Smaller critical size and enhanced strength by nano-laminated structure in nickel

Abstract

Because of a shift in the dominant deformation mechanisms, the strength/hardness of metals increases with decreasing grain size down to a critical value, then decreases with further grain refinement. Here, laminated structure with low-angle grain boundaries was found to have smaller critical size and enhanced strength when compared to the equiaxed grain structure, through a series of large-scale molecular dynamics simulations. Then, the corresponding atomistic mechanisms were investigated by checking and comparing the rotations of grains and equivalent strain partitioning in grain boundary atoms. In laminated structure, grain boundary activies were found to be promoted with decreasing lamellar thickness. More importantly, when compared to the equiaxed grain structure at the same length scale (lamellar thickness/grain size), grain boundary activities were found to be inhibited by the laminated structure, which is the main reason for the enhanced strength and the smaller critical size. Besides grain boundary activities, formation of extended dislocations, formation of deformation twins, partial dislocations interacting with formed TBs, partial dislocation blocking by and even transmission through low-angle grain boundaries were also found to play important roles in plastic deformation of nano-laminated structure. The current findings should provide insights for designing stronger and more stable nanostructured metals.