Decoupling dislocation multiplication and velocity effects in metals at extreme strain rates

TL;DR

This study decouples the effects of dislocation velocity and multiplication on metal strengthening at extreme strain rates, revealing their distinct roles depending on initial microstructure.

Contribution

It demonstrates that dislocation velocity mainly causes the SRS upturn, while microstructure evolution influences hardening based on initial dislocation density.

Findings

Dislocation velocity governs the SRS upturn at high strain rates.

Microstructure evolution impacts hardening depending on initial dislocation density.

Dislocation multiplication is negligible in high-density microstructures but significant in low-density ones.

Abstract

The dynamic behavior of metals is governed by collective dislocation motion and interactions that strongly depend on the applied strain rate. Metals exhibit weak strain rate sensitivity (SRS) below a certain threshold, followed by a distinct SRS upturn at higher loading rates. While this upturn is typically attributed to increased glide resistance at high dislocation velocity due to mechanisms such as phonon drag, the role of strain-rate-dependent dislocation multiplication and microstructural evolution under these extreme conditions remains elusive. Here, we decouple these two strengthening effects and show that, while dislocation velocity primarily governs the SRS upturn, the hardening due to microstructure evolution depends strongly on the initial dislocation density. Our investigation of hardness evolution across ten decades of strain rates in a quenched and tempered martensitic low-carbon steel (LCS) using laser-induced projectile impact tests (LIPIT) and nanoindentation reveals SRS upturn around 10^7 1/s. By performing in situ re-indentation of the formed craters, we probe the contribution of dislocations generated during initial deformation at different strain rates. We show that while dislocation multiplication plays a negligible role in fine-grained LCS with high dislocation density, a pronounced dislocation multiplication contributes to the hardness increase in pure iron with lower initial dislocation density. Our results show that, depending on the initial microstructure of metals, dislocation multiplication significantly governs high-strain-rate plasticity, in addition to dislocation velocity effects.