Mobility of dislocations in Aluminum: Faceting and asymmetry during nanoscale dislocation shear loop expansion

Abstract

Dislocation loop expansion in Al is studied with a hierarchical multiscale approach via atomistic and discrete dislocation dynamics (DDD) simulations. First, mobility laws for straight screw, 30°, 60°, and edge dislocations are calculated using molecular dynamics (MD) simulations. Each atomistic mobility law is described by an empirical piecewise function with a character angle dependent power law exponent to capture the nonlinear damping regime. For the dislocation velocity range considered in this work, the mobilities of screw and 60° dislocations are lower than edge and 30° dislocations. The mobilities of mixed dislocations, such as 30° and 60°, are found to depend on the character of the leading Shockley partial. Second, MD simulations of dislocation loop expansion under a constant external Schmid stress are performed. The expanded dislocation loop is found to facet in the 60° and screw segments due to low mobilities. The dislocation loop is also found to have reflection asymmetry about the edge axis due to mixed dislocation mobility asymmetry. Finally, the nonlinear atomistic mobility laws are implemented into DDD simulations to illustrate that atomistic mobility laws for straight dislocations can describe faceting and asymmetric behaviors observed during dislocation loop expansion in higher length scale simulations.