Ole Bøcker Pedersen

Atomistic simulations of cross-slip of jogged screw dislocations in copper

We have performed atomic-scale simulations of cross-slip processes of screw dislocations in copper, simulating jog-free dislocations as well as different types of jogged screw dislocations. Minimum-energy paths and corresponding transition state energies are obtained using the nudged-elastic-band path technique. We find low barriers and effective masses for the conservative motion along the dislocations of elementary jogs on both ordinary {111}<110> and nonoctahedral {110}<110> slip systems. The jogs are found to be constricted and therefore effectively act as pre-existing constrictions; the cross-slip activation energy is thereby dramatically reduced, yielding values in agreement with experiment.

Simulation of structure and annihilation of screw dislocation dipoles

Abstract Large scale atomistic simulations are used to investigate the properties of screw dislocation dipoles in copper. Spontaneous annihilation is observed for dipole heights less than 1 nm. Equilibrated dipoles of heights larger than 1 nm adopt a skew configuration due to the elastic anisotropy of Cu. The equilibrium splitting width of the screw dislocations decreases with decreasing dipole height, as expected from elasticity theory. The energy barriers, and corresponding transition states for annihilation of stable dipoles are determined for straight and for flexible dislocations for dipole heights up to 5.2 nm. In both cases the annihilation is initiated by cross-slip of one of the dislocations. For straight dislocations the activation energy shows a linear dependence on the inverse dipole height, and for flexible dislocations the dependence is roughly linear for the dipoles investigated.

Analytical determination of the average Eshelby tensor for transversely isotropic fiber orientation distributions

The analytical expression for the average S tensor in Eshelbys equivalent inclusion method of internal stress determination in composite materials is derived for any transversely isotropic fiber orientation distribution function specified by a generic histogram. The analytical average provides a basis for direct application of experimentally measured fiber ODFs in the modelling of internal stresses and effective properties of thermomechanical fiber composites. Upper and lower bound estimates utilizing the analytical expression for the average S tensor are presented and discussed.

Texture and grain-size effects on cyclic plasticity in copper and copper-zinc

Abstract A study of plastic strain controlled fatigue of copper and copper-zinc shows that polycrystalline Cu-30%Zn does not display true cyclic saturation and that texture has a major effect on the cyclic stress-strain (CSS) behaviour, whereas grain size has a minor effect. The self-consistent Sachs estimate of the CSS curve for polycrystalline Cu-30%Zn lies within 20% of the experimental curve for plastic strain amplitudes up to about 5 × 10 −3 , as compared with 1 × 10 −3 for copper. The increased range of validity of the Sachs model is correlated with slip planarity.

Internal stresses and dislocation dynamics in cyclic plasticity and fatigue of metals

Abstract Theories of the role of internal stresses in cyclic plasticity and in the origin of fatigue cracks are discussed in the light of a quantitative phenomenological characterization of the shapes of hysteresis loops and recent advances in atomistic modelling of cross slip and annihilation of screw dislocations. Part of the phenomenological ‘back stress’ derived from hysteresis loop shapes by the Cottrell procedure can be substantiated as a real stress associated with lattice strains. According to the present ‘static-dynamic’ theory this stress can destabilize accumulated edge dipole arrays. Subsequent recovery by annihilation of screw dipoles with atomistically calculated heights then accounts quantitatively for the observed slip heterogeneity. Brown’s theory of homogeneously slipping PSBs with a surface stress singularity makes no indications of the observed loop shapes or PSB slip heterogeneity. The implications for fatigue of industrial materials are discussed.

On the work hardening of fiber reinforced copper

Keywords: Copper ; Dislocations (crystals) ; Elastic moduli ; Infiltration ; Mathematical models ; Metallic matrix composites ; Microstructure ; Plastic flow ; Residual stresses ; Strain hardening ; Stress relaxation ; Thermal cycling ; Composite flow stress ; Fiber clustering ; Fiber reinforced copper ; Stress strain curve ; Tensile deformation ; Fiber reinforced metals Note: Dept. of Mat. Sci. and Engineering, Massachusetts Inst. of Technology, Cambridge, MA, United States Materials Department, Ris¯ National Laboratory, P.O. Box 49, DK-4000, Roskilde, Denmark Department of Materials, Swiss Fed. Institute of Technology, CH-1015, Lausanne, Switzerland13596462 (ISSN) Reference LMM-ARTICLE-1998-002View record in Web of ScienceView record in Scopus Record created on 2006-10-09, modified on 2017-05-10

Atomistic simulations of jog migration on extended screw dislocations

Abstract We have performed large-scale atomistic simulations of the migration of elementary jogs on dissociated screw dislocations in Cu. The local crystalline configurations, transition paths, effective masses, and migration barriers for the jogs are determined using an interatomic potential based on the Effective Medium Theory. The minimum energy path through configuration space and the corresponding transition state energy are obtained using the Nudged Elastic Band path technique. We find very similar migration properties for elementary jogs on the (110){111} octahedral slip systems and the (110){110} non-octahedral slip systems, with energy barriers in the 15–19 meV range.

A nanotheory of the intense slip localization causing metal fatigue

Abstract Recent breakthroughs in computer simulation of the dislocation dynamics of glide and cross-slip give strong hints for a nanotheory of nucleation of persistent slip bands (PSBs) as the reco...

Atomic-scale modeling of the annihilation of jogged screw dislocation dipoles

Using atomistic simulations we investigate the annihilation of screw dislocation dipoles in Cu. In particular we determine the influence of jogs on the annihilation barrier for screw dislocation dipoles. The simulations involve energy minimizations, molecular dynamics, and the Nudged Elastic Band method. We find that jogs on screw dislocations substantially reduce the annihilation barrier, hence leading to an increase in the minimum stable dipole height.

Atomic structure and energetics of constricted screw dislocations in copper

Atomistic simulations of cross slip of a dissociated screw dislocation have been performed. Shapes and energetics of different dislocation configurations relevant to cross slip in an f.c.c. metal (Cu) are determined. The minimum stress-free activation energy and activation length in the Friedel-Escaig cross-slip mechanism are determined. The simulations reveal a new energetically favourable configuration of a dissociated screw dislocation not previously considered. The importance of this configuration to surface nucleated cross slip is discussed.