Objective molecular dynamics study of cross slip under high-rate deformation

In this paper, we investigate high-strain-rate deformations in crystalline materials using a novel implementation of Objective Molecular Dynamics (OMD). The OMD method is exact and has a rigorous foundation based on the fundamental invariance of the underlying potential energy surface: all atoms out to infinity satisfy the equations of molecular dynamics to high accuracy. Using this OMD method, we compute how dislocations filling all of the space in a crystalline material undergo time-dependent, three-dimensional motions during deformation. We apply this method to investigate the dynamics of screw dislocations in FCC nickel. Our key finding is that the macroscopic motion (i.e., loading conditions) and initial conditions greatly affect the atomic scale deformation mechanisms—such as the formation, motion, multiplication, annihilation, and abrupt changes of the slip plane and Burgers vector of dislocations. Small changes in the macroscopic loading conditions generate a rich variety of atomic deformation pathways. In certain macroscopic motions, we observe the growth of a stacking fault into a mechanical twin, which subsequently thickens by a process of step motion. In other macroscopic motions, we observe the initiation and subsequent development of cross-slip by the Friedel–Escaig (FE) or Fleischer mechanisms (FL). Under mixed loading conditions, a novel mechanism, with a combination of both FE and FL mechanisms was also observed. Our findings on the effect of external strain rate and temperature on the critical stress for homogeneous cross-slip quantitatively agree with a version of transition state theory with a stress-dependent activation barrier. Beyond dislocation motion, we demonstrate the modeling of sliding surfaces using the OMD framework. These examples highlight potential applications of the OMD framework to the mechanisms underlying plastic deformation and friction in material systems.