Clear band formation simulated by dislocation dynamics: Role of helical turns and pile-ups

We present dislocation dynamics simulations of the glide of dislocations in random populations of Frank loops. Specific local rules of interaction are developed to reproduce elementary interaction mechanisms obtained from molecular dynamics simulations. We show that absorption of Frank loops as helical turns on screw dislocations is at the heart of the process of clear band formation because (1) it transforms the loops into jogs on dislocations, (2) when the dislocations unpin, the jogs are transported along the dislocation lines, leading to a progressive clearing of the band and (3) the dislocations are re-emitted in a glide plane different from the initial one, allowing for a broadening of the band. We also show that isolated dislocations cannot form a clear band of finite thickness because the clearing process would be limited to one plane tilted with respect to the { 111 } primary plane. Rather, a pile-up of dislocations is needed, leading to collective effects between dislocations that are analyzed in details.