On the origin of ultrahigh cryogenic strength of nanocrystalline metals

At liquid nitrogen temperature, the yield strength of nanocrystalline $\mathrm{Ni}$ and $\mathrm{Co}$ increases by as much as 50%–80% over the already-impressive ($\sim 1\mathrm{GPa}$ ) room-temperature values. This unusual strength ratio as well as the remarkable magnitude of flow stress reached (as high as $2.5\mathrm{GPa}$ ) are unexpected for conventional close-packed pure metals. The strong temperature dependence is attributed to the unusually small activation volume measured in strain rate change tests. Grain boundary dislocation nucleation is proposed as the thermally activated deformation mechanism in nanocrystalline grains.