A three-dimensional multiple-slip dislocation density-based crystalline formulation, specialized finite-element formulations and Voronoi tessellations adapted to martensitic orientations were used to investigate large strain inelastic deformation modes and dislocation density evolution in martensitic microstructures. The formulation is based on accounting for variant morphologies and orientations, retained austenite and initial dislocation densities that are uniquely inherent to martensitic microstructures. The effects of parent austenite orientation and retained austenite were also investigated for heterogeneous fcc/bcc crystalline structures. Furthermore, the formulation was used to investigate microstructures mapped directly from SEM/EBSD images of martensitic steel alloys. The analysis indicates that variant morphology and orientations have a direct effect on dislocation density accumulation and inelastic localization in martensitic microstructures, and that lath directions, orientations and arrangements are critical characteristics of high strength martensitic deformation and behavior.

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Coincidence Site Lattice (CSL) grain boundaries (GBs) are believed to be low-energy, resistant to intergranular fracture, as well as to hydrogen embrittlement. Nevertheless, the behavior of CSL-GBs are generally confused with their angular deviations. In the current study, the effect of angular deviation from the perfect Σ3 (111) [110] GBs in α-iron on the hydrogen diffusion and the susceptibility of the GB to hydrogen embrittlement is investigated through molecular static and dynamics simulations. By utilizing Rice-Wang model it is shown that the ideal GB shows the highest resistance to decohesion below the hydrogen saturation limit. Finally, the hydrogen diffusivity along the ideal GB is observed to be the highest.

A three-dimensional multiple-slip dislocation-density-based crystal plasticity formulation, and specialised finite element formulations were used to determine the initial dislocation-densities associated with martensitic transformations in steel alloys. The analysis is based on modelling the shear part from the phenomenological theory of martensitic transformation to obtain both the transformation mobile and immobile dislocation-densities. The model was validated with experiments related to the transformation of lath martensite in high-strength low-alloying steels.