Anomalous High Strain Rate Compressive Behavior of Additively Manufactured Copper Micropillars

TL;DR

This study investigates the dynamic compressive behavior of additively manufactured copper micropillars, revealing a transition in deformation mechanisms at high strain rates with implications for microscale material modeling.

Contribution

It provides the first detailed analysis of high strain rate behavior of 3D printed copper micropillars, highlighting a transition in deformation mechanisms at elevated strain rates.

Findings

Microcrystalline copper deforms with weak strain rate dependence.

UFG copper shows strong rate dependence up to 0.1 s$^{-1}$.

Yield stress saturates at high strain rates, indicating a change in deformation mechanism.

Abstract

Microscale dynamic testing is vital to the understanding of material behavior at application relevant strain rates. However, despite two decades of intense micromechanics research, the testing of microscale metals has been largely limited to quasi-static strain rates. Here we report the dynamic compression testing of pristine 3D printed copper micropillars at strain rates from $\sim0.001$ s$^{-1}$ to $\sim500$ s$^{-1}$. It was identified that microcrystalline copper micropillars deform in a single-shear like manner exhibiting a weak strain rate dependence at all strain rates. Ultrafine grained (UFG) copper micropillars, however, deform homogenously via barreling and show strong rate-dependence and small activation volumes at strain rates up to $\sim0.1$ s$^{-1}$, suggesting dislocation nucleation as the deformation mechanism. At higher strain rates, yield stress saturates remarkably, resulting in a decrease of strain rate sensitivity by two orders of magnitude and a four-fold increase in activation volume, implying a transition in deformation mechanism to collective dislocation nucleation.