The development of high-strength metals has driven the endeavor of pushing the limit of grain size reduction according to the Hall-Petch law. However, a continued grain refinement is technologically particularly challenging and incorporates issues concerning microstructural stability and raises also the problem of a possible inverse Hall-Petch effect. However, utilizing structural transformations at – or of grain boundaries as well as more complex interactions of dislocations at / with grain boundaries can provide additional options and new pathways for optimizing the mechanical properties and / or the stability of ultrafine grained - or nanocrystalline materials. Importantly, a set of examples now exist that indicate the applicability of such strategies also for commercial, complex and / or multi-component alloys. Advanced approaches might be based on different strategies; one example is given by strengthening nanocrystalline materials via dislocation exhaustion through a localized structural transformation at grain boundaries that decreases the density of residual lattice dislocations considerably. Thus, the plasticity in the dislocation-exhausted nanomaterial becomes dominated by the activation of grain boundary-dislocation sources, leading to an ultra-hardening effect.
Another approach is based on dislocation-twin boundary interactions. Twins are generally regarded as obstacles to dislocations in face-centered cubic metals and can modify individual dislocations by locking them in twin boundaries or obliging them to dissociate. Yet, as recently shown, dislocations can also be transported by twin lamella via periodic twinning and detwinning at the atomic scale. Our results reveal a novel evolution route of dislocations by a dislocation–twin interaction where the twins act as transport vehicles rather than as obstacles.Further options are based on transformations of the grain boundary core structure itself, which has been shown to affect precipitation behavior and, most likely, the local thermo-mechanical coupling during severe straining. The coupled analyses by tracer diffusion and high-resolution transmission electron microscopy indicate that a metastable grain boundary transition occurred in a deformed Ni-base multicomponent alloy, which is explained by a concomitant relaxation of transformation-induced elastic strains occurring on a longer time scale in comparison to those for grain boundary diffusion. These analyses provide solid evidence towards the existence of grain boundary phase transitions, even in complex, multi-component alloys. In this contribution, recent examples for advanced strategies that utilize grain boundary transformations or grain boundaries as a vehicle for transitions or defect interactions are highlighted.