Dislocation cores have long dominated the electronic and optical behaviors of semiconductor devices and detailed atomic characterization is required to further explore their effects. Miniaturization ...of semiconductor devices to nanometre scale also puts emphasis on a material's mechanical properties to withstand failure due to processing or operational stresses. Sessile junctions of dislocations provide barriers to propagation of mobile dislocations and may lead to work‐hardening. The sessile Lomer–Cottrell and Hirth lock dislocations, two stable lowest elastic energy stair‐rods, are studied in this paper. More specifically, using atomic resolution high‐angle annular dark‐field imaging and atomic‐column‐resolved X‐ray spectrum imaging in an aberration‐corrected scanning transmission electron microscope, dislocation core structures are examined in zinc‐blende CdTe. A procedure is outlined for atomic scale analysis of dislocation junctions which allows determination of their identity with specially tailored Burgers circuits and also formation mechanisms of the polar core structures based on Thompson's tetrahedron adapted to reactions of polar dislocations as they appear in CdTe and other zinc‐blende solids. Strain fields associated with the dislocations calculated via geometric phase analysis are found to be diffuse and free of `hot spots' that reflect compact structures and low elastic energy of the pure‐edge stair‐rods.
An accurate description of the evolution of dislocation networks is an essential part of discrete and continuum dislocation dynamics models. These networks evolve by motion of the dislocation lines ...and by forming junctions between these lines via cross slip, annihilation and junction reactions. In this work, we introduce these dislocation reactions into continuum dislocation models using the theory of de Rham currents. We introduce dislocations on each slip system as potentially open lines whose boundaries are associated with junction points and, therefore, still create a network of collectively closed lines that satisfy the classical relations α=curlβp and divα=0 for the dislocation density tensor α and the plastic distortion βp. To ensure this, we leverage Frank’s second rule at the junction nodes and the concept of virtual dislocation segments. We introduce the junction point density as a new state variable that represents the distribution of junction points within the crystal containing the dislocation network. Adding this information requires knowledge of the global structure of the dislocation network, which we obtain from its representation as a graph. We derive transport relations for the dislocation line density on each slip system in the crystal, which now includes a term that corresponds to the motion of junction points. We also derive the transport relations for junction points, which include source terms that reflect the topology changes of the dislocation network due to junction formation.
An application of high-energy wide angle synchrotron X-ray diffraction to investigate the tensile deformation of 9Cr ferritic/martensitic (F/M) ODS steel is presented. With tensile loading and ...in-situ X-ray exposure, the lattice strain development of matrix was determined. The lattice strain was found to decrease with increasing temperature, and the difference in Young's modulus of six different reflections at different temperatures reveals the temperature dependence of elastic anisotropy. The mean internal stress was calculated and compared with the applied stress, showing that the strengthening factor increased with increasing temperature, indicating that the oxide nanoparticles have a good strengthening impact at high temperature. The dislocation density and character were also measured during tensile deformation. The dislocation density decreased with increasing of temperature due to the greater mobility of dislocation at high temperature. The dislocation character was determined by best-fit methods for different dislocation average contrasts with various levels of uncertainty. The results shows edge type dislocations dominate the plastic strain at room temperature (RT) and 300 °C, while the screw type dislocations dominate at 600 °C. The dominance of edge character in 9Cr F/M ODS steels at RT and 300 °C is likely due to the pinning effect of nanoparticles for higher mobile edge dislocations when compared with screw dislocations, while the stronger screw type of dislocation structure at 600 °C may be explained by the activated cross slip of screw segments.
•The tensile deformation of 9Cr ODS steel was studied by synchrotron irradiation.•The evolution of internal mean stress was calculated.•The evolution of dislocation character was determined by best-fit method.•Edge type dominates plasticity at RT and 300 °C, while screw type dominates at 600 °C.
The proposed existence of the edge and screw dislocation in the 1930s, and the subsequent work showing that dislocation theory could explain the plastic deformation of crystals, represent an ...important step in developing our understanding of materials into a science. The continued work involved with characterization of dislocations and linking them to a variety of physical properties in both single and poly crystals have made enormous progress over the past 50 years. It is rare to find a technical application involving a material with any crystal structure that is not impacted by dislocations; mechanical properties, massive phase transformations, interphases, crystal growth, electronic properties, the list goes on. In many systems the properties is controlled by the formation of partial dislocations separated by a stacking fault; for example plastic deformation via deformation twinning. And finally, giant strides have been made in characterization and modeling of systems containing dislocations.
The Special Issue on “Crystal Dislocations” is intended to provide a unique international forum aimed at covering a broad range of results involving dislocations and their importance on crystal properties and crystal growth. Scientists working in a wide range of disciplines are invited to contribute to this cause.
