Academic literature on the topic 'Dislocation 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).

An understanding of the crack initiation and crack growth in metals spanning the entire spectrum of conventional and advanced has long been a major scientific challenge. It is known that dislocations are involved both in the initiation and propagation of cracks in metals and alloys. In this review, we first describe the experimental observations of dislocation emission from cracks under stress. Then the role played by these dislocations in fatigue and fracture is considered at a fundamental level by considering the interactions of crack and dislocations emitted from the crack. We obtain precise expression for the equilibrium positions of dislocations in an array ahead of crack tip. We estimate important parameters, such as plastic zone size, dislocation free zone and dislocation stress intensity factor for the analysis of crack propagation. Finally, we describe very recent novel and significant results, such as residual stresses and relatively large lattice rotations across a number of grains in front of the crack that accompanies fatigue process.

Multiplication and rearrangement of dislocations in face-centered cubic (FCC) metals during tensile deformation are affected by grain size, stacking fault energy (SFE), and solute elements. X-ray diffraction (XRD) line-profile analysis can evaluate the dislocation density (ρ) and dislocation arrangement (M) from the strength of the interaction between dislocations. However, the relationship between M and ρ has not been thoroughly addressed. In this study, multiplication and rearrangement of dislocations in FCC metals during tensile deformation was evaluated by XRD line-profile analysis. Furthermore, the effects of grain size, SFE, and solute elements on the extent of dislocation rearrangement were evaluated with varying M values during tensile deformation. M decreased as the dislocation density increased. By contrast, grain size and SFE did not exhibit a significant influence on the obtained M values. The influence of solute species and concentration of solute elements on M changes were also determined. In addition, the relationship between dislocation substructures and M for tensile deformed metals were also explained. Dislocations were loosely distributed at M > 1, and cell walls gradually formed by gathering dislocations at M < 1. While cell walls became thicker with decreasing M in metals with low stacking fault energy, thin cell walls with high dislocation density formed for an M value of 0.3 in metals with high stacking fault energy.

Hexagonal close-packed metals and alloys show significant creep behavior with extremely low activation energies at and below ambient temperature even below their 0.2% proof stresses. It is caused by straightly-aligned dislocation arrays in a single slip system without any dislocation cuttings. These dislocation arrays should, then, pile up at grain boundary (GB) because of violation of von Mises' condition in H.C.P. structure. The piled-up dislocations have to be accommodated by GB sliding. Electron back scatter diffraction (EBSD) analyses and atomic force microscope (AFM) observations were performed to reveal the mechanism of GB sliding below ambient temperature in H.C.P. metals as an accommodation mechanism of ambient temperature creep. EBSD analyses revealed that crystal lattice rotated near GB, which indicates the pile up of lattice dislocations at GB. AFM observation showed a step caused by GB sliding. GB sliding below ambient temperature in H.C.P. metals are considered to compensate the incompatibility between neighboring grains by dislocation slip, which is called slip induced GB sliding.

The sub-grain size, d, during steady-state dislocation creep of polycrystalline metals is theoretically formulated to be inversely proportional to the dislocation density, ρ, which is defined as the number of dislocations swept out of a sub-grain divided by the cross-sectional area of the sub-grain. This dislocation density differs from the typically observed dislocation density inside a sub-grain after unloading, ρ_ob. In the current work, the ρ_ob values inside sub-grains in steadily crept specimens of Al, Cu, Fe, Fe–Mo alloy, austenitic stainless steel, and high-Cr martensitic steel reported in the literature were used to evaluate the relation ρ_ob=ηρ. It was confirmed that η≈1 for pure metals (regardless of the type of metal) crept at high temperatures and low stresses or for long durations and η>1 for Mo-containing alloys and martensitic steel crept at low temperatures and/or high stresses. Moreover, it is suggested that the condition η>1 corresponds to a state of excess immobile dislocations inside the sub-grain. The theoretical relation d_ob (≈d)∝η∙〖ρ_ob〗^(-1), where d_ob is the observed sub-grain size, essentially differs from the well-known empirical relation d_ob∝〖ρ_ob〗^(-0.5).

Dislocations in metals affect their properties on the macro- and the microscales. For example, they increase a metal’s hardness and strength. Dislocation outcrops exist on the surfaces of such metals, and atoms in the proximity of these outcrops are more loosely bonded, facilitating local chemical corrosion and reactivity. In this study, we present a unique autocatalytic mechanism by which a system of inorganic semiconducting gold(I) cyanide nanowires forms within preexisting dislocation lines in a plastically deformed Au-Ag alloy. The formation occurs during the classical selective dealloying process that forms nanoporous Au. Nucleation of the nanowire originates at the surfaces of the catalytic dislocation outcrops. The nanowires are single crystals that spontaneously undergo layer-by-layer one-dimensional growth. The continuous growth of nanowires is achieved when the dislocation density exceeds a critical value evaluated on the basis of a kinetic model that we developed.

Quantum-based atomistic simulations are being used to study fundamental deformation and defect properties relevant to the multiscale modeling of plasticity in bcc metals at both ambient and extreme conditions. Ab initio electronic-structure calculations on the elastic and ideal-strength properties of Ta and Mo help constrain and validate many-body interatomic potentials used to study grain boundaries and dislocations. The predicted Σ5 (310) [100] grain boundary structure for Mo has recently been confirmed in HREM measurements. The core structure, γ surfaces, Peierls stress, and kink-pair formation energies associated with the motion of a/2〈111〉 screw dislocations in Ta and Mo have also been calculated. Dislocation mobility and dislocation junction formation and breaking are currently under investigation.

Dislocations play a central role in determining strength and flow properties of metals and alloys. Diffusionless phase transformation of β→α in Ti6Al4V during the Direct Metal Laser Sintering (DMLS) process produces martensitic microstructures with high dislocation densities. However, heat treatment, such as stress relieving and annealing, can be applied to reduce the volume of these dislocations. In the present study, an analysis of the X-ray diffraction (XRD) profiles of the non-heat-treated and heat-treated microstructures of DMLS Ti6Al4V(ELI) was carried out to determine the level of defects in these microstructures. The modified Williamson–Hall and modified Warren–Averbach methods of analysis were used to evaluate the dislocation densities in these microstructures. The results obtained showed a 73% reduction of dislocation density in DMLS Ti6Al4V(ELI) upon stress relieving heat treatment. The density of dislocations further declined in microstructures that were annealed at elevated temperatures, with the microstructures that were heat-treated just below the β→α recording the lowest dislocation densities.

Dislocations are known to be among the most deleterious performance-limiting defects in multicrystalline silicon (mc-Si) based solar cells. In this work, we propose a method to remove dislocations based on a high temperature treatment. Dislocation density reductions of >95% are achieved in commercial ribbon silicon with a double-sided silicon nitride coating via high temperature annealing under ambient conditions. The dislocation density reduction follows temperature-dependent and time-dependent models developed by Kuhlmann et al. for the annealing of dislocations in face-centered cubic metals. It is believed that higher annealing temperatures (>1170°C) allow dislocation movement unconstrained by crystallographic glide planes, leading to pairwise dislocation annihilation within minutes.

The influence of moving dislocations on mass-transfer and the phenomena, accompanying it in pulse-deformed metals is studied in a real-time. Transport of self-interstitial atoms (SIAs) by mobile edge dislocations in crystal with FCC lattice is investigated by molecular dynamics. A strain rate (106s-1) and dislocation density (1010– 1012cm-2) in simulated crystal corresponds to a laser effect in a Q-factor mode. The experimental investigations in a real-time are performed by recording of electrical signal induced by the laser pulse irradiation of metal foils of different crystal structures.