Slip band formation and mobile dislocation density generation in high rate deformation of single fcc crystals

The mechanisms for the nucleation, thickening, and growth of crystallographic slip bands from the sub-nanoscale to the microscale are studied using three-dimensional dislocation dynamics. In the simulations, a single fcc crystal is strained along the [111] direction at three different high strain rates: 10⁴, 10⁵, and 10⁶?s^??1. Dislocation inertia and drag are included and the simulations were conducted with and without cross-slip. With cross-slip, slip bands form parallel to active (111) planes as a result of double cross-slip onto fresh glide planes within localized regions of the crystal. In this manner, fine nanoscale slip bands nucleate throughout the crystal, and, with further straining, build up to larger bands by a proposed self-replicating mechanism. It is shown that slip bands are regions of concentrated glide, high dislocation multiplication rates, and high dislocation velocities. Cross-slip increases in activity proportionally with the product of the total dislocation density and the square root of the applied stress. Effects of cross-slip on work hardening are attributed to the role of cross-slip on mobile dislocation generation, rather than slip band formation. A new dislocation density evolution law is presented for high rates, which introduces the mobile density, a state variable that is missing in most constitutive laws.     

Dislocational kinetics in the intense deformation of fcc single crystals

The stability of the dislocational subsystem of fcc single crystals deformed in dynamic conditions is investigated. It is shown that, depending on the deformational conditions, the system may have one or two steady states, one of which is ρ _s ⁽¹⁾ . When the system has one trivial steady state, it may be stable or unstable. In some conditions, a second unstable point ρ _s ⁽²⁾ appears; in this case, ρ _s ⁽¹⁾ is stable. Tomsk State Architectural and Building Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 43–48, August, 1997.     

Modelling the dynamic deformation and patterning in fcc single crystals at high strain rates: dislocation dynamics plasticity analysis

The deformation process in copper and aluminium single crystals under shock loading is investigated using a multiscale model of plasticity that couples discrete dislocation dynamics and finite element analyses. Computer simulations are carried out to mimic loading condition of high strain rates ranging from 10⁵ to 10⁷?s^?1, and short pulse durations of few nanoseconds involved in recent laser based experiments. The effects of strain rate, shock pulse duration and the nonlinear elastic properties are investigated. Relaxed configurations using dislocation dynamics show formation of dislocation micro bands and weak dislocation cells. Statistical analyses of the dislocation microstructures are preformed to study the characteristics of the local dislocation densities and the distribution of the instantaneous dislocations velocities.     

Temperature dependence of the hardening factor and the microstrain stage for polycrystalline substitutional fcc solid solutions

Russian Physics Journal -     

Effect of stacking-fault energy on the development of a dislocation substructure,strain hardening,and plasticity of fcc solid solutions

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 35–46, March, 1991.     

Study of the atomic structure of bcc and fcc crystals upon instantaneous plastic deformation

The evolution of the atomic structure of face-centered cubic (fcc) and body-centered cubic (bcc) crystals under the conditions of pulsed external loads and large plastic strains is investigated on the basis of computer experiments. The crystals are strained in steps to 32%. After each deformation step (2%), the system is relaxed by molecular dynamics to a new equilibrium state at 300 K. The results of the computer experiments show that plastic deformation can take place under instantaneous external loads either as a result of the motion of partial dislocations, twinning, or the turning and displacement of atomic planes, depending on the stage of the process. The laws governing the variation of the potential energy of the system and the rotation angle of the atomic planes as functions of the degree of plastic strain of the crystal are found. Zh. Tekh. Fiz. 67, 100–102 (December 1997)     

Low-temperature fatigue behaviour and development of dislocation structure in aluminium single crystals with single-slip orientation

Al single crystals oriented for single slip were cyclically deformed under constant plastic strain amplitudes between 1?×?10^?3 and 5?×?10^?2 at 77?K. Al single crystals showed hardening to saturation at all applied shear stress amplitudes. The resultant cyclic stress–strain curve (CSSC) showed a stress plateau in a range of plastic strain amplitude from 2?×?10^?3 to 2?×?10^?2. Surface observation revealed that multiple slip systems were active even at the strain amplitude in the plateau region. At plastic strain amplitudes corresponding to the plateau of the CSSC, persistent slip bands (PSBs) were formed parallel to the primary slip plane. In the PSBs, well-developed dislocation walls parallel to the {100} planes were observed. The microstructure in the PSBs was explained by the fact of multiple activation of the primary and critical slip systems. The above results indicate that the high stacking fault energy of Al is an important factor affecting the fatigue behaviour even at 77?K.     

