Ayrat A. Nazarov

On the structure, stress fields and energy of nonequilibrium grain boundaries

Abstract The changes of grain boundary (GB) structure caused by the absorption of lattice dislocations are analysed. The typical result of the absorption is the disordering of networks of disclination dipoles/grain boundary dislocations (GBDs). It is shown that the variance of GBD spacings is a good quantitative measure of the degree of GB structure nonequilibrium. By the use of Monte Carlo technique the abating law of x − 1 2 for long range stress fields of disordered GBD networks is obtained. Excess energies of nonequilibrium boundaries are also calculated. In strongly excited state of GBs characterized by the maximum value of the variance of GBD spacings the excess energy can exceed the equilibrium GB energy. Such high-nonequilibrium boundaries can exist in ultrafine grained materials.

On the nature of high internal stresses in ultrafine grained materials

Abstract We present a model to account for high internal elastic strains ultra-fine grained materials. The model is based on the concept of highly nonequilibrium grain boundaries containing random extrinsic grain boundary dislocations (EGBDs) networks. Root mean square elastic strain calculated on the basis of this model is in good agreement with experimental data, if reasonable values of EGBD density are assumed. The excess energy of grain boundaries caused by the internal stresses is estimated. The volume expansion of ultrafine grained materials is evaluated by the use of non-linear elasticity approach. The calculated value of the dilatation also agrees well with experimental results reported in the literature.

Microstructures and hardness of ultrafine-grained Ni3Al

Abstract The microstructural evolution of the ultrafine-grained intermetallic compound Ni 3 Al is studied as a function of annealing at different temperatures. The ultrafine microstructure is produced by a high plastic torsional straining. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to characterize the microstructural evolution and microhardness is used to determine mechanical behaviour. The as-deformed microstructure exhibits an almost fully disordered crystalline structure with coherent domain size of about 18 nm, a strong torsional texture and high internal elastic strains. On annealing the as-deformed samples at different temperatures, the recrystallization of the material into a granular type structure containing non-equilibrium grain boundaries is first observed. This is followed by the transformation from non-equilibrium grain boundaries with simultaneous grain growth. This transformation is correlated with an increase of hardness. A new concept of non-equilibrium grain boundaries transparency is presented to interpret this singular behaviour. The results are compared to those obtained on an ultrafine-grained Al-1.5% Mg alloy produced by the same technique and which exhibits the same mechanical behaviour.

Models of the defect structure and analysis of the mechanical behavior of nanocrystals

Abstract The structure of nanocrystals is modelled using the concept of nonequilibrium grain boundaries (GBs). The latter are considered to contain disordered arrays of dislocations and junction disclinations. The root mean square (rms) elastic strain, excess GB energy, and volume expansion of material caused by these defects are calculated for uniform-randomly distributed dislocations and for disclinations of random strengths and coupled into quadrupole configurations. The role of random internal stresses in the yield stress of nanocrystals is studied in a percolation approach.

Principles of Fabrication of Bulk Ultrafine-Grained and Nanostructured Materials by Multiple Isothermal Forging

On the basis of generalization of research results obtained at the Institute for Metals Superplasticity Problems, principles of fabrication of bulk ultrafine-grained and nanostructured materials by multiple isothermal forging are formulated. Multiple isothermal forging is shown to be a universal high-performance deformation technique for the grain refinement in metals and alloys maximally exploiting the potential of dynamic recrystallization.

Long-range stress fields of disordered dislocation arrays: Two types of disorder, and two decay laws

Abstract Two different decay laws for fluctuations of the long-range stress field induced by disordered planar dislocation ensembles have been reported in the literature: σ ∝ x −3/2 and σ ∝ x −1/2. The limits of these laws are established and corresponding physical processes are discussed.

Drift of dislocation tripoles under ultrasound influence

Numerical simulations of dynamics of different stable dislocation tripoles under influence of monochromatic standing sound wave were performed. The basic conditions necessary for the drift and mutual rearrangements between dislocation structures were investigated. The dependence of the drift velocity of the dislocation tripoles as a function of the frequency and amplitude of the external influence was obtained. The results of the work can be useful in analysis of motion and self-organization of dislocation structure under ultrasound influence.

Microstructure changes in ultrafine-grained nickel processed by high pressure torsion under ultrasonic treatment

HIGHLIGHTSEffect of ultrasound on the microstructure of severely deformed nickel is studied.Ultrasound exerts a relaxing effect on the structure of nanostructured materials.Effect of ultrasound on the structure of nanomaterials depends on its amplitude. ABSTRACT Commercially pure nickel was processed by high pressure torsion (HPT) and subjected to ultrasonic treatment (UST) with different amplitudes of compression‐tension stresses in the zone of stress antinode of a standing wave. It was found that microstructure parameters such as the dislocation density, low‐ and high‐angle grain boundary fractions, microhardness, and the stored excess energy as well, non‐monotonically depend on the ultrasound amplitude. A structure relaxation leading to a reduction of internal stresses and stored energy and increase of the fraction of high‐angle boundaries was observed at some intermediate amplitudes of the oscillating stress. The maximum relaxation effect was observed in the samples after UST with the amplitude of 60 MPa. Possible mechanisms of the influence of ultrasound on the microstructure of deformed materials are discussed.

Effect of Ultrasonic Treatment on the Microstructure and Properties of Nanostructured Nickel Processed by High Pressure Torsion

The effect of ultrasonic treatment on the microstructure, microhardness and thermal stability of pure nickel after high pressure torsion (HPT) was studied. It was shown that the ultrasonic treatment reduces internal stresses induced by severe plastic deformation. The higher the intensity of ultrasound in the range studied, the stronger is this effect. Also it was revealed that grain growth in nickel processed by HPT followed by ultrasonic treatment occurs at higher temperatures than that in nickel as-processed by HPT, i.e. the thermal stability of nanostructured nickel is increased.

Relaxation of the residual defect structure in deformed polycrystals under ultrasonic action

Using numerical computer simulation, the behavior of disordered dislocation systems under the action of monochromatic standing sound wave has been investigated in the grain of the model two-dimensional polycrystal containing nonequilibrium grain boundaries. It has been found that the presence of grain boundaries markedly affects the behavior of dislocations. The relaxation process and changes in the level of internal stresses caused by the rearrangement of the dislocation structure due to the ultrasonic action have been studied.