Metal deformation and phase transitions at extremely high strain rates
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Introduction Materials driven at extremely high strain rates constitute an important frontier of materials science. In applications such as laser-driven inertially confined fusion, materials can be driven so fast that many conventional materials concepts must be reconsidered. The high-powered lasers used in this work can compress materials to densities many times their ambient density over a period on the order of 10 nanoseconds. Since the drive is typically uniaxial, at least locally, the high compression rate is accompanied by a high shear strain rate, and the rates affect material behavior both in the kinetics of phase transitions and the mechanisms of plastic deformation. The processes of damage and fracture in the associated rarefaction waves are also affected. In the past, these extraordinary strain rates were associated with shock waves—compressive waves with a front that steepens due to nonlinear material response that causes the wave velocity to increase with pressure. More recently, the high strain rates can be attained in strong ramp compression, in which nonsteady-state waves are tailored to have a rapid rise but not as abrupt as a shock wave at the same pressure. This distinction is important for materials dynamics because shock waves generate a great deal of heat. Shock waves with pressures greater than a few hundred GPa typically melt solids, so the ensuing dynamics are the purview of fluid dynamics. The heat production is a consequence of conservation of mass,
momentum, and energy at the shock front. This heat production follows the Rankine-Hugoniot equations, and it has been known for some time that a pressure reached through two smaller shock waves results in less heat production than if the same pressure were reached in a single shock. Ramp compression can be viewed as the limit in which the pressure increase is broken up into many small shocks and has been called quasi-isentropic. It is not actually isentropic, since plastic work and other sources of dissipation generate heat. Ramp compression opens up new applications; it also facilitates the study of solid materials at extremely high strain rates and high pressures. From a materials viewpoint, high rates affect the mechanisms of deformation. Some mechanisms are too slow to respond on the time scale of an experiment. Slow processes such as creep are irrelevant to high-rate deformation. Processes such as dislocation motion are sufficiently fast and are understood to play an important role in metal deformation at moderate to high rates. As the rate increases, the dislocations must flow faster, so according to the dislocation-mobility law, the shear stress must be higher. At low stresses, dislocations move from one lattice site to the next through a thermally activated hopping over the lattice hurdle known as the Peierls barrier. If the material is to respond at high rates, however, the shear stress must be high enough that dislocations can glide across the Peierls barrier rapidly without waiting for a thermal fluctuation.
R.E. Rudd, Lawrence Livermo
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