In recent years, the demand for ultrasonic vibration-assisted forming has been increasing because of its energy efficiency and high precision. The acoustic softening effect induced by ultrasonic loading reduces flow stress and promotes metal softening. However, the underlying microstructural mechanisms remain unclear. This study investigates the mechanism of acoustic energy absorption by dislocations in body-centered cubic (BCC) iron. The kinetic states of atoms surrounding an edge and a screw dislocation under ultrasonic excitation are analyzed using molecular dynamics simulations. The results demonstrate that atoms around edge dislocations absorb ultrasonic energy more efficiently than those in a perfect crystal. This is because the reduced atomic density along the slip direction enhances atomic mobility. In contrast, screw dislocations, whose interatomic spacing along the slip direction remains uniform, absorb energy through a different mechanism: local lattice distortion reduces the interatomic spacing perpendicular to the slip plane, enhancing atomic mobility along the slip direction under vibration. Under ultrasonic loading, both edge and screw dislocations reduce yield stress by 47.0% and 15.9%, respectively, consistent with their preferential acoustic energy absorption mechanisms. Furthermore, the combination of the shear strain components and applied ultrasonic vibration significantly influences the degree of stress reduction, depending on how the vibration couples with the slip direction of the dislocation. These findings provide atomistic insights into the interaction between BCC dislocations and ultrasonic vibrations and clarify the origin of ultrasonic-induced softening in metals.
ASAZUMA et al. (Wed,) studied this question.