Ductile mechanisms of metals containing pre-existing nanovoids

Abstract

The void growth in monocrystalline Cu and Fe are investigated by molecular dynamics simulations to reveal the ductile mechanisms based on dislocation emission and propagation. The results show that the void growth in Cu is governed by the collective interaction of stacking faults along four (111) planes. Three dominant mechanisms of void growth in Fe are identified: (i) for small voids, nucleation of twinning boundaries; (ii) for intermediate voids, emission of shear loops; (iii) for large voids, stacking faults nucleate at the void surface and then degenerate into shear loops. The slip-twinning transition rate of Fe at room temperature calculated according to Zerrili-Armstrong model is in the range measured by our atomistic simulations. Vacancy generation which promotes void growth results from the intersection of more than two stacking faults in Cu, while in Fe it is attributed to the jog dragging of screw dislocations. An analytical model based on nudged elastic band calculation is developed to include the strain rate dependence of the nanovoid-incorporated incipient yielding. This new model demonstrates that the critical radius of shear loop in Cu under a strain rate of 108 s-1 is on the order of Burgers vector. For both metals, the dislocation density has been calculated to elucidate the plastic hardening coupled with void growth. This work sheds new lights in exploring the atomistic origins of the void size and strain rate dependent mechanisms associated with dislocation activities close to void surface.