Strength Of Dislocation Junctions Research Articles

Dislocation energy and line tension in molecular crystal cyclotetramethylene tetranitramine (β-HMX)

Cyclotetramethylene tetranitramine (β-HMX) is an energetic molecular crystal often used in plastic bonded explosives. Its decomposition reaction may be triggered by plastic deformation. Efforts have been made in recent years to evaluate the mechanisms of plasticity in these crystals and to develop constitutive descriptions that can be used to represent plastic deformation on the microstructural level. In this work, we use atomistic simulations to evaluate the dislocation self-energy, core energy, and line tension in four slip systems previously identified as being the most active. The cores are compact and the anisotropic elasticity solution applies at distances from the dislocation line larger than approximately one Burgers vector. Core energies between 0.3 and 0.5 eV/Å result. The line tension varies rapidly when the character of the dislocation is modified due to the strong elastic anisotropy of the crystal, with maxima at approximately ±40° relative to the screw orientation. The line tension also varies from slip system to slip system. These quantities enter many models of elementary mechanisms of dislocation motion such as cross-slip, dislocation nucleation from stress concentrators, the strength of dislocation junctions and other dislocation structures, and the critical stress for the operation of Frank–Read dislocation sources. The data reported here can be used to evaluate the conditions in which these processes operate and as an input to dislocation dynamics simulations.

Effect of the Deformation Axis Orientation on the Strength of Dislocation Junctions in FCC Single Crystals

The effect of the deformation axis orientation on the length and the strength of dislocation junctions in fcc single crystals has been studied within the model of interdislocation contact interactions, which includes the formation of a dislocation reaction and a change in a dislocation configuration under stress in three-dimensional space. The probabilities of failure of a dislocation junction under stress, the formation of indestructible dislocation junctions, and the formation of long strength dislocation barriers capable of limiting the shear zone have been determined.

Effects of the isotropic and anisotropic hardening within each grain on the evolution of the flow stress, the r-value and the deformation texture of tensile tests for AA6016 sheets

Both the isotropic and anisotropic hardening laws implemented in a crystal elastic visco-plastic finite element model (CEPFEM) were tested for the prediction of the flow stress, the r-value and the deformation texture for tensile tests of the AA6016 sheet. The isotropic hardening law provides good predictions of the flow stress anisotropy. The dislocation hardening was found to be dominant in the hardening behavior. By accounting for the strength variety of different dislocation junctions, predictions of the anisotropic hardening law are sensitive to the activity/strength of different dislocation junctions. Both the plastic shear of slip systems and the dominant dislocation junctions are grain orientation dependent. The flow stress level correlates with the strength of particular dislocation junctions. Both hardening models give similar predictions of the r-value and the deformation texture. Although the physics of crystal plasticity is implemented by including the strength difference of different dislocation junctions, the predictions have not significantly improved. This demonstrates that the strength of dislocation junctions is not responsible for explaining the limitation of the isotropic hardening model. Since the latter does not suffer from neglecting the strength variety of different dislocation junctions, the less complicated isotropic hardening model may still be used as such.

Influence of the Orientation of a Reacting Slipping Dislocation on the Strength of Dislocation Junctions in FCC Crystals

A change in the dislocation configuration formed as a result of dislocation reaction under the action of stress in the slip plane is considered. The influence of the geometry of intersection of reacting dislocations, change in the orientation of reacting segments of an expanding dislocation loop, and change in the spectrum of forest dislocations on the destruction stress of dislocation junction is investigated. Theoretical values of the contact interdislocation interaction parameter are obtained and their correlation with the experimental values is estimated.

Asymmetric dislocation junctions exhibit a broad range of strengths

The strength of dislocation junctions with segments of unequal length is studied. Such structures are common, while the equivalent symmetric junctions are exceptions. It is shown that varying the length of junction “arms” leads to significant variation of the junction strength and to changes of the failure mechanism. The relative strength of Lomer and glissile junctions may be reversed as the junctions become asymmetric. This increases the variability of the strength of pinning sites a mobile dislocation encounters during glide.

The Effects of Solute Segregation on the Evolution and Strength of Dislocation Junctions

In this study, the role of solute segregation on the strength and the evolution behavior of dislocation junctions is studied by utilizing kinetic Monte Carlo and 3D dislocation dynamics simulations. The different solute concentrations and the character of the junctions are all included in the simulations in an effort to make a parametric investigation. The results indicate that solute segregation can lead to both strengthening and weakening behavior depending upon the evolution of the dislocation junctions.

