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.

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Multiphase alloys such as advanced high strength steels show limited ductility due to interface decohesion at internal boundaries. This interface decohesion is caused by dislocations that pile-up at interfaces in the material, where they cause a stress concentration. This stress concentration in turn can lead to interface decohesion, resulting in the formation of voids, which, when they coalesce, can form a macroscopic crack. In order to understand the process of interface decohesion and the factors facilitating this, in this thesis interface decohesion at interfaces between the soft iron matrix of steel and hard precipitates is studied at the nano-scale with molecular dynamics simulations. From the nano-scale simulations cohesive laws are derived that relate the tractions at the interface to the separations at the interface. These cohesive laws can be used to describe interface decohesion in material models at the next larger length scale (micro-scale), such as discrete dislocation plasticity. @en

The behaviour of 11 differently oriented iron-precipitate interfaces under a shear load is studied with molecular dynamics simulations. We find that the behaviour depends not only on the interface orientation but also on the shear direction. Furthermore, for many interfaces the presence of a dislocation at the interface triggers a structure change in the interface, and with that completely modifies the shear behaviour. Several interface characteristics are inspected for their possibly decisive influence on the observed loading curves. However, none of these characteristics is found to correlate conclusively with the shear behaviour of the different interfaces. This indicates that actual shear behaviour is rooted in a deeper level of complexity than just depending on the properties of the initial interface. Clearly the time evolution of the interface during shearing is crucial. From the observations a comprehensive cohesive law is derived that represents the shear behaviour for every interface and for both shear directions. This cohesive law can be used in numerical methods at a larger length sale, such as discrete dislocation plasticity.@en

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.

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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.

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