The origin of strain avalanches in sub-micron plasticity of fcc metals

Numerous experimental observations have recently demonstrated two aspects of sub-micron plasticity in nano pillars: (1) the statistical nature of the yield strength, and (2) the existence of dynamic temporal oscillations in the hardening regime. The present work is focused on the second aspect, where we study strain avalanches in nickel nano pillars via a series of 3-D dislocation dynamics (DD) simulations. The computer simulations reveal the existence of a key mechanism that explains the origin of oscillations in post-yield plastic flow in fcc metals. Small dipolar-loops easily form through several cross-slip mechanisms, and then act as internal glide dislocation sources. When dislocations exit pillar boundaries, they leave many dipolar-loops behind. At this stage, the pillar is starved from dislocations and an increase in stress is necessary to keep up with the applied strain rate (hardening stage). At some critical stress level, dipolar-loops become effective Frank-Read sources of new glide dislocations that collectively move in a burst mode, producing strain avalanches (softening stage). The time delay between these two states, which is determined by the average dislocation velocity and pillar diameter, leads to plastic strain oscillations and intermittent dislocation avalanches, consistent with experimental observations.