Revealing twinning from triple lines in nanocrystalline copper via molecular dynamics simulation and experimental observation

Abstract

As the intersecting line of three grain boundaries (GBs), triple line is in non-equilibrium state with large stress concentration and extra energy, which makes it the optimal site for twin nucleation. However, it is very difficult to verify this conjecture by direct experiments, so no one has done it so far. In present work, we try to use molecular dynamics (MD) simulation on nanocrystalline Cu to confirm such hypothesis. We found that 25 out of 28 twins were emitted from triple lines. The related atomic micro-mechanism has been analysed. Stress relaxation and energy reduction were considered to be the driving factors for twinning from triple lines. This could be proved by changes of dihedral angles, relative GB energy sum, average atomic von Mises shear stress and average atomic energy. Experimentally, the transmission Kikuchi diffraction (TKD) characterization on nanocrystalline Cu revealed that the fraction of the triple junction-twin intersection was 81.7% in all twinned nanograins. The underestimation of experimental results might be caused by invisible triple lines in the orientation mapping.