The behaviour of 11 differently oriented iron-precipitate interfaces under mixed loading conditions is studied with molecular dynamics simulations. We find that the interface structure and the change in this structure play a key role in the response to the loading. The structure change is influenced by both the loading history and the loading direction. Depending on the interface and the loading direction, the presence of a dislocation at the interface may have an additional influence on the structure change. We update our previously derived cohesive laws for pure shear and pure tensile loading to take into account the influence of the other loading direction on the behaviour during mixed loading conditions. However, not for every interface a unique relation exists between the separations at the interface and the tractions. In those cases our cohesive laws give no exact prediction, but rather a range of possible values. The cohesive laws are intended to be used in numerical methods at the next larger length scale, such as discrete dislocation plasticity.

The friction behavior of atomically stepped metal surfaces under contact loading is studied using molecular dynamics simulations. While real rough metal surfaces involve roughness at multiple length scales, the focus of this paper is on understanding friction of the smallest scale of roughness: atomic steps. To this end, periodic stepped Al surfaces with different step geometry are brought into contact and sheared at room temperature. Contact stress that continuously tries to build up during loading, is released with fluctuating stress drops during sliding, according to the typical stick-slip behavior. Stress release occurs not only through local slip, but also by means of step motion. The steps move along the contact, concurrently resulting in normal migration of the contact. The direction of migration depends on the sign of the step, i.e., its orientation with respect to the shearing direction. If the steps are of equal sign, there is a net migration of the entire contact accompanied by significant vacancy generation at room temperature. The stick-slip behavior of the stepped contacts is found to have all the characteristic of a self-organized critical state, with statistics dictated by step density. For the studied step geometries, frictional sliding is found to involve significant atomic rearrangement through which the contact roughness is drastically changed. This leads for certain step configurations to a marked transition from jerky sliding motion to smooth sliding, making the final friction stress approximately similar to that of a flat contact.

Thermal degradation under wet conditions is considered as an important aging mechanism in polyamide 6,6 (PA 6,6). The effect of water on thermal degradation of amorphous PA 6,6 is investigated at relatively low temperatures, varying from 1000 to 2000 K, using reactive force field molecular dynamics (MD) and collective variable-driven hyperdynamics simulations. The simulation of the related long-term chemical reactions is made possible thanks to the self-learning accelerated MD concept of hyperdynamics in combination with the corresponding accurate reproduction of the correct dynamics, consistent with unbiased MD simulations. The kinetics of cleavage reactions of the amide bonds in the backbone of the polymer chains, responsible for the thermal degradation of the polymer, is studied, and the influence of water content on the activation energy and pre-exponential factor of the cleavage reactions is explored. The results show that activation energy and pre-exponential factor are in agreement with experimental data. The proposed simulation framework not only estimates kinetic properties of thermal degradation that are consistent with experimental observations but also provides a predictive tool for studying long-term thermal degradation of PA 6,6.

The impingement of edge dislocations on nano-scale interfaces formed when bringing in contact aluminum crystals is investigated using molecular dynamics simulations. Dislocations, inserted in the bottom crystal, glide towards the contact when the two crystals are pressed together. There, dislocations are absorbed and upon further loading new dislocations are nucleated from the impinging site. Absorption and nucleation are events that affect the length of dislocation pile-ups and therefore the plastic behavior of crystals under contact loading. While it is possible to track absorption and nucleation at the nano-scale with molecular dynamics simulations, larger scale models, which are suitable to study plasticity, do not have the right resolution and neglect these events. The goal of this work is to gain a better understanding of dislocation impingement and to assess to which extent absorption and re-nucleation would play a role at the larger scale. The contacts are here characterized by their initial atomic scale roughness for which a simple, novel definition is introduced. Results show for the first time that roughness controls dislocation nucleation from the contact. This is true for both dislocation-free crystals and for crystals containing one or more dislocations before application of contact loading. In dislocation-free crystals nucleation occurs at decreasing load for increasing roughness. When a dislocation impinges on the contact, it affects its local roughness, by that decreasing the load necessary for dislocation nucleation. Only when the initial roughness of the contact is above a given threshold, dislocation impingement does not affect the load required for nucleation. If instead of a single dislocation, a train of dislocations impinges on the same site, dislocation nucleation is even more facilitated. However, even in this case the contact pressure required to nucleate dislocations is in the order of one GPa, rather high compared with the pressure required to sustain plastic deformation when macro-scale bodies are in contact.

Single dislocations and dislocation pile-ups at crystalline interfaces cause stress concentrations. This can lead to decohesion of the interfaces under tensile loading. To describe this decohesion quantitatively, a cohesive law for planar crack growth at iron/precipitate interfaces is developed. This cohesive law is based on a universal description of the adhesive energy for different interfaces. Only a single scaling factor c must be applied to convert the prediction based on this universal adhesive energy into the actual traction-separation behaviour during crack growth. This scaling factor takes into account the atomic strain and relaxations. The cohesive law derived here can be implemented in a Discrete Dislocation Plasticity framework to describe crack growth at a larger scale. The method presented in this paper for the derivation of a cohesive law can be applied to other material combinations showing brittle crack growth as well.

Molecular dynamics simulations are performed to obtain a better understanding of the interactions of single dislocations and dislocation pile-ups with interfaces between iron and a precipitate. The material properties of the precipitate material and the iron-precipitate interaction are varied to understand the influence of interface structure, interface strength and precipitate stiffness on these interactions under shear loading. Our main findings are: (1) the interface adhesion is determined by a combination of the atomic interactions across the interface and the interface structure, (2) the interface structure is the key factor determining the dislocation accommodation capability of the interface: very strong semi-coherent interfaces do accommodate dislocations, while only very weak coherent interfaces are capable of doing this, and (3) a strong precipitate prevents slip transfer into the precipitate. Results of this study combined with those of a forthcoming study under tensile loading can be used to improve the description of interface decohesion in existing larger-scale models, such as discrete dislocation plasticity.

Results are presented of large-scale Molecular Dynamics simulations of low-energy He bombardment of W nanorods, or so-called 'fuzz' structures. The goal of these simulations is to see if ballistic He penetration through W fuzz offers a more realistic scenario for how He moves through fuzz layers than He diffusion through fuzz nanorods. Instead of trying to grow a fuzz layer starting from a flat piece of bulk W, a new approach of creating a fully formed fuzz structure 0.43 μm thick out of ellipsoidal pieces of W is employed. Lack of detailed experimental knowledge of the 3D structure of fuzz is dealt with by simulating He bombardment on five different structures of 15 vol% W and determining the variation in He penetration for each case. The results show that by far the most important factor determining He penetration is the amount of open channels through which He ions can travel unimpeded. For a more or less even W density distribution He penetration into fuzz falls off exponentially with distance and can thus be described by a 'half depth'. In a 15 vol% fuzz structure, the half depth can reach 0.18 μm. In the far sparser fuzz structures that were recently reported, the half depth might be 1 μm or more. This means that ballistic He penetration offers a more likely scenario than He diffusion through nanorods for how He moves through fuzz and may provide an adequate explanation for how He penetrates through the thickest fuzz layers reported so far. Furthermore, the exponential decrease in penetration with depth would follow a logarithmic dependence on fluence which is compatible with experiments. A comparison of these results and molecular dynamics calculations carried out in the recoil interaction approximation shows that results for W fuzz are qualitatively very different from conventional stopping power calculations on W with a similarly low but homogeneous density distribution.