, Hf) is considered in detail. This phenomenon is established to have a multifactorial nature.

Dislocation and diffusion deformation mechanisms in ultrafine grained materials and free volume role

The effect of average grain size on the mechanisms of polycrystal deformation are considered in the range of grain sizes from 1nm to 1cm. Critical grain sizes are selected. The activity of dislocation and diffusion mechanisms is analyzed. The deformation distribution in grains of different sizes is considered for a polycrystal as a whole within the grain size distribution.