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

Area of Science:

• Materials Science
• Mechanical Engineering
• Solid Mechanics

Background:

• At extreme strain rates (>10^6 s^-1), pure metals exhibit antithermal hardening, where strength increases with temperature.
• This phenomenon is attributed to phonon drag impeding dislocation motion within the crