At Extreme Strain Rates, Pure Metals Thermally Harden while Alloys Thermally Soften

When materials are deformed at extreme strain rates, >10^{6} s^{-1}, a counterintuitive mechanical response is seen where the strength and hardness of pure metals increases with increasing temperature. This antithermal hardening is due to dislocations meeting resistance to their motion from phonons in the crystal lattice. However, here, using optically driven microballistic impact testing to measure dynamic strength and hardness, we show that when the composition is systematically varied away from high purity, the mechanical response of metals transitions from phonon drag of dislocations back to thermally activated pinning of dislocations, even at the highest strain rates. This boundary from "hotter-is-stronger" to "hotter-is-softer" is observed and mapped for nickel, titanium, and gold. The ability to tune between deformation mechanisms with very different temperature dependencies speaks to new directions for alloy design in extreme conditions.