We report the first ab initio density-functional study of <111> screw dislocation cores in the bcc transition metals Mo and Ta. Our results suggest a new picture of bcc plasticity with symmetric and compact dislocation cores, contrary to the presently accepted picture based on continuum and interatomic potentials. Core energy scales in this new picture are in much better agreement with the Peie...

The microstructure characterization of tempered nanocrystalline bainitic steel is investigated in the present study. It is found that two types of plastic relaxation, dislocation debris and nanotwin, occurs in the displacive transformation due to relatively low transformation temperature and high carbon content. Because most carbon atoms trap in the dislocation, high dislocation density can be ...

Lattice strain and defect formation in oxygen-implanted silicon (SIMOX) were investigated by high-resolution x-ray diffraction and transmission electron microscopy. At doses of 1 X 10” and 3 X IO” cmp2 a high density of vacancy-type defects formed in a uniaxially compressed layer at the surface of the as-implanted wafer. Annealing at 900 “C for 0.5 h reduced this negative strain as the defects ...

The jump condition at a possibly non-material interface for geometricallynecessary dislocation density in field dislocation mechanics (FDM) and its averaged approximation, phenomenological mesoscopic FDM (PMFDM), is derived. In the context of grain boundaries, the condition implies a tensorial constraint on all five grain boundary parameters, slip transmission at the boundary, possible grain bo...

The problem of the collective behavior of straight parallel edge dislocations is investigated. Starting from the equation of motion of individual dislocations a continuum description is derived. It is shown that the influence of the short range dislocation-dislocation interactions on the dislocation dynamics can be well described by a local back stress which scales like the square root of dislo...

Combining through-focus high-resolution transmission electron microscopy and hierarchical multiscale simulations consisting of density-functional theory, analytical empirical potentials, and continuum elastic theory we demonstrate the existence of a new dislocation type in GaN. In contrast with all previously identified or suggested dislocation structures in GaN, all core atoms are fully coordi...

It is argued in this paper that there are three distinctive physical situations in which the continuum descriptions of crystal plasticity are conceptually different. They are determined by the properties of the random field of dislocation density. One of the cases corresponds to conventional plasticity theory. In this case the system energy possesses the energy density, and a formula for energy...

The dislocation density and subgrain size were determined in the base material and friction-stir welds of 6061-T6 aluminum alloy. High-resolution X-ray diffraction measurement was performed in the base material. The result of the line profile analysis of the X-ray diffraction peak shows that the dislocation density is about 4.5 9 10 m 2 and the subgrain size is about 200 nm. Meanwhile, neutron ...

The high dislocation density (ρd) in Fe-C and/or Fe-N martensite is known to be one of the main causes its overall yield strength. Experiments indicate that and, therefore, strength low and medium C steels, follows a well defined behavior with respect background concentration (co). In present work, we introduce model based on finding after γ→α′ sufficiently accommodate/trap most available inter...

We consider a static theory of dislocations with moment stress in an anisotropic or isotropic elastoplastical material as a T (3)-gauge theory. We obtain Yang-Mills type field equations which express the force and the moment equilibrium. Additionally, we discuss several constitutive laws between the dislocation density and the moment stress. For a straight screw dislocation, we find the stress ...