*5.2. Dislocation Emission from Grain Size Less than 20nm*

By now, extensive experiments and simulations exist to show the important role that stress-assisted nanograin rotation play in GB-mediated deformation mode in NC materials. In fact, it is clear that the rotation of nanograins has a close correlation with the enhanced dislocation activity and plasticity in NC metals and alloys.

Li et al. [48] propose a theoretical model to investigate the dislocation emission behavior in NC face-centered cubic crystals. Noting that the nanograin rotation dominates the deformation in these materials, they investigate energy characteristics and the critical shear stress that is required to trigger the dislocation emission from GBs. Their important results show that the "nanograin rotation process can make the originally energetically unfavorable dislocation emission process favorable". They show that the critical stress can also be extraordinarily reduced as compared with the rotation-free case. For example, the proposed dislocation emission process can reduce the required external stress to almost zero if the rotation magnitude can reach 8.5 and 2.9 degrees for Al and Pt, respectively. The findings thus establish that nanograin rotation is a very effective dislocation-emission mechanism in NC materials. Thus, it possibly explains the experimentally observed rotation-dislocation correlation in NC materials. For example, Wang et al. [49] experimentally investigated the atomic-scale deformation dynamic behaviors of Pt nanocrystals with size of ∼18 nm in situ. They used a homemade device in a high-resolution TEM for this purpose. They found that the plastic deformation of the nanosized single crystalline Pt commenced first with dislocation "appearance" then followed by a dislocation "saturation" phenomenon. It was also clear that the magnitude of strain plays a key role on dislocation behaviors in these materials. More recently, these results were confirmed by extensive experiments and molecular dynamics simulations by Liu et al. [50]. They also find that stress-driven grain growth can greatly contribute to enhanced dislocation emission and tensile ductility in NC metals. What is the underlying mechanism behind this correlation? Liu et al. [50] propose a theoretical model based on the cooperative nanograin rotation and GB migration for grain growth to explain the enhanced dislocation emission from GBs.

One should also note the important results obtained by Swygenhoven and Derlet. They investigated [51], GB sliding using MD computer simulations. They used a model of Ni NC sample with a mean grain size of 12 nm under uniaxial tension. They found atomic shuffling and stress-assisted free-volume migration. The activated accommodation

processes under high-stress and room temperature conditions are shown to be GB and triple-junction migration, and dislocation activity.

From these studies, it becomes abundantly clear that GB dislocations in GB and their emission plays a very significant role sliding or rotation of the GB. The sliding or rotation are necessary to explain the deformation behavior and dislocation emission of grains about 15 nm or less in NC materials.
