Assessment of interatomic potentials for atomistic analysis of static and dynamic properties of screw dislocations in W

Screw dislocations in bcc metals display non-planar cores at zero temperature which result in high lattice friction and thermally activated strain rate behavior. In bcc W, electronic structure molecular statics calculations reveal a compact, non-degenerate core with an associated Peierls stress between 1.7 and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations can only be gained by using more efficient atomistic simulations based on semiempirical interatomic potentials. In this paper we assess the suitability of five different potentials in terms of static properties relevant to screw dislocations in pure W. As well, we perform molecular dynamics simulations of stress-assisted glide using all five potentials to study the dynamic behavior of screw dislocations under shear stress. Dislocations are seen to display thermally-activated motion in most of the applied stress range, with a gradual transition to a viscous damping regime at high stresses. We find that one potential predicts a core transformation from compact to dissociated at finite temperature that affects the energetics of kink-pair production and impacts the mechanism of motion. We conclude that a modified embedded-atom potential achieves the best compromise in terms of static and dynamic screw dislocation properties, although at an expense of about ten-fold compared to central potentials

Analytical integration of the forces induced by dislocations on a surface element

An analytical formulation of the nodal forces induced by a dislocation segment on a surface element is presented. The determination of such nodal forces is a critical step when associating dislocation dynamics simulations with continuum approaches to simulate the plastic behaviour of finite domains. The nodal force calculation starts from the infinite-domain stress field of a dislocation and involves a triple integration over the dislocation ensemble and over the surface element at the domain boundary. In the case of arbitrary oriented straight segments of dislocations and a linear rectangular surface element, the solution is derived by means of a sequence of integrations by parts that present specific recurrence relations. The use of the non-singular expression for the infinite-domain stress field ensures that the traction field is finite everywhere even at the dislocation core. A specific solution is provided for virtual semi-infinite segments that can be used to enforce global mechanical equilibrium in the infinite domain. The proposed model for nodal forces is fully analytical, exact and very efficient computationally. A discussion on how to adapt the proposed methodology to more complex shape functions and surface element geometry is presented at the end of the paper. © 2014 IOP Publishing Ltd

Linking atomistic, kinetic Monte Carlo and crystal plasticity simulations of single-crystal tungsten strength

Understanding and improving the mechanical properties of tungsten is a critical task for the materials fusion energy program. The plastic behavior in body-centered cubic (bcc) metals like tungsten is governed primarily by screw dislocations on the atomic scale and by ensembles and interactions of dislocations at larger scales. Modeling this behavior requires the application of methods capable of resolving each relevant scale. At the small scale, atomistic methods are used to study single dislocation properties, while at the coarse-scale, continuum models are used to cover the interactions between dislocations. In this work we present a multiscale model that comprises atomistic, kinetic Monte Carlo (kMC) and continuum-level crystal plasticity (CP) calculations. The function relating dislocation velocity to applied stress and temperature is obtained from the kMC model and it is used as the main source of constitutive information into a dislocation-based CP framework. The complete model is used to perform material point simulations of single-crystal tungsten strength. We explore the entire crystallographic orientation space of the standard triangle. Non-Schmid effects are inlcuded in the model by considering the twinning-antitwinning (T/AT) asymmetry in the kMC calculations. We consider the importance of ?111?{110} and 111 {112} slip systems in the homologous temperature range from 0.08Tm to 0.33Tm, where Tm =3680 K is the melting point in tungsten.</p

Comparison of Atomistic and Phase Field Descriptions of [001] Symmetric Tilt Grain Boundary in Ni

During thermomechanical treatments, polycrystalline microstructures undergo a complex and dynamic evolution for which no real predictive model exists. We propose here to combine phase field simulations and molecular dynamics simulations to eventually fill this gap. We employ The Phase Field model of Admal and cowrokers [Int. J. Plast. 2018], which is derived from the socalled Kobayachi-Warren-Carter model, and is connected to crystal plasticity to include the mutual interactions of grain boundary migration with the presence of crystal dislocations. In the model, the grain boundary is itself a distribution of geometrically necessary dislocations. Comparison with atomistic data, shows that this PF model naturally captures some key features required to predict microstructure evolutions, such as grain boundary energy, shear coupling effect and even some mobility trends under the application of a driving force to migration. At the end of the paper, a (semi-) analytical model is proposed to quantitatively connect the phase field simulations to the reference Molecular Dynamics data

Analytical integration of the forces induced by dislocations on a surface element

An analytical formulation of the nodal forces induced by a dislocation segment on a surface element is presented. The determination of such nodal forces is a critical step when associating dislocation dynamics simulations with continuum approaches to simulate the plastic behaviour of finite domains. The nodal force calculation starts from the infinite-domain stress field of a dislocation and involves a triple integration over the dislocation ensemble and over the surface element at the domain boundary. In the case of arbitrary oriented straight segments of dislocations and a linear rectangular surface element, the solution is derived by means of a sequence of integrations by parts that present specific recurrence relations. The use of the non-singular expression for the infinite-domain stress field ensures that the traction field is finite everywhere even at the dislocation core. A specific solution is provided for virtual semi-infinite segments that can be used to enforce global mechanical equilibrium in the infinite domain. The proposed model for nodal forces is fully analytical, exact and very efficient computationally. A discussion on how to adapt the proposed methodology to more complex shape functions and surface element geometry is presented at the end of the paper. © 2014 IOP Publishing Ltd

The influence of the cube component on the mechanical behaviour of copper polycrystalline samples in tension

International audienceCopper tensile samples were prepared by various thermomechanical treatments (rolling and annealing) producing different initial textures, characterized mainly by an increasing Cube component. These samples were characterized microstructurally (orientation, grain boundary and grain size distributions) and mechanically (tensile stress-strain curves, hardness, dislocation density). It is found that an increase of the Cube percentage leads to an increase of the initial yield stress and to a decrease of the hardening rate at high strains. The macroscopic mechanical behaviour is compared with simulations performed with a simple Taylor type model including anisotropic plastic behaviour and dislocation-based constitutive equations. We show that the proposed modeling allows to reproduce all experimental curves with a limited number of adjusted parameters and that dynamic softening is more active in the Cube orientation than in any other present orientation. This is also confirmed by the EBSD and X-Ray data. As a result, the Cube orientation is shown to be softer in terms of texture and hardening rate. The opposite strengthening effect observed on the initial yield stress is shown to be due to heterogeneous grain size distributions resulting from complex recrystallization mechanisms

H induced decohesion of an Al grain boundary investigated with first principles: General conditions for instant breakage and local delayed fracture

International audienceThe uniaxial tensile test response of a H decorated Σ5 [100] twist grain boundary (GB) in face-centred-cubic Al has been examined with first principles. The impurity shows a strong tendency to relocate during loading. To capture these H movements, the standard model framework was extended to probe loading-unloading hysteresis. Due to the strong monotonic decrease in the H formation energy with rising GB elongation, the maximum tensile stress accepted by the H decorated GB in the slow fracture limit generally is reached before breakage becomes thermodynamically favourable. For any intact GB configuration visited upon exceeding this stress, the assumption of global chemical equilibrium is argued to be violated. Moreover, while breakage remains ensured from the slow fracture limit considerations, it may in practice require the influx of H to the GB vicinity. A quantitative analysis of GB destabilisation in this scenario requires multi-scale modelling even in the slow fracture limit

Plastic strain-induced grain boundary migration (SIBM) in pure aluminum: SEM in-situ and AFM examinations

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