Twin boundaries (TB) as perfectly symmetric grain boundaries (GB) have received lots of attention for their joint effect on ductility and strength. A simplified dislocation-coherent twin boundary (CTB) simulation was preformed using molecular dynamics (MD) and discrete dislocation dynamics (DDD) to analyse the evolution of microstructure and quantify the size effects associated with CTB. Single crystalline pillars and bicrystalline pillars containing a vertical CTB with different sample size were simulated. Size effects on flow stress were observed in single crystals and bicrystals in the range of 145 nm to 3 micron , then disappeared in larger samples. In addition, the yield stresses of the bicrystals lager than 3 microns was slightly higher than that of the single crystals, which stems from the larger density of immobile dislocations in the bicrystals.
