Elastic Fields Of Dislocations Research Articles

Contributions of polarized dislocation walls, internal stresses and vacancies on hydrogen trapping processes in tensile strengthening (100) nickel single crystal

Multiscale impact of dislocation patterns on hydrogen diffusion and trapping mechanisms has been investigated in multi-slip tensile strengthening nickel single crystal. A thorough analysis of dislocation densities and distribution, resulting long-range internal stresses and vacancies concentration was performed at different levels of plastic deformation to precisely characterize the different trapping sites and their impact on apparent diffusion coefficient. Both factors, which govern the hydrogen distributions, have been exanimated using an original analyze combining electrochemical permeation and thermal desorption mass spectroscopy. The results revealed a slowing of the hydrogen diffusion resulting from the phenomenon of trapping by dislocations at different scales. An energetic approach made it possible to associate the reversible trapping with the elastic field of dislocations, and the irreversible trapping with the core of dislocations and vacancies. Moreover, the analyses highlight an implication of the long-range internal stress in the increase of the apparent hydrogen solubility in relation with dislocation cells formation. Finally, a contribution of vacancies to hydrogen trapping is demonstrated with a competition between the formation of vacancies induced by plastic strain and hydrogen ingress.

INSTABILITY IN A DISLOCATION ENSEMBLE AT PLASTIC DEFORMATION IN METALS

A problem related to the development of instability of a homogeneous state in an ensemble of screw dislocations under plastic deformation of metals is considered . The study of the development of instability and structure formation in the dislocation ensemble is carried out on the basis of the method developed for charged particles in plasma and associated with the correlation interaction of electrons and positively charged ions. Accordingly, the screw dislocation ensemble is represented as a system of dislocations with an opposite Burgers vector, i.e., as a plasma-like medium with opposite dislocation charges. The total dislocation charge of the dislocation ensemble is equal to zero due to the law of conservation of the Burgers vector. In this situation, the elastic field of dislocations is “cut off”. The stress field of a single dislocation is shielded by a uniformly distributed dislocation “background” and is characterized by a certain effective potential. It is found that at long distances it decreases exponentially. Therefore, the value in the argument of the falling potential can be considered as the radius of screening of the elastic field of dislocations. It is shown that the screening radius is equal to the correlation radius, which makes it possible to construct a two-particle correlation function and find the energy of the correlation interaction of dislocations. A system of kinetic equations for a dislocation ensemble is formulated, taking into account the elastic and correlation interaction of dislocations, as well as the processes of their generation and annihilation. The criterion of instability of the homogeneous distribution of dislocations for the formulated system of equations is established. The instability criterion is met under the condition that the dislocation density exceeds a certain critical value that depends on the square of the flow stress and material constants (such as the Burgers vector modulus and shear modulus, as well as indirectly, the packing defect energy). In the framework of linear analysis, it is shown that when one system of sliding screw dislocations is taken into account, a one – dimensional periodic dislocation dissipative structure is formed at the moment of instability occurrence, and when multiple sliding is taken into account, solutions appear in the form of various variants of polyhedral lattices (cellular structures). It is established that the characteristic size of the cellular structure coincides with the experimental dependence both qualitatively and quantitatively ( the cell size is proportional to the square root of the dislocation density, and the proportionality coefficient is about ten). It is shown that the origin of spatially inhomogeneous dislocation structures, based on correlation instability, depends mainly on the features of the elastic interaction of dislocations and is not critical to the choice of the mechanisms of their kinetics (i.e., the mechanisms of generation, annihilation, and runoff of dislocations).

Kinetics of segregation formation in elastic field of edge dislocation in bcc iron

We study the kinetics of the redistribution of impurity atoms in the elastic fields of dislocations. Study is based on taking into account the influence of elastic fields on diffusion fluxes of interstitial impurity atoms. The study is performed by computer simulation methods and consists of several stages. The first is the simulation of a dislocation core structure with a Burgers vector along [100] direction by using the modified molecular static method. The second is obtaining the coefficients that determine the influence of the components of the strain tensor on the diagonal elements of the matrix of diffusion coefficients and in the equations for fluxes of carbon atoms in bcc iron. The third stage is associated with modeling the diffusion characteristics of carbon atoms by the method of molecular dynamics in a crystal without defects. The fourth and final stage uses the data about atomic structure that we obtained at first stage, as well as the characteristics calculated in the second and third, is a simulation of the formation of segregations of interstitial atoms, which is based on solving diffusion equations that take into account the elastic deformations created by the dislocation. The complex of developed models is used to analyze the kinetics of segregation formation. 3D graphs illustrate the distribution of interstitial atoms in the vicinity of a dislocation for different times at certain temperatures.

