Probing microplasticity in small scale FCC crystals via Dynamic Mechanical Analysis

TL;DR

This study uses Dynamic Mechanical Analysis to detect and analyze microplasticity and dislocation avalanches in small-scale copper crystals, revealing how dislocation activity influences material response before yielding.

Contribution

It introduces a novel application of AC load perturbations during quasi-static compression to probe pre-yield dislocation dynamics in nano-crystals, supported by a new continuum dislocation model.

Findings

Dislocation avalanches cause an evolving dissipative response before yield.

Dynamic modulus depends on loading rate and static stress.

Model agrees with experimental frequency response data.

Abstract

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.