Connecting strain rate dependence of fcc metals to dislocation avalanche signatures

TL;DR

The paper investigates how strain rate controls plastic deformation in fcc metals by linking the rate-dependent plastic response to dislocation avalanche signatures. Using large-scale 3D discrete dislocation dynamics that incorporate phonon-drag mobility and thermally activated cross-slip, the authors show that higher strain rates activate stronger, shorter dislocation configurations, yielding larger, more simultaneous avalanches and increased dislocation storage. Avalanche statistics, modeled as truncated power laws, exhibit rate-driven shifts of exponents and upper cut-offs, connecting microscopic avalanche dynamics to macroscopic flow stress and hardening. A crystal-plasticity interpretation reveals how forest hardening remains the dominant regime in the studied range, while cross-slip activity weakens at higher rates, providing a mesoscopic mechanism that reconciles experimental observations across scales. Overall, the work offers a mechanistic link between strain-rate sensitivity and dislocation avalanche behavior, informing predictions of rate-dependent plasticity in bulk metals and guiding interpretation of experimental data.

Abstract

Strain rate sensitivity is a key feature of material deformation, whose importance is growing both because miniaturized components experience higher effective rates and because small scale simulations increasingly probe such conditions. As a dynamical characteristic, strain rate dependence is shown to be intimately connected to dislocation avalanches, which are a fundamental mechanism of dislocation dynamics. Using carefully designed, state of the art dislocation dynamics simulations in the intermediate range strain rate from 5 to 1000, we show that increasing strain rate promotes the activation of a growing number of stronger sites. The dislocation microstructure progressively rearranges into configurations with shorter segments. Dislocation avalanches become larger through the superposition of simultaneous events and because stronger obstacles are required to arrest them. As a result, the avalanche statistics are strongly affected by strain rate, with a reduced power law regime and an increasing power law exponent. Larger avalanches, in turn, lead to an enhanced dislocation storage rate. Contribution from collinear systems to avalanches and cross slip activity decreases, altering the fraction of screw dislocations and the resulting microstructure. These results provide an original mesoscopic picture of rate sensitivity in this strain rate range and offer a mechanistic interpretation of existing observations and findings from experiments and simulations.

Figures (11)

• Figure 1: impact of the strain rate on the plastic deformation at the mesoscale as simulated by DDD simulations. (a) Typical evolution of the resolved shear stress $\tau$ as a function of the total shear strain $\gamma$ for increasing strain rates $\dot{\epsilon}$ for [001] single crystals. (b) Corresponding evolutions of the dislocation density $\rho$ as a function of the strain $\gamma$. (c) Evolution of the flow stress $\tau_y$ with imposed strain rate and comparison with experimental data from edington1969influence.
• Figure 2: Evolution of the average interaction coefficient $\bar{\alpha}(\dot\epsilon)$ during deformation and impact of the strain rate.
• Figure 3: a-b) Density histograms of activation stresses as function of strain rate. c) Cumulative histograms of the same data. d) Evolution of the parameters of the GEV distributions modeling the simulation data.
• Figure 4: Relative contribution of individual slip systems 's' to dislocation 'a' (see main text) for different slip systems (columns). Each line correspond to a different imposed strain rate: 5/s (top row), 50 /s (middle row) and 500 /s (bottom row).
• Figure 5: Correlation of avalanche strain burst $\Delta \gamma$ (a) and stress drop $\Delta \sigma$ (b) with the avalanche duration $T$ for the different strain rate considered.