  In small-scale metallic systems, collective dislocation activity has been
correlated with size effects in strength and with a step-like plastic response
under uniaxial compression and tension. Yielding and plastic flow in these
samples is often accompanied by the emergence of multiple dislocation
avalanches. Dislocations might be active pre-yield, but their activity
typically cannot be discerned because of the inherent instrumental noise in
detecting equipment. We apply Alternate Current (AC) load perturbations via
Dynamic Mechanical Analysis (DMA) during quasi-static uniaxial compression
experiments on single crystalline Cu nano-pillars with diameters of 500 nm, and
compute dynamic moduli at frequencies 0.1, 0.3, 1, and 10 Hz under
progressively higher static loads until yielding. By tracking the collective
aspects of the oscillatory stress-strain-time series in multiple samples, we
observe an evolving dissipative component of the dislocation network response
that signifies the transition from elastic behavior to dislocation avalanches
in the globally pre-yield regime. We postulate that microplasticity, which is
associated with the combination of dislocation avalanches and slow viscoplastic
relaxations, is the cause of the dependency of dynamic modulus on the driving
rate and the quasi-static stress. We construct a continuum mesoscopic
dislocation dynamics model to compute the frequency response of stress over
strain and obtain a consistent agreement with experimental observations. The
results of our experiments and simulations present a pathway to discern and
quantify correlated dislocation activity in the pre-yield regime of deforming
crystals.

Materials Science;Statistical Mechanics