Christopher Kohler

Atomistic simulation of precipitation hardening in α-iron: influence of precipitate shape and chemical composition

Classical molecular dynamics simulations of the interaction of edge dislocations with precipitates in α-iron are performed. The critical resolved shear stress (CRSS) is determined for various morphologies of precipitates: pure copper and nickel precipitates, ordered and unordered copper/nickel precipitates, copper and nickel precipitates with substitutional iron atoms, copper precipitates of ellipsoidal shape, and copper precipitates with nickel shells. The dependence of the CRSS on the nature of the precipitate is explained by considering the Burgers vector distribution within the precipitates. It is shown that, except for the ordered precipitates, chemical inhomogeneities of the precipitates lower the CRSS with respect to the precipitates consisting of the pure phases.

Nanocrystalline silicon from hot-wire deposition : a photovoltaic material?

Abstract Nanocrystalline silicon (nc-Si:H) attracts a great deal of attention due to the hope for more efficient and stable solar cells, as well as better thin-film transistors and optical sensors. In this study, we report on improvements in the structural and electronic qualities of intrinsic nc-Si:H grown from hot-wire chemical vapour deposition. For examining a wide range of deposition parameters, we use a design-of-experiments approach. In contrast to our previous films obtained from tungsten and tantalum filaments, a novel type of filament greatly enhances preferential growth in the 〈110〉 direction over a wide range of deposition conditions. General considerations on the orientation and electronic activity of grain boundaries in polycrystalline silicon explain why the electronic quality of this 〈110〉-oriented film is remarkably higher than the one previously grown, mixed-phase nanocrystalline silicon. Mobility–lifetime products of films from our novel filaments are two orders of magnitude higher than those of samples from Ta wires. In photoluminescence spectra, no band tail contributions occur, and the amorphous and defect peaks are greatly reduced. Moreover, the transverse optical Raman signal is red-shifted, and thereby indicates a reduction in mechanical strain in our novel nanocrystalline silicon films.

Line defects in solid continua and point particles in (2 + 1)-dimensional gravity

Solutions of a topological gauge theory of the group are identified with geometries of rectilinear topological defects in solid continua. The defect solutions are classified by isolated singularities of holomorphic functions and correspond to disclinations and dislocations, both of planar and screw type. Dislocations are represented as dipoles of disclinations. The interrelationship of the defect solutions with geometries of elementary particles in (2 + 1)-dimensional gravity is discussed.

Point particles in 2+1 dimensional gravity as defects in solid continua

The analogy between point particles in 2+1 dimensional gravity and linear defects in solid continua is reconsidered within the framework of Chern-Simons gauge theory. Explicit field configurations for particles/defects are given. In particular, a new solution for the spinning particle is proposed.

Dissociation ofa〈100〉 edge superdislocations in theγ′-phase of nickel-base superalloys

Dissociation of a⟨100⟩ edge superdislocations in Ni3Al, the hardening γ′-phase of nickel-base superalloys, was investigated using molecular dynamics simulations and theory of elasticity. It was shown that these dislocations dissociate either symmetrically or asymmetrically when they are close to the ⟨011⟩ orientation. The symmetric dissociation, called Hirth lock, has the lowest energy. The reasons for the dissociation are the strong energy reduction due to the core splitting and the relaxation of elastic strains within the dissociation area. The dissociation of a⟨100⟩ edge superdislocations is the reason for their alignment in ⟨011⟩ orientation in the γ′-rafts of superalloys. However, the dissociation does not block the movement of the dislocation because they penetrate the γ′-rafts by climbing. Under loading conditions, typical for creep tests of nickel-base superalloys at high temperatures (≥1000°C), the Hirth lock slightly expands but remains stable. The asymmetric configuration is less stable and can transform into the lower energy Hirth lock.

Atomistic simulations of strain distributions in quantum dot nanostructures

Strain distributions around a Ge quantum dot (QD) buried in a Si spacer layer are investigated theoretically by means of classical molecular dynamics simulations using the Tersoff potential. Applying periodic boundary conditions laterally, two-dimensional superlattices of QDs are obtained. Strain distributions in systems of different sizes and lattice misorientations are computed in order to explain possible vertical correlations in self-organized three-dimensional QD superstructures. Generally, the strain of relaxed systems displays an oscillatory behaviour as a function of the distance from the QD. For QD systems with growth direction [001], a simple fitting function is used to describe the strain along a vertical path above the QD by an oscillation and a decay according to a power law. For QDs with the shape of a truncated pyramid, the planar strain decays by a power of approximately −3. The period of the oscillation is nearly proportional to the QD superlattice constant and decreases with increasing coordination number of the QD superlattice. In misoriented systems with a small tilt angle about the [110] axis, the region of tensile planar strain above the QD is bent in the direction opposite to the misorientation causing a vertical correlation with lateral shift. For a tilt angle ≈55°, no strain oscillation is found which implies a perfect vertical correlation.

