Discrete dislocation simulation of the ultrasonic relaxation of non-equilibrium grain boundaries in a deformed polycrystal

•Ultrasonic relaxation of non-equilibrium grain boundaries is simulated.•Columnar polycrystal contains three non-parallel slip systems.•Ultrasonic treatment causes a significant rearrangement of lattice dislocations.•Rearrangement of lattice dislocations results in a decrease in stress fields of non-equilibrium grain boundaries.•The model predicts an optimal amplitude with the maximum relaxing effect. For the first time, the relaxation of disordered dislocation arrays in a model 3 × 3 columnar polycrystal under ultrasonic action is studied using the discrete dislocation approach. All grains contain three non-parallel slip systems located at an angle of 60° to each other. The non-equilibrium state of the grain boundaries is modeled using two finite edge dislocation walls with Burgers vector of opposite signs, which are equivalent to a wedge junction disclination quadrupole. It is shown that ultrasonic treatment causes a significant rearrangement of the lattice dislocations and their gliding towards the grain boundaries. It results in a decrease in the internal stress fields associated with the presence of non-equilibrium grain boundaries and relaxation of dislocation structure. The model predicts an existence of optimal amplitude, at which the maximum relaxing effect can be achieved. Dependence of the relaxation of dislocation structure on the grain size is also investigated.