The dominant deformation mechanism of nanocrystalline materials with the finest grains: Grain boundary sliding or grain boundary migration?

This paper proposes a new composite model which contains two types of grains: the normal nanograins (ranging from 20 to 100 nm) and the finest grains (ranging from 2 to 4 nm). The finest grains were considered to be a part of the triple junctions, denoted as super triple junctions (STJs). The model describes the competition of deformation mechanism between stress-driven migration of grain boundary (GB) and GB sliding in mechanically loaded nanocrystalline materials. In the model, the enhanced strengthening occurs due to the effects of STJs as obstacles for GB sliding; the stress-driven migration of GB lead to the grain growth, which gives rise to a good ductility for nanocrystalline samples under plastic deformation. The dependences of the critical shear stress and energy difference for GB dislocations traversing through STJs and GB migration on some characteristic parameter rs were calculated and analyzed. The analysis demonstrates that the existence of the finest grains will simultaneously lead to good strength and good ductility.