B. V. Petukhov

Effect of solid-solution softening of crystalline materials: Review

The phenomenon of softening of crystalline materials by alloying, which is promising for controlling mechanical properties of materials, is due to the occurrence of an additional channel of facilitated formation of dislocation kinks at impurity centers. This effect is related to the kink mechanism of dislocation motion. Hence, a range of materials that are capable of softening can be singled out: metals with a bcc structure, semiconductors, ceramic materials, intermetallic compounds, etc. A unified basis for description of softening regularities is given by the phenomenological theory. This theory predicts many properties that are general for all materials under consideration: the range of strength characteristics of impurities capable of softening crystals, limiting possibilities of softening at the optimal choice of the components, etc. The theory, supplemented with the knowledge of some material constants (determined from microscopic calculations or, even better, from experiment), makes it possible to calculate the temperature and concentration dependences of the yield stress and other parameters measured in mechanical tests.

Dynamic interaction of dislocations with impurity subsystem in crystalline materials

The entrainment of impurities by moving dislocations results in the accumulation of impurities in dislocation cores, which eventually significantly modifies the dynamic properties of dislocations. In the framework of the kink mechanism, the possible modes of motion are found self-consistently and the conditions for dislocation immobilization are determined. The dependence of the immobilization stress (the parameter that is most important for “defect engineering” in semiconductors) on the material parameters and experimental conditions is calculated.

On hardening of crystals by immobilization of dislocations due to mobile impurities

A theory describing the effect that impurities have on the regularities of the plastic deformation of crystals in terms of dynamic strain aging is developed. This theory is a combination of two models, one of which describes the immobilization kinetics of individual dislocations due to the entrainment of impurities, while the second considers the collective dynamics of the dislocation ensemble. The unified model proposed here makes it possible to calculate the concentration and temperature dependences of the contribution that impurity atoms make to the flow stress. This theory explains the anomalous temperature dependence of the yield stress, which is experimentally observed in a number of materials.

Deformation of covalent crystals in the vicinity of the yield drop

The theory for description of the stress peak (yield drop) on the stress-strain curves of covalent crystals with low and zero dislocation densities has been developed. The kinetics of the variation of the dislocation density and the shape of the stress peak in the vicinity of the upper yield stress is described analytically within the framework of the generalized Alexander-Haasen model. The character of the elastoplastic transition is analyzed in detail and the model is compared with the experimental data for silicon.

Analytical Description of the Yield Drop in the Alexander-Haasen Model

The analytical solution of the equations of the Alexander-Haasen model, which describes the shape of the deformation-curve peak (the so-called yield drop), has been obtained for the case of a low initial dislocation density. It is shown that the self-development of the dislocation structure results in the specific kinetic transition with a dramatic decrease of the elastic-deformation rate and an increase of the plastic-flow rate. It is natural to interpret this phenomenon as an elastoplastic transition, and to consider the corresponding stress as the yield stress despite the fact that, being considered in the traditional way, its value does not coincide with either the upper or lower yield stress. The conditions of the existence of the yield drop are studied, and the quantitative criterion of the corresponding change in the deformation-curve shape is proposed.

Dynamic aging of dislocations in materials with a high crystalline relief: Competition between diffusion and impurity entrainment

A model of the dynamic interaction of dislocations with the impurity subsystem of crystals that have a high lattice potential relief (Peierls barriers) has been developed. It is shown that the microscopic structure of migration barriers for impurities near a dislocation core may cause qualitatively different behavior of the impurity atmosphere on a moving dislocation. It is justified that the impurity kinetics during atmosphere formation includes two stages. The first (initial) stage is fast and significantly nonequilibrium; it is followed by the second stage, characterized by a slower approach to equilibrium. The initial stage manifests itself at a sufficiently fast dislocation motion and may lead to an anomalous increase in the driving force (or the yield strength of the material) with an increase in the temperature in some range. Blocking of the dislocation motion by impurities may cause inverse brittle-ductile transition, which is observed in some materials with an increase (rather than the usual decrease) in temperature.

Hardening of crystals caused by the dynamic aging of dislocations

A model relating the pronounced effect of mobile impurities on the processes of plastic deformation in crystals to the dynamic aging of dislocations is proposed. The model is based on the concept of internal stresses depending individually on the age of dislocations due to the difference in the impurity environment. The concentration dependence of the impurity contribution to the hardening of materials is calculated. The theory is illustrated by the experimental data for a GeSi solid solution.

Kinetics of State Switching in Quasi-One-Dimensional Nanosystems. Statistical Model of the Influence of Defects

The role of native and radiation-induced defects in the state switching kinetics for one-dimensional systems (nanowires, molecular single-chain magnets, biological macromolecules, and many others) has been studied. An analytical approach to the description of the influence of randomly located defects on the dynamics of new-phase domains is developed using an analogy with stochastic processes in queueing theory. This method makes it possible to calculate threshold phenomena and the dependence of switching fields on material parameters.

State switching kinetics for quasi-one-dimensional nanosystems: Effects of Finite length and irradiation

The state switching in an extended quasi-one-dimensional material is modeled by the stochastic formation of local new-state nuclei and their subsequent growth along the system axis. An analytical approach is developed to describe the influence of defects, dividing a sample into an ensemble of finite-length segments, on its state switching kinetics. As applied to magnetic systems, the method makes it possible to calculate magnetization curves for different defect concentrations and parameters of material.

Influence of impurity atmosphere on the deformation of silicon crystals

The Alexander–Haasen theory, which describes the deformation kinetics of silicon crystals, has been generalized for impurity crystals. The deformation kinetics of an impurity sample is calculated in a wide range of parameters, including the cases of partial and complete entrainment of impurities by moving dislocations. The developed model, despite its simplicity, adequately describes the qualitative transformation of the stress–strain curves of impurity silicon crystals in dependence of the impurity concentration and other material parameters. The manifestation of negative velocity dependence of the yield stress, observed in natural experiments, is analyzed.