Explanation of the discrepancy between the measured and atomistically calculated yield stresses in body-centered cubic metals

We propose a mesoscopic model that explains the factor of two to three discrepancy between experimentally measured yield stresses of BCC metals at low temperatures and typical Peierls stresses determined by atomistic simulations of isolated screw dislocations. The model involves a Frank-Read type source emitting dislocations that become pure screws at a certain distance from the source and, owing to their high Peierls stress, control its operation. However, due to the mutual interaction between emitted dislocations the group consisting of both non-screw and screw dislocations can move at an applied stress that is about a factor of two to three lower than the stress needed for the glide of individual screw dislocations.Comment: 4 pages, 2 figures; RevTex4; submitted to PR

Generalized stacking fault energy surfaces and dislocation properties of aluminum

We have employed the semidiscrete variational generalized Peierls-Nabarro model to study the dislocation core properties of aluminum. The generalized stacking fault energy surfaces entering the model are calculated by using first-principles Density Functional Theory (DFT) with pseudopotentials and the embedded atom method (EAM). Various core properties, including the core width, splitting behavior, energetics and Peierls stress for different dislocations have been investigated. The correlation between the core energetics and dislocation character has been explored. Our results reveal a simple relationship between the Peierls stress and the ratio between the core width and atomic spacing. The dependence of the core properties on the two methods for calculating the total energy (DFT vs. EAM) has been examined. The EAM can give gross trends for various dislocation properties but fails to predict the finer core structures, which in turn can affect the Peierls stress significantly (about one order of magnitude).Comment: 25 pages, 12 figure

Electronic Selection Rules Controlling Dislocation Glide in bcc Metals

The validity of the structure-property relationships governing the deformation behavior of bcc metals was brought into question with recent {\it ab initio} density functional studies of isolated screw dislocations in Mo and Ta. These existing relationships were semiclassical in nature, having grown from atomistic investigations of the deformation properties of the groups V and VI transition metals. We find that the correct form for these structure-property relationships is fully quantum mechanical, involving the coupling of electronic states with the strain field at the core of long $a/2$ screw dislocations.Comment: 4 pages, 2 figure

Screw dislocation in zirconium: An ab initio study

Plasticity in zirconium is controlled by 1/3 screw dislocations gliding in the prism planes of the hexagonal close-packed structure. This prismatic and not basal glide is observed for a given set of transition metals like zirconium and is known to be related to the number of valence electrons in the d band. We use ab initio calculations based on the density functional theory to study the core structure of screw dislocations in zirconium. Dislocations are found to dissociate in the prism plane in two partial dislocations, each with a pure screw character. Ab initio calculations also show that the dissociation in the basal plane is unstable. We calculate then the Peierls barrier for a screw dislocation gliding in the prism plane and obtain a small barrier. The Peierls stress deduced from this barrier is lower than 21 MPa, which is in agreement with experimental data. The ability of an empirical potential relying on the embedded atom method (EAM) to model dislocations in zirconium is also tested against these ab initio calculations

Atomic structure of (111) twist grain boundaries in f.c.c. metals

In this paper we have studied the atomic structures of (111) twist boundaries and investigated the applicability of the structural unit model which has previously been established for tilt boundaries and (001) twist boundaries. The calculations were carried out using two different descriptions of interatomic forces. A pair potential for aluminium, for which the calculations were made at constant volume, and a many-body potential for gold, for which the calculations were performed at constant pressure. The atomic structures of all the boundaries studied were found to be very similar for both the descriptions of atomic interactions. This suggests that the principal features of the structure of (111) twist boundaries found in this study are common to all f.c.c. metals. At the same time it supports the conclusion that calculations employing pair potentials are fully capable of revealing the generic features of the structure of grain boundaries in metals. The results obtained here, indeed, show that structures of all the boundaries with misorientations between 0° and 21.79° (Σ=21) are composed of units of the ideal lattice and/or the 1/6<112>stacking fault on (111) planes, and units of the Σ=21 boundary. Similarly, structures of boundaries with misorientations between 21.79° and 27.8° (Σ=13), 27.8° and 38.21° (Σ=7) and 38.21° and 60° (Σ=3) can all be regarded as decomposed into units of the corresponding delimiting boundaries. Therefore we conclude that the atomic structure of (111) twist boundaries can well be understood in the framework of the structural unit model. A related aspect analysed here in detail is the dislocation content of these boundaries. This study shows both the general relation between dislocation content and atomic structure of the boundaries, which is an integral part of the structural unit model, and features specific to the dislocation networks present in the (111) twist boundaries. Furthermore, the dislocation content revealed by the atomistic calculations can be compared in several cases with transmission electron microscope (TEM) observations and the results are discussed in this context

Dislocation core field. II. Screw dislocation in iron

The dislocation core field, which comes in addition to the Volterra elastic field, is studied for the screw dislocation in alpha-iron. This core field, evidenced and characterized using ab initio calculations, corresponds to a biaxial dilatation, which we modeled within the anisotropic linear elasticity. We show that this core field needs to be considered when extracting quantitative information from atomistic simulations, such as dislocation core energies. Finally, we look at how dislocation properties are modified by this core field, by studying the interaction between two dislocations composing a dipole, as well as the interaction of a screw dislocation with a carbon atom