Metals strengthen with increasing temperature at extreme strain rates

The strength of materials is inherently dependent on the strain rate applied during testing. At lower strain rates, the movement of dislocations (defects in the crystal lattice) is the primary mechanism controlling plasticity, and this movement is thermally activated: higher temperatures facilitate dislocation motion, leading to material softening. However, at extremely high strain rates (above 10⁴ s⁻¹), additional strengthening mechanisms come into play, including dislocation drag. This phenomenon is of critical importance in various applications, such as high-speed manufacturing, hypersonic transport, and meteorite impacts, where materials are subjected to extreme conditions. Traditional micromechanical strength measurements have limitations in accessing the high strain rate regime, hindering our understanding of material behavior in these scenarios. This research aims to address this gap by utilizing microballistic impact testing to quantify the strength of metals at strain rates greater than 10⁶ s⁻¹ and to investigate the unusual temperature dependence of strength at these rates. This is particularly interesting because decades of research at lower strain rates firmly establish thermal softening as the norm.

Existing literature extensively documents the thermal softening of metals across a wide range of strain rates (10⁻¹⁰ to 10⁴ s⁻¹). Studies using various techniques like Kolsky bars, flyer plates, and shock impacts have provided valuable data, but these methods often involve macroscale samples and high impact velocities, making it challenging to achieve strain rates above 10⁴ s⁻¹ without the confounding influence of strong shock effects. While theoretical models have predicted a possible transition to a regime where metals might not soften at high temperatures due to the dominance of dislocation drag at extreme strain rates, there has been a scarcity of experimental data to validate these predictions. This study addresses this research gap.

The researchers employed laser-induced particle impact tests using alumina microparticles (12.5 ± 1 µm) as impactors onto copper, titanium, and gold substrates. The high hardness of the alumina particles ensured that plasticity was primarily confined to the substrate. High-speed cameras tracked the impact and rebound trajectories of the particles, providing data on the coefficient of restitution (CoR), which is the ratio of rebound to impact velocity. Additionally, 3D laser scanning confocal microscopy measured the size and depth of the impact craters formed on the substrate. Two independent strength measures were extracted from the experimental data: dynamic yield strength (Yd), calculated from the CoR data using a power-law relation established in previous work, and dynamic hardness (Hd), determined from the impact energy and crater volume. The strain rate for each experiment was calculated as the ratio of impact velocity to particle diameter. Furthermore, the researchers developed a model to separate the thermal, athermal, and drag strengthening components of the total strength to gain insights into the underlying deformation mechanisms.

The key finding is that at strain rates greater than 10⁵ s⁻¹, the strength of copper, titanium, and gold increases with increasing temperature. This contrasts with the typical thermal softening observed at lower strain rates. Specifically, for copper, the dynamic yield strength increased by approximately 30% over a temperature range of 150 °C. Both the increased rebound velocity (higher CoR) and decreased crater volume independently confirmed the anomalous thermal strengthening. The model developed by the authors showed that while thermal and athermal strengthening components exhibit conventional thermal softening, the dislocation drag component increases significantly with temperature at extreme strain rates, outweighing the softening contributions and leading to a net strengthening effect. Analysis of the apparent activation energy (Qapp) for plasticity further supports the transition from thermally activated softening (positive Qapp) at lower strain rates to dislocation drag strengthening (negative Qapp) at higher strain rates, with the transition occurring around 10⁴ s⁻¹.

The observed anomalous thermal strengthening at extreme strain rates directly addresses the long-standing research question regarding the behavior of materials under such conditions. The findings validate theoretical predictions about the transition in deformation mechanisms at high strain rates and provide experimental evidence for the dominance of dislocation drag strengthening at these rates. The quantitative agreement between the experimental data and the model further strengthens the interpretation of the results. These findings have implications for materials design for extreme conditions, as extrapolating low-strain-rate data to high-strain-rate scenarios can lead to inaccurate predictions of material strength and temperature dependence.

This study presents the first quantitative experimental evidence of anomalous thermal strengthening in pure metals at strain rates exceeding 10⁵ s⁻¹. The research utilizes a novel microballistic impact testing technique to obtain accurate strength measurements without the interference of shock effects. The findings provide crucial insights into the deformation mechanisms at extreme strain rates, emphasizing the significance of dislocation drag and highlighting the need for revised strategies in materials design and optimization for applications involving extreme conditions. Future research could focus on exploring this phenomenon in a wider range of materials and investigating the microstructural factors that influence the transition in deformation mechanisms.

The study primarily focused on pure metals. The results may not directly generalize to alloys or composites. The range of temperatures investigated was relatively limited. Further studies could investigate a broader temperature range and explore the effect of alloying elements on the observed phenomenon.