The key role played by dislocation core radius and energy in hydrogen interaction with dislocations

It is generally believed that the H-induced reduction in dislocation energy plays a key role in modifying dislocation behaviors in the process of hydrogen embrittlement. Here, we examine the factors that lead to H reducing the line energies of the edge and screw dislocations in bcc Fe by atomistic simulations. Grand canonical Monte Carlo simulations are conducted to obtain the distribution of H around the dislocations. We find that H mainly aggregates at the dislocation cores and the H concentration in the elastic field of dislocations is extremely low. The direct consequences of such a distribution pattern of H are as follows. (i) In contrast with previous studies, H induces no change in the shear modulus of the systems containing dislocations. (ii) H increases the core radii and decreases the core energies of the dislocations, which are the only factors leading to the reduction of dislocation line energy by H. (iii) H brings little effect on the stress field of either the edge or screw dislocation, implying that H induces almost no stress-shielding effect on dislocations. A linear relation between the critical shear stress for homogeneous dislocation nucleation and logarithmic bulk H concentration is thus revealed, based on the atomistic result of the H-induced increase in the core radius and decrease in the core energy of the dislocations. The present results indicate that the dislocation-dislocation interaction in the presence of H, which is the key ingredient for the H-enhanced localized plasticity mechanism for hydrogen embrittlement, can be easily evaluated by the linear elastic theory of dislocations if the core radius and energy of dislocations are properly described.

The Key Role Played by Dislocation Core Radius and Energy in Hydrogen Interaction with Dislocations

It is generally believed that the H-induced reduction in dislocation energy plays a key role in modifying dislocation behaviors in the process of hydrogen embrittlement. Here, we examine the factors that lead to H reducing the line energies of the edge and screw dislocations in bcc Fe by atomistic simulations. Grand canonical Monte Carlo simulations are conducted to obtain the distribution of H around the dislocations. We find that H mainly aggregates at the dislocation cores and the H concentration in the elastic field of dislocations is extremely low. The direct consequences of such a distribution pattern of H are as follows. (i) In contrast with previous studies, H induces no change in the shear modulus of the dislocation systems. (ii) H increases the core radii and decreases the core energies of the dislocations, which are the only factors leading to the reduction of dislocation line energy by H. (iii) H brings little effect on the stress field of either the edge or screw dislocation, implying that H induces almost no stress-shielding effect on dislocations. A linear relation between the critical shear stress for homogeneous dislocation nucleation and logarithmic bulk H concentration is thus revealed, based on the atomistic result of the H-induced increase in the core radius and decrease in the core energy of the dislocations. The present results indicate that the dislocation-dislocation interaction in the presence of H, which is the key ingredient for the H-enhanced localized plasticity mechanism for hydrogen embrittlement, can be easily evaluated by the linear elastic theory of dislocations if the core radius and energy of dislocations are properly described.

Effect of elastic fields of dislocations of the equilibrium configurations of self-interstitial atoms in cubic crystals. Part II. FCC copper crystal

Effect of elastic fields of dislocations of the equilibrium configurations of self-interstitial atoms in cubic crystals. Part II. FCC copper crystal

Effect of elastic fields of dislocations of the equilibrium configurations of self-interstitial atoms in cubic crystals. Part I. BCC iron crystal

On the basis of the results from our discussion of the elastic interaction of a selfinterstitial atom with screw and edge dislocations in a bcc iron crystal, we can make the following conclusions.

Elastic fields of dislocations piercing the interface of an anisotropic bicrystal

AbstractThe elastic field of a dislocation piercing the interface of an anisotropic bicrystal is obtained for the general case of a dislocation line refracted at the interface. The theory is based on the method of dislocation‐line force fans which reduce the three‐dimensional elastic field of the dislocation in the bicrystal to the superposition of two‐dimensional fields of straight dislocations and line forces in an infinite homogeneous medium. The suggested method should be suitable for numerical calculations of the elastic fields of dislocations in arbitrary bicrystals.

Addenda to elastic fields of dislocations and disclinations in an isotropic micropolar continuum

Addenda to elastic fields of dislocations and disclinations in an isotropic micropolar continuum

Investigation of boundary structure of copper phthalocyanine crystal grains in resolving crystallographic planes

Deformed crystals of copper phtalocyanine have been investigated. The crystals were deformed in the process of grinding. Large angle grain boundaries with good resolution of dislocations and crystallographic planes (001) have been detected in the electron microscope JEM-100 C.Fig1 shows the grain dislocation boundary with disorientation angles ≈ll.5°.Attention should be paid to the asymmetry of the boundary. The boundary for the left side grain is almost parallel to the plane (100), while the atomic planes of the right side grain break off at the boundary at a distance of ≈65 Å from each other. The measured Burgers vector (b=12.6 Å) and the distance between the dislocations in the wall correspond to the measured disorientation angle.If the image of the region of ‘perfect’ lattice is projected from the same negative under the photographic enlarger on the printed photograph, it will be easy to see that the elastic fields of dislocation forming the boundary given in fig. 1 are also asymmetric.

On simple analytic solutions for anistropic elastic fields of dislocations

On simple analytic solutions for anistropic elastic fields of dislocations