Dr. K. Peter D. Lagerlof, Associate Professor of Ceramics
Guest Editor
Atomistic simulations of dislocation mobility reveal that bodycentered cubic (BCC) high-entropy alloys (HEAs) are distinctly different from traditional BCC metals. HEAs are concentrated solutions in ...which composition fluctuation is almost inevitable. The resultant inhomogeneities, while locally promoting kink nucleation on screw dislocations, trap them against propagation with an appreciable energy barrier, replacing kink nucleation as the rate-limiting mechanism. Edge dislocations encounter a similar activated process of nanoscale segment detrapping, with comparable activation barrier. As a result, the mobility of edge dislocations, and hence their contribution to strength, becomes comparable to screw dislocations.
The continuum dislocation dynamics framework for mesoscale plasticity is intended to capture the dislocation density evolution and the deformation of crystals when subjected to mechanical loading. It ...does so by solving a set of transport equations for dislocations concurrently with crystal mechanics equations, with the latter being cast in the form of an eigenstrain problem. Incorporating dislocation reactions in the dislocation transport equations is essential for making such continuum dislocation dynamics predictive. A formulation is proposed to incorporate dislocation reactions in the transport equations of the vector density-based continuum dislocation dynamics. This formulation aims to rigorously enforce dislocation line continuity using the concept of virtual dislocations that close all dislocation loops involved in cross slip, annihilation, and glissile and sessile junction reactions. The addition of virtual dislocations enables us to accurately enforce the divergence free condition upon the numerical solution of the dislocation transport equations for all slip systems individually. A set of tests were performed to illustrate the accuracy of the formulation and the solution of the transport equations within the vector density-based continuum dislocation dynamics. Comparing the results from these tests with an earlier approach in which the divergence free constraint was enforced on the total dislocation density tensor or the sum of two densities when only cross slip is considered shows that the new approach yields highly accurate results. Bulk simulations were performed for a face centered cubic crystal based on the new formulation and the results were compared with discrete dislocation dynamics predictions of the same. The microstructural features obtained from continuum dislocation dynamics were also analyzed with reference to relevant experimental observations.
Dislocations of the sternoclavicular joint (anterior/posterior) and acromioclavicular joint (SCJ and ACJ, respectively) are rare injuries in childhood/adolescence, each having its own special ...characteristics. In posterior SCJ dislocation, the concomitant injuries in the upper mediastinum are most important complication, while in anterior SCJ dislocation there is a risk of permanent or recurrent instability.
In a retrospective analysis from seven pediatric trauma centers under the leadership of the Section of Pediatric Traumatology of the German Trauma Society, children (<18 years) were analyzed with focus on age, gender, trauma mechanism, diagnostics, treatment strategy and follow-up results. Additional epidemiological big data analysis from routine data was done.
In total 24 cases with an average age of 14.4 years (23 boys, 1 girl) could be evaluated (7x ACJ dislocation type ≥ Rockwood III; 17x SCJ dislocation type Allman III, including 12 posterior). All ACJ dislocations were treated surgically. Postoperative immobilization lasted 3-6 weeks, after which a movement limit of 90 degrees was recommended until implant removal. Patients with SCJ dislocation were posterior dislocations in 75%, and 15 of 17 were treated surgically. One patient had a tendency toward sub-dislocation and another had a relapse. Conservatively treated injuries healed without complications. Compared to adults, SCJ injuries were equally rarely found in children (< 1% of clavicle-associated injuries), while pediatric ACJ dislocations were significantly less frequent (p<0.001).
In cases of SCJ dislocations, our cohort analysis confirmed both the heterogeneous spectrum of the treatment strategies in addition to the problems/complications based on previous literature. The indication for the operative or conservative approach and for the specific method is not standardized. In order to be able to create evidence-based standards, a prospective, multicenter-study with a sufficiently long follow-up time would be necessary due to the rarity of these injuries in children. The rarity was emphasized by our routine data analysis.
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•The edge dislocation dipole pairs are first characterized in Ni-based single crystal superalloys.•The interacting mechanism between the edge dislocation dipole pair and heterogeneous ...interfacial misfit dislocation network is clarified by molecular dynamics simulation.•A network of interfacial misfit dislocation can impede, accommodate and pile up edge dislocation dipoles in Ni matrix.•A theoretical model is proposed to depict the loading rate effect on a stimulating period of edge dislocation dipole pairs.
Understanding the intrinsic strengthening mechanism is of great significance for microstructural design of Ni-based single crystal superalloys. In this paper, from an atomistic perspective, the interacting mechanism between edge dislocation dipole pair and interfacial misfit dislocation network has been elaborated. It is shown that a network of interfacial misfit dislocations can effectively impede the movement of matrix dislocations, accommodate and pile up the edge dislocation dipole pairs in Ni matrix. Furthermore, we have systematically elucidated the influence of loading rates on the stimulating period of edge dislocation dipole pairs and stiffness of Ni/Ni3Al substrate. These findings provide a better understanding of the interfacial strengthening mechanism of Ni-based single crystal superalloys.
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