Atomistically informed dislocation dynamics in fcc crystals

We develop a nodal dislocation dynamics (DD) model to simulate plastic processes in fcc crystals. The model explicitly accounts for all slip systems and Burgers vectors observed in fcc systems, including stacking faults and partial dislocations. We derive simple conservation rules that describe all partial dislocation interactions rigorously and allow us to model and quantify cross-slip processes, the structure and strength of dislocation junctions, and the formation of fcc-specific structures such as stacking fault tetrahedra. The DD framework is built upon isotropic non-singular linear elasticity and supports itself on information transmitted from the atomistic scale. In this fashion, connection between the meso and micro scales is attained self-consistently, with all material parameters fitted to atomistic data. We perform a series of targeted simulations to demonstrate the capabilities of the model, including dislocation reactions and dissociations and dislocation junction strength. Additionally we map the four-dimensional stress space relevant for cross-slip and relate our findings to the plastic behavior of monocrystalline fcc metals.

Strain rate sensitivity of the commercial aluminum alloy AA5182-O

The mechanical behavior of the commercial aluminum alloy AA5182-O is investigated at temperatures ranging from −120 to 150 ◦ C and strain rates from 10 −6 to 10 −1 s −1 . The strain rate sensitivity parameter is determined as a function of temperature and plastic strain, and the strain rate and temperature range in which dynamic strain aging leads to negative strain rate sensitivity is mapped. The effect of dynamic strain aging on ductility and strain hardening is investigated. The sensitivity of the measured quantities to the experimental method employed and their dependence on grain shape are discussed. The experimental data are compared with the predictions of a model constructed based on a recently proposed mechanism for dynamic strain ageing. The mechanism is based on the effect solute clustering at forest dislocations has on the strength of dislocation junctions. The model is shown to reproduce qualitatively the experimental trends. © 2004 Elsevier B.V. All rights reserved.

A study of dislocation junctions in FCC metals by an orientation dependent line tension model

The formation and strength of dislocation junctions in FCC crystals have been calculated using an orientation-dependent line tension model. The structure of the different types of junctions existing in FCC metals in the absence of an applied stress is examined with particular emphasis on the Lomer–Cottrell lock, the Hirth lock and the glissile junction. We have determined the ‘yield surface’ in stress space corresponding to the dissolution of junctions. Although this model represents a huge simplification of the physics of dislocations, the comparison with more sophisticated models shows that it is able to satisfactorily reproduce both the structure of junctions as well as their response to an applied stress. Moreover, it is in qualitative agreement with available experimental data. It is claimed that this simple model can provide useful parameters related to junction strength in higher level models of single crystal plasticity.

Formation and strength of dislocation junctions in FCC metals : A study by dislocation dynamics and atomistic simulations

The structure and strength of three-dimensional Lomer-Cottrell junctions are studied using four different models. These models represent increasing levels of sophistication, with each case including some new physical effect. The line tension approximation makes use only of the elastic self energy of the dislocation lines, The edge-screw dislocation dynamics model increases the sophistication by including the elastic interactions between all of the dislocation segments, At the next level of sophistication, the nodal dislocation dynamics method accounts for the dissociation of the core into partial dislocations. Finally, the quasicontinuum method takes into account the core effects at the atomic level, The results show that although the line tension model represents a huge simplification of the physics of dislocations, it is able to reproduce the equilibrium structure of any type of Lomer-Cottrell interaction, both in the absence and presence of an externally applied stress. The pseudo-analytical description can thus be used as a bench mark for the development of dislocation dynamics simulations.

Mesoscopic analysis of structure and strength of dislocation junctions in fcc metals

We develop a finite-element-based nodal dislocation dynamics model to simulate the structure and strength of dislocation junctions in fcc crystals. The model is based on anisotropic elasticity theory supplemented by the explicit inclusion of the separation of perfect dislocations into partial dislocations bounding a stacking fault. We demonstrate that the model reproduces in precise detail the structure of the Lomer-Cottrell lock already obtained from atomistic simulations. In light of this success, we also examine the strength of junctions culminating in a stress-strength diagram which is the locus of points in stress space corresponding to dissolution of the junction.

Structure and Strength of Dislocation Junctions: An Atomic Level Analysis

The quasicontinuum method is used to simulate three-dimensional Lomer-Cottrell junctions both in the absence and in the presence of an applied stress. The simulations show that this type of junction is destroyed by an unzipping mechanism in which the dislocations that form the junction are gradually pulled apart along the junction segment. The calculated critical stress needed for breaking the junction is comparable to that predicted by line tension models. The simulations also demonstrate a strong influence of the initial dislocation line directions on the breaking mechanism, an effect that is neglected in the macroscopic treatment of the hardening effect of junctions.