The intra-granular deformations of polycrystalline materials with high stacking fault energy occur predominantly by (full) dislocation slip. However, under extreme conditions, such as in nano-sized grains, intra-granular dislocation activity may become limited and deformation twinning may emerge as the dominant deformation mechanism [1], [2]. Furthermore, as the volume fraction of the grain boundaries increases with decreasing grain size, additional deformation mechanisms may emerge from grain boundaries, such as, grain boundary migration, grain rotation, or grain boundary sliding, resulting in a complex interplay of various deformation mechanisms. Experimental studies of nanocrystalline materials have shown that twin and grain boundaries may both evolve at similar time-scales suggesting that there exists interplay between intra-granular twinning and grain boundary migration [3]. A computational model at the meso-length scale that incorporates these mechanisms is important to obtain the effective material deformation response, which may not be accurately represented by traditional non-linear constitutive models. In this work, a phase field approach has been developed to migrate both twin and grain boundaries. The main aim is to study the simultaneous evolution of grain and twin boundaries in 3D columnar grain microstructures of nanocrystalline FCC materials under mechanical load. We adopted a multiple-order-parameter phase field model previously used for modelling martensitic transformations in shape memory alloys to model the intra-granular deformation twinning and the Kobayashi-Warren-Carter (KWC)-type phase field model to model grain boundaries. These models are coupled through the phase field order parameter describing the local crystal orientation assuming only tilt grain boundaries with respect to the columnar (grain) direction. The simulations revealed a rich interplay between deformation twin and grain boundary evolution when nanocrystalline palladium is subjected to tensile load and we will discuss these effects on the resulting non-linear constitutive behavior.Keywords: plasticity, nanocrystalline material, twinning, grain boundaries, phase field modelReferences:[1] Yamakov, V., et al. (2002) Dislocation processes in the deformation of nanocrystalline aluminium by molecular-dynamics simulation, Nature Materials, 45-48.[2] Chen, M., et al. (2003) Deformation Twinning in Nanocrystalline Aluminum, Science 300, 1275-1277. [3] Kobler, A., et al. (2013) Deformation-induced grain growth and twinning in nanocrystalline palladium thin films, Beilstein Journal of nanotechnology 4, 554-566.