Stress-assisted grain-rotation-induced dislocation emission from grain boundaries in nanocrystalline face-centered-cubic metals

Extensive experiments have shown that stress-assisted nanograin rotation is an important grain-boundary-mediated deformation mode in nanocrystalline materials. The nanograin rotation is observed to has close correlation with the enhanced dislocation activity and plasticity in nanocrystalline metals and alloys. However, how nanograin rotation affects the dislocation behaviour remains unclear. In the present study, a theoretical model is proposed to investigate the dislocation emission behaviour in nanocrystalline face-centered cubic crystals as nanograin rotation dominates the deformation. The energy characteristics and the critical shear stress that is required to trigger the dislocation emission from grain boundaries are analysed. The results show that the nanograin rotation process can make the originally energetically unfavourable dislocation emission process favourable. The critical stress can also be extraordinarily reduced as compared with the rotation-free case. As a result, 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 suggest nanograin rotation as an effective dislocation-emission mechanism in nanocrystalline materials and possibly explain the experimentally observed rotation-dislocation correlation in nanocrystalline materials.