Krister O. E. Henriksson

Analytical interatomic potential for modeling nonequilibrium processes in the W-C-H system

A reactive interatomic potential based on an analytical bond-order scheme is developed for the ternary system W–C–H. The model combines Brenner’s hydrocarbon potential with parameter sets for W–W, W–C, and W–H interactions and is adjusted to materials properties of reference structures with different local atomic coordinations including tungsten carbide, W–H molecules, as well as H dissolved in bulk W. The potential has been tested in various scenarios, such as surface, defect, and melting properties, none of which were considered in the fitting. The intended area of application is simulations of hydrogen and hydrocarbon interactions with tungsten, which have a crucial role in fusion reactor plasma-wall interactions. Furthermore, this study shows that the angular-dependent bond-order scheme can be extended to second nearest-neighbor interactions, which are relevant in body-centered-cubic metals. Moreover, it provides a possibly general route for modeling metal carbides.

Difference in formation of hydrogen and helium clusters in tungsten

The experimentally observed large difference in the depths of hydrogen and helium clusters formed in tungsten still lacks a fundamental explanation. Using density functional theory calculations, molecular dynamics simulations, and kinetic Monte Carlo calculations, we show that the fundamental mechanism behind the different clustering depths is significantly different behaviors of interstitial H and He atoms in W: H–H states are unstable for small interatomic distances whereas He–He states are strongly bound.

Carbides in stainless steels: Results from ab initio investigations

The useful properties of steels are due to a complicated microstructure containing iron and chromium carbides. Only some basic physical properties of these carbides are known with high precision, although the carbides may have a vital impact on the performance and longevity of the steel. To improve on this situation, we have performed extensive density-functional theory calculations of several carbides. The quantitative results are in perfect agreement with the relative empirical stability of the carbides. Also, in contradiction with experimental data, we find that Cr23C6 responsible for the hardness of stainless steels is not the most stable chromium-dominated carbide.

Interatomic potentials for the Be–C–H system

Analytical bond-order potentials for beryllium, beryllium carbide and beryllium hydride are presented. The reactive nature of the formalism makes the potentials suitable for simulations of non-equilibrium processes such as plasma-wall interactions in fusion reactors. The Be and Be-C potentials were fitted to ab initio calculations as well as to experimental data of several different atomic configurations and Be-H molecule and defect data were used in determining the Be-H parameter set. Among other tests, sputtering, melting and quenching simulations were performed in order to check the transferability of the potentials. The antifluorite Be(2)C structure is well described by the Be-C potential and the hydrocarbon interactions are modelled by the established Brenner potentials.

THE DEPTHS OF HYDROGEN AND HELIUM BUBBLES IN TUNGSTEN: A COMPARISON

Abstract The role of self-trapping and defect trapping of hydrogen and helium implanted into tungsten has been investigated using density functional theory (DFT) calculations, molecular dynamics simulations, and kinetic Monte Carlo simulations (KMCSs). The potential energy curves of hydrogen or helium pairs were obtained by molecular dynamics, and the energy of the most essential states was checked with DFT. Under assumptions of bubble formation due to trapping by similiar impurity atoms (self-trapping) or defects, KMCSs were carried out using parameters from implantation experiments. The results indicate that self-trapping plays no (or a very small) role in hydrogen bubble formation, whereas helium bubbles form due to strong self-trapping.

Atomistic simulations of stainless steels: a many-body potential for the Fe–Cr–C system

Stainless steels found in real-world applications usually have some C content in the base Fe-Cr alloy, resulting in hard and dislocation-pinning carbides-Fe3C (cementite) and Cr23C6-being present in the finished steel product. The higher complexity of the steel microstructure has implications, for example, for the elastic properties and the evolution of defects such as Frenkel pairs and dislocations. This makes it necessary to re-evaluate the effects of basic radiation phenomena and not simply to rely on results obtained from purely metallic Fe-Cr alloys. In this report, an analytical interatomic potential parameterization in the Abell-Brenner-Tersoff form for the entire Fe-Cr-C system is presented to enable such calculations. The potential reproduces, for example, the lattice parameter(s), formation energies and elastic properties of the principal Fe and Cr carbides (Fe3C, Fe5C2, Fe7C3, Cr3C2, Cr7C3, Cr23C6), the Fe-Cr mixing energy curve, formation energies of simple C point defects in Fe and Cr, and the martensite lattice anisotropy, with fair to excellent agreement with empirical results. Tests of the predictive power of the potential show, for example, that Fe-Cr nanowires and bulk samples become elastically stiffer with increasing Cr and C concentrations. High-concentration nanowires also fracture at shorter relative elongations than wires made of pure Fe. Also, tests with Fe3C inclusions show that these act as obstacles for edge dislocations moving through otherwise pure Fe.

Melting temperature effects on the size of ion-induced craters

Recent work on the sizes of craters produced by ion impacts of solids has shown that the size of the crater scales with the inverse square of the cohesive energy. This observation is in contrast to the size of craters produced in macroscopic impacts, which scale directly with the inverse of the cohesive energy. It has relied on the assumption that the melting temperature is proportional to the cohesive energy. Using computer simulations, we now show that the size scales in fact with the inverse of the product of the melting temperature and cohesive energy. This provides direct proof that the reason to the different behavior of macroscopic and ion-induced cratering is flow of the liquid produced by the ions.

A Be?W interatomic potential

In this work, an interatomic potential for the beryllium-tungsten system is derived. It is the final piece of a potential puzzle, now containing all possible interactions between the fusion reactor materials beryllium, tungsten and carbon as well as the plasma hydrogen isotopes. The potential is suitable for plasma-wall interaction simulations and can describe the intermetallic Be(2)W and Be(12)W phases. The interaction energy between a Be surface and a W atom, and vice versa, agrees qualitatively with ab initio calculations. The potential can also reasonably describe Be(x)W(y) molecules with x, y = 1, 2, 3, 4.

Tensile testing of Fe and FeCr nanowires using molecular dynamics simulations

Using molecular dynamics, we have studied the behaviour of cylindrical [001]-oriented Fe and FeCr nanowires under uniaxial tensile strain with both an embedded atom method (EAM) and a Tersoff-like bond order potential. The mechanical properties were analysed and the deformation mechanism was studied and compared between the potentials. The effects of chromium content and size of the wire were studied. Both potentials show elongation by deformation twinning in the 〈111〉/{211} system resulting in a significantly stiffer and stronger [110]-axial nanowire. The pure iron nanowires are elastically softer than bulk iron and an addition of chromium has both a softening and weakening effect. The bond order potential shows a strong dependence on chromium concentration, while the dependence is considerably weaker for the EAM potential.

Polymer conformations in internal (polyspherical) coordinates

The small‐amplitude conformational changes in macromolecules can be described by the changes in bond lengths and bond angles. The descriptors of large scale changes are torsions. We present a recursive algorithm, in which a bond vector is explicitly written in terms of these internal, or polyspherical coordinates, in a local frame defined by two other bond vectors and their cross product. Conformations of linear and branched molecules, as well as molecules containing rings can be described in this way. The orientation of the molecule is described by the orientation of a body frame. It is parametrized by the instantaneous rotation angle, and the two angles that parametrize the orientation of the instantaneous rotation axis. The reason not to use more conventional Euler angles is due to the fact that Euler angles are not well‐defined in gimbal lock (i.e., when a body axis becomes aligned with its space fixed counter part). The position of the molecule is parametrized by its center of mass. Original and calculated positions are compared for several proteins, containing up to about 100,000 atoms.