Grain-size-independent plastic flow at ultrahigh pressures and strain rates.

A basic tenet of material science is that the flow stress of a metal increases as its grain size decreases, an effect described by the Hall-Petch relation. This relation is used extensively in material design to optimize the hardness, durability, survivability, and ductility of structural metals. This Letter reports experimental results in a new regime of high pressures and strain rates that challenge this basic tenet of mechanical metallurgy. We report measurements of the plastic flow of the model body-centered-cubic metal tantalum made under conditions of high pressure (>100 GPa) and strain rate (∼10(7) s(-1)) achieved by using the Omega laser. Under these unique plastic deformation ("flow") conditions, the effect of grain size is found to be negligible for grain sizes >0.25 µm sizes. A multiscale model of the plastic flow suggests that pressure and strain rate hardening dominate over the grain-size effects. Theoretical estimates, based on grain compatibility and geometrically necessary dislocations, corroborate this conclusion.

The trianvil test apparatus: measurement of shear strength under pressure.

An experimental apparatus has been developed for performing shear tests on specimens held under moderately high hydrostatic pressures (up to the order of 10 GPa). This testing procedure experimentally determines the pressure dependent shear strength of thin foil specimens. This information is necessary for models of materials subjected to extreme pressures and can assist in model validation for models such as discrete dislocation dynamics simulations, among others. This paper reports the development of the experimental procedures and the results of initial experiments on thin foils of polycrystalline Ta performed under hydrostatic pressures ranging from 2 to 4 GPa. Subsequent characterization of the samples held under pressure established that the procedure described herein represents a reliable method to impose nearly uniform hydrostatic pressure on thin foil specimens. Both yielding and hardening behavior of Ta are observed to be sensitive to the imposed pressure.