Metallic materials exhibit a variety of response mechanisms during deformation processes, including dislocation production and motion, diffusion, stress-induced phase transformations, and twinning in the crystal structure. Which mechanism will dominate the deformation process depends upon temperature, pressure, and strain rate. While the movement and interaction of dislocations may dominate the overall response of metals, changes in the deformation mechanism create a fundamentally different environment in which these dislocations move and interact. For example, the appearance of a twinned lattice structure creates additional obstacles to dislocation motion.
Characterizing the mechanisms of deformation under extreme conditions of pressure, temperature, and strain rate is challenging. Although detailed, real-time observations at the relevant time and length scales of the dynamic process are not presently possible, we use detailed recovery-based observations on metals, along with hydrodynamic and molecular dynamic simulations, to infer the deformation behavior under extreme conditions. Our goal is to investigate material response to shocks and determine how different deformation mechanisms affect material properties. We will also compare material behavior across a wide range of loading conditions, utilizing multiple test platforms such as a gas gun, high explosives, and lasers.
|Figure 1. Transmission electron micrographs show residual deformation signatures from samples loaded to (a) 16 GPa, (b) 25 GPa, and (c) 42 GPa. A gradual shift from dislocation to twinning is evident.|
Relevance to PLS Research Themes
Our stockpile stewardship mission demands a fundamental understanding of the behavior of solid metals under extreme conditions. To achieve this, new experimental and theoretical frameworks are required. Correctly modeling metal response to shock-wave passage and subsequent high-strain-rate deformation requires an understanding of the material response mechanisms across a wide range of pressure, temperature, and strain rate. Any numerical model used to predict this behavior must reflect the operative deformation mechanism, while changes in this underlying mechanism may require changes in the form of the model. Detailed, recovery-based experimental observations play a vital role in these efforts. The experimental and modeling methodology we use will help advance the development of robust numerical models of metal deformation under extreme conditions.
Major Accomplishments in 2004
|Figure 2. Transmission electron micrographs from a sample loaded to 25-GPa peak pressure show (a) dislocation cells under shockless loading and (b) stacking faults under shock loading.|
We have conducted experiments utilizing different quasi-isentropic loading paths on a gas gun and a laser-based platform using both single-crystal and polycrystalline copper samples. Single-crystal copper samples (<100> orientation) recovered from the laser-based platform have shown a gradual transition, from a dislocation-dominated to a twin-dominated (although still dislocation-assisted) deformation response, over a pressure range of 20–50 GPa, as illustrated in Figure 1. Polycrystalline samples have shown an even more gradual shift, as might be expected from the relatively few crystallites oriented favorably in an easily twinned direction. Analysis of the gas-gun-driven material is currently in progress.
Our laser-based system, the Omega laser at the University of Rochester, utilizes short loading pulses and has rapid thermal transients. One advantage of this platform is that the shock wave forms over a relatively small distance, which allows us to investigate both the isentropic (shockless) and shock response in a single sample at a fixed peak pressure. We have made the first observations of a remarkable change in material response over this transition. In isentropic loading, the residual microstructure is dominated by dislocation cells, and in shock loading, it is dominated by stacking faults (Figure 2). Because the temperature and pressure differences between these loading paths are very small, we conclude that this behavior is due to strain-rate effects.
|Figure 3. Molecular dynamics simulations show twin formation in single-crystal copper during (a) a 50-GPa shock in a perfect crystal and (b) a 20-GPa shock in a crystal containing a void (which collapses during shock passage).|
These experimental results support the molecular dynamics simulations conducted as a component of this work. The results have indicated that there is a pressure threshold approximately equal to 50 GPa for twin formation in <100> copper. This twin nucleation threshold is lowered, in the case of a 20-GPa shock, by the presence of a void defect (Figure 3). These simulations also support the theory that the twin formation mechanism is one of coalescence wherein stacking faults on three adjacent planes join to form a “nanotwin” that may subsequently rapidly grow.
We have established a viable path toward understanding the dynamic response mechanisms of technically relevant materials. In addition, we have found that for these specimen-recovery experiments, a laser-based system minimizes the disturbance of the microstructure and allows for retention of some deformation structures that would be unstable on more traditional platforms. Comparison of results across platforms provides a rational basis for assessing the use of laser-based material testing strategies to provide technically valuable strength data. Additionally, the use of multiple platforms, possessing distinct differences in strain rate and hold time at peak pressure, yields a more complete understanding of the evolving features that take place in the microstructure during these transients.
L. Davila, et al., “Shock-Induced Void Collapse in fcc Metals,” App. Phys. Lett., in press.
J.M. McNaney, et al., “High-Pressure, Laser-Driven Deformation of an Aluminum Alloy,” Mettal. Trans. A, 35A, 2625–2631 (2004).
B.A. Remington, et al., “Materials Science under Extreme Conditions of Pressure and Strain Rate,” Mettal. Trans. A, 35A, 2587–2608 (2004)
Looking ahead, we can hope to establish the upper and lower bounds on material constitutive response by measuring the shape of the recovered sample. In addition, the recovery techniques used here may be leveraged to provide containment capability in cases where it is desirable to contain the debris emitted from the test assembly.
Methodologies developed under this work are the first steps in providing valuable data to address the challenges of larger-scale experiments that measure the constitutive behavior of metals at ultrahigh pressures. For example, the current technique for measuring metal strength results in the presence of a partially ionized plasma in contact with the sample surface. A heat shield is used to prevent the associated thermal wave from affecting the experiment. Multilayer deposits along with recovery-based experimentation will allow us to assess the effectiveness of these heat-shield materials.