Molecular dynamics simulation of the relaxation of a grain boundary disclination dipole under ultrasonic stresses

Abstract

Effect of oscillating tension-compression stresses on the atomic structure of a [112] tilt grain boundary (GB) containing a disclination dipole in nickel is simulated by molecular dynamics method. Initial system for simulations was constructed by joining together bicrystals containing symmetric tilt GBs Σ = 11/62.96° and Σ = 31/52.20°, so the disclination dipole had a strength ω = 10.76°. The as-constructed system was relaxed at temperature T = 300 K for time interval 150 ps. Then a sinusoidal uniaxial stress was applied along the normal to tilt axis in GB plane and its effect on the structure and energy of the bicrystal was studied. The period of the stress was equal to 160 ps, up to 5 cycles of deformation were applied. The model used is convenient for studies of dislocation absorption or generation by GBs, since only edge dislocations of a single slip system can nucleate and glide in each grain. The simulations have shown that the oscillating stress results in a generation of lattice dislocations by the GB, their glide across grains and sink at appropriate surfaces. The dislocations are nucleated at GB regions where the stress field of the disclination dipole facilitates this process, so the latter results in a compensation of the disclination stress fields and recovery of an equilibrium GB structure and energy. It is concluded that lattice dislocation generation by nonequilibrium GBs can be an underlying mechanism of a relaxation effect of ultrasound on the structure of heavily deformed metals.