A molecular-dynamics investigation of single dislocations

The kinematics and kinetics of edge and screw dislocations in FCC materials were studied by molecular dynamics (MD) simulations. In the first part, dislocations in single crystal Cu were accelerated to steady state velocities upon applying shear stress. It was found that screw dislocations enter into the transonic regime continuously with increasing stress. Edge dislocations were limited by the lowest transverse sound velocity (ca. 1.6 km/s) at low stresses and discontinuously crossed into the transonic regime at higher stresses. For sufficiently long edge dislocations the subsonic-transonic transition was initiated by an athermal nucleation process. Finally the velocity dependence of the dislocation mobility was derived. In the second part the kinetics of cross-slip and annihilation of a screw dislocation dipole in Cu, Al, and Ni were studied by multiple MD simulations of long (200b) dislocations at selected stresses and temperatures with the aim to account for the thermally activated nature of the cross-slip process. The cross- slip mechanism that was identified required the formation of a finite length constriction before cross-slip could be initiated. It was shown that point constrictions are not the transition state of cross-slip. The long dislocations in this study allowed multiple cross-slip events to occur independently along the dislocation line, leading to termination of the cross-slip process by formation of sessile loops. A study of the kinetics confirmed that cross-slip is a first-order process. The generated statistical data allowed the calculation of cross-slip activation energy and volume.