Thermal and Mechanical Fatigue Loading: Mechanisms of Crack Initiation and Crack Growth

The present contribution is focused on the experimental investigations and numerical simulations of the deformation behaviour and crack development in the austenitic stainless steel X6CrNiNb18-10 (AISI–347) under thermal and mechanical cyclic loading in HCF and LCF regimes. The main objective of this research is the understanding of the basic mechanisms of fatigue damage and development of simulation methods, which can be applied further in safety evaluations of nuclear power plant components. In this context the modelling of crack initiation and crack growth inside the material structure induced by varying thermal or mechanical loads are of particular interest. The mechanisms of crack initiation depend among other things on the art of loading, microstructure, material properties and temperature. The Nb-stabilized austenitic stainless steel in the solution-annealed condition was chosen for the investigations. Experiments with two kinds of cyclic loading — pure thermal and pure mechanical — were carried out and simulated.The fatigue behaviour of the steel X6CrNiNb18-10 under thermal loading was studied within the framework of the joint research project [1]. Interrupted thermal cyclic tests in the temperature range of 150 °C to 300 °C combined with non-destructive residual stress measurements (XRD) and various microscopic investigations, e.g. in SEM, were used to study the effects of thermal cyclic loading on the material. This thermal cyclic loading leads to thermal induced stresses and strains. As a result intrusions and extrusions appear inside the grains (at the surface), at which micro-cracks arise and evolve to a dominant crack. Finally, these micro-cracks cause continuous and significant decrease of residual stresses.The fatigue behaviour of the steel X6CrNiNb18-10 under mechanical loading at room temperature was studied in the framework of the research project [2]. With a combination of interrupted LCF tests and EBSD measurements the deformation induced transformation of a fcc austenite into a bcc α′-martensite was observed in different stages of the specimen lifetime. The plastic zones develop at the crack tips, in which stress and strain amplitudes are much higher than the nominal loading, and enable martensitic transformation in the surrounding of the crack tip. The consequence of this is that cracks grow in the “martensitic tunnels”. The short and long crack growth behaviours of the steel X6CrNiNb18-10 under mechanical loading at room temperature and T = 288 °C were studied for different loading parameters. Moreover, the R-ratio was modified in order to study the effect of crack closure at the crack tip for long cracks.Several FE-models of specimens with different geometries and microstructures were created and cyclically loaded according to the experimental boundary conditions. A plastic constitutive law based on a Chaboche type model was implemented as a user subroutine in the FE software ABAQUS. The corresponding material parameters were identified using uniaxial LCF tests of X6CrNiNb18-10 with different strain amplitudes and at different temperatures. These calculations aimed in the estimation of stress and strain distributions in the critical areas in which the crack initiation was expected.Copyright

Recent Developments in IMD: Interactions for Covalent and Metallic Systems

We describe the recent developments of IMD (ITAP Molecular Dynamics), a general purpose pro gram for classical molecular dynamics simulations on workstations and massively parallel supercomputers. As pair potentials are not entirely suitable for many classes of materials, several further types of interactions with many body forces have been implemented, thereby extending the range of applicability of IMD. IMD now supports, in particular, also EAM (Embedded Atom Method) potentials for the simulation of metals, and Stillinger-Weber and Tersoff potentials for the simulation of covalent systems, such as ceramics and semiconductors.

Computation of Strain Distributions in Quantum Dot Nanostructures by Means of Atomistic Simulations

Strain distributions around Ge quantum dots embedded in a Si matrix are computed by means of classical molecular dynamics simulations using the molecular dynamics code IMD. The Tersoff potential is employed in order to model covalent bonds. Two crystal lattice structures are considered, the cubic and hexagonal diamond structure. The distributions of the planar strain are studied for a large number of system sizes and lattice misorientations. In a second part, the scaling of IMD is analyzed for different parallelization schemes and machine architectures.

Simulation of Dislocations in Icosahedral Quasicrystals with IMD

We report on recent investigations performed with IMD (ITAP Molecular Dynamics), a general purpose program for classical molecular dynamics simulations on workstations and massively parallel supercomputers. Especially the simulations of dislocations in icosahedral quasicrystals are described. The quasiperiodic structure leads to new interesting properties. The visualization of a dislocation is much more complicated than in periodic crystals and is presented in detail. An overview of the software used is also provided.