Extended planar boundary inclinations in fcc single crystals and polycrystals subjected to plane strain deformation

When fcc single crystals with high-symmetry crystal orientations are deformed to moderate strains by rolling, tension or channel die compression, long dislocation boundaries inclined to the extension axis form. Similarly, long dislocation boundaries are often found in grains embedded in polycrystals deformed in the same manner. These extended planar boundaries (EPBs) are characteristically -30-40° from the extension direction and contain the transverse specimen axis. The objective of the present article is to demonstrate that EPBs formed during plane strain deformation are parallel to equivalent slip planes, a pair of hypothetical slip systems used for analyses of the strain and crystal rotation components in place of the larger number of physical slip systems. The coincidence of EPBs and equivalent slip plane inclinations is shown to account for persistent observations of EPBs in the angle range -30-40° from the rolling direction, in rolled single crystals of various initial orientations. The tendency of EPBs towards tilt or twist boundary character can also be rationalized on the basis of the equivalent slip system concept and consideration of the dislocation types available to be incorporated into EPBs.     

Multiphysical simulation analysis of the dislocation structure in germanium single crystals

To grow high-quality germanium crystals is one of the most important problems of growth industry. The dislocation density is an important parameter of the quality of single crystals. The dislocation densities in germanium crystals 100 mm in diameter, which have various shapes of the side surface and are grown by the Czochralski technique, are experimentally measured. The crystal growth is numerically simulated using heat-transfer and hydrodynamics models and the Alexander–Haasen dislocation model in terms of the CGSim software package. A comparison of the experimental and calculated dislocation densities shows that the dislocation model can be applied to study lattice defects in germanium crystals and to improve their quality.     

Effect of pretreatment on microdeformation development in hcf polycrystals of substitutional solid solutions

The effect of small plastic deformation and subsequent free-state aging even under a load on microplastic deformation development in hcp polycrystals of substitutional solid solutions is investigated on Cu-Al and Cu-Ni alloys. It is established that in the temperature interval 77–300°K the effect of deformation pretreatment on subsequent microdeformation development is independent of the temperature at which it occurs, but is determined only by the temperature of secondary deformation. At the same time a clear correlation is observed between the change in the macroscopic elasticity limit and the shape of the microdeformation curve. If the macroscopic elasticity limit (σ″)after deformation pretreatment is larger than the friction strain (σ _F), the stress-strain diagram is a second-order parabola, as in the case of annealed alloys. If the macroscopic elasticity limit after deformation pretreatment equals the friction strain, the shape of the curveσ=f (?) changes considerably. A stage of plastic deformation, on which the linear dependence ofσ on ?_P is conserved, is observed on the stress-strain curves after the range of elastic deformation. The mechanism of microplastic deformation development in predeformed alloys is treated by the statistical theory of microdeformation of polycrystals.     

Contribution from dislocation to deformation resistance in polycrystals of copper-based solid solutions

The evolution of dislocation structure in solid solutions of Cu-Al and Cu-Mn systems with different grain sizes and at different test temperatures is studied by means of transmission electron diffraction microscopy. The scalar density of dislocations is measured and its relationship to the flow stress of alloys is determined. Changes in the contribution from dislocation hardening to deformation resistance upon variations in the contributions associated with changes in grain size, solid-solution hardening, and test temperature are analyzed.     

Evolution of dislocation structure during deformation of γ-irradiated LiF crystals

The effect of γ irradiation on the mechanical characteristics and dislocation structure of slip bands in LiF crystals is studied at doses D⩽7.3×10⁸ R. Irradiation causes a substantial increase (up to a factor of 30) in the yield stress τ _y of the crystals, with τ _y ∝ D ^0.4 in the first approximation. The deformation shear increases in the slip bands of irradiated crystals, as do the densities of the screw and edge dislocation components, while the dislocation mean free paths decrease. Irradiation also raises the probability of twinning cross slip for screw dislocations. The observed effects are assumed to be related to the formation of a different kind of defects in the irradiated crystals, primarily clusters of implanted atoms. Fiz. Tverd. Tela (St. Petersburg) 39, 1072–1075 (June 1997)     

Evolution of a dislocation structure in the surface layers of single crystals during high-temperature annealing

The dislocation structure of KCl crystals annealed in a forevacuum has been shown to display a nonmonotonic variation of the density of dislocations in the direction from the surface. Our experiments and estimates showed that the observed redistribution of dislocations near the surface is due to the diffusion of impurities from the atmosphere and the generation of dislocations in the diffusion zone.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 48–52, September, 1987.     

Determination of the dislocation structure in paratellurite single crystals using the piezo-optic effect

It is shown theoretically and experimentally that dislocations in paratellurite single crystals can be revealed using the piezo-optic effect. The mechanical-stress distribution near dislocations is calculated. Dislocations in the samples are observed using the photoelastic method and chemical etching. Data on the spatial distribution of dislocations in Czochralski-grown paratellurite crystals are reported.     

Complexes of impurity atoms in diluted substitutional solid solutions

The equilibrium concentration of complexes composed of two B atoms is calculated for an A–B diluted substitutional solid solution with a body-centered cubic lattice. The conditions under which the fraction of the complexes becomes significant are determined.     

Electric conductivity of electron-bombarded single crystals of KCl-RbCl solid solutions

We present the results of investigations of the influence of divalent cation impurities on the radiative changes in the electric conductivity of RbCl crystals and KCl-RbCl solid solutions bombarded by electrons. It was observed that a temperature dependence of the electic conductivity of the bombarded crystals has a complicated form in this case. In the case of crystals containing no cation impurities of different valence, there is no radiative decrease of the electric conductivity due to the action of the Smoluchowski mechanism, and a sharp increase is observed instead. We consider physical properties that lead to the appearance of a maximum in the temperature dependence of the electric conductivity of the bombarded crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 92–96, December, 1970.     

Intersection of screw dislocations in fcc crystals during torsional deformation

Dislocation reactions, including dislocation intersections during various processes in crystals, play an important and often crucial role. This is most pronounced during plastic deformation of crystalline solids, which attracts particular interest from researchers. Intersection of screw dislocations in fcc crystals during their deformation by uniaxial tension and compression was studied by A. Cottrell [1]. It was shown that the intersection of similar screw dislocations moving toward each other results in the formation of interstitial thresholds on them; in the case of intersection of opposite screw dislocations, vacancy thresholds are formed on them.     

Temperature dependence,at low temperatures,of macroscopic elastic limit of polycrystals of substitutional solid solutions with fcc lattice

The influence of temperature within the range of 77–293°K on the macroscopic elastic limit σ″ of polycrystals of solid solutions Cu-Ni and Cu-Al was studied. The ratio of the macroscopic elastic limit of the polycrystals was obtained. The contribution of various factors to σ″ and its temperature dependence was analyzed. It was demonstrated that the latter is determined by the temperature dependence of the interaction of dislocations with atoms of the alloying element.     

Thermodynamics of fe-based ternary solid solutions containing nitrogen and a substitutional solute

Simple two-energy level models for ternary solid solutions containing a substitutional (u) solute and an interstitial solute (i) can be used to extract from thermodynamic data and energy needed to insert an i-atom into an interstitial site whose nearest neighbor shell of lattice atoms contains only one U-atom. However, such models are only valid, even in the high-temperature limit, when the U-concentration is small. It has been shown that such models can be extended so as to include all interstitial site types and that the energy parameters extracted from solutions dilute in the U-component may be used to predict the solubility of the i-component up to U-contentrations of more than 10 at.%.