Atomic mechanisms of high-speed migration of symmetric tilt grain boundaries in nanocrystalline Ni

Molecular dynamics simulations of structural rearrangements in nanocrystalline Ni with the symmetric tilt grain boundary (GB) ∑5 (310) [001] under shear loading were conducted. It was found that GB can be displaced in the direction perpendicular to the shear loading direction. To activate the displacement, it is necessary to reach the threshold value of the shear stress. The GB displacement is abrupt and is due to a certain sequence of displacements of the atomic planes adjacent to the GB. These planes are successively rebuilt from the structure of one grain to the structure of another grain in the process of GB migration. The velocity of GB migration can reach several hundred meters per second and depends on the rate of shear loading. The use of periodic boundary conditions prevents the rotations of the grains. As the simulated tilt GB is symmetric, both of the crystallite grains will have the same shear moduli in the direction of the applied loading. The shear loading of the crystallite with such a structure does not lead to any volume driving forces. The GB displacement was entirely due to the coupling effect. The shear stress curve as a function of time has a sawtooth shape. The GB experiences displacement upon reaching the maximum value of the applied shear stresses. Despite the high stress values, the GB displacement did not cause the nucleation of the defect structure in the crystallite. The GB migration is accompanied by a change in the volume of atoms involved in structural rearrangements.