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 ($>100\mathrm{GPa}$) and strain rate ($\sim 10^7{\mathrm{s}}^{-1}$) 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 $>0.25\mu \mathrm{m}$ 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.

• Received 17 March 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.065502