GRETHE WINTHER Department of Civil and Mechanical Engineering, Technical University of Denmark DK-2800 Kgs. Lyngby, Denmark grwi@dtu.dk Plastic deformation induces self-organization of dislocations into patterns. In metals of medium to high stacking fault energy, e.g. aluminium and copper, planar dislocation boundaries extending over tens of micrometre in each dimension evolve. They form a pattern of parallel boundaries covering entire grains often in two intersecting sets. Our understanding of the origin and mechanical consequences of the preferred alignment of these boundaries has developed over the last decades due to the interplay between experimental characterization by transmission electron microscopy and recently synchrotron based techniques and modelling/simulation. The process has been iterative; each advance has involved inclusion of new parameters characterising the boundaries. This contribution highlights the journey and provides an outlook to the near future. In the beginning the preferred alignment was related to the macroscopic deformation axes. Subsequently crystallographic information was added resulting in experimental maps of alignment in orientation space. This grain orientation dependence was further analysed in terms of slip systems, leading to empirical predictive relations across deformation modes. As the next step, characterisation of the dislocation content in the boundaries added a deeper understanding of these relations. The current understanding allows for successful discrete dislocation dynamics simulations of boundary formation [1]. Concurrently, experimental advances in in-situ studies of the dynamics of the early stages of boundary formation and alignment enable validation of such simulations [2]. The potential for direct comparison of experimental movies and dislocation dynamics simulations is illustrated and discussed. REFERENCES [1] Frankus F., Pachaury Y., El-Azab A., Devincre B., Poulsen H.F. , Winther G., Investigating the formation of a geometrically necessary boundary using discrete dislocation dynamics, J. Mech. Phys. Solids, Vol. 199, p. 106069, 2025. [2] Zelenika A., Yildirim C. , Detlefs C. , Rodriguez-Lamas R., Grumsen F.B., Poulsen H.F., Winther G., 3D microstructural and strain evolution during the early stages of tensile deformation, Acta Materialia, Vol. 270, p. 119838, 2024.