At extreme strain rates, pure metals thermally harden while alloys thermally soften

Abstract: When materials are deformed at extreme strain rates, greater than $10^6 \text{ s}^{-1}$, a counterintuitive mechanical response is seen where the strength and hardness of pure metals increases with increasing temperature. The anti-thermal hardening is due to defects in the material becoming pinned by phonons in the crystal lattice. However, here, using optically driven microballistic impact testing to measure the dynamic strength and hardness, we show that when the composition is systematically varied away from high purity, the mechanical response of metals transitions from ballistic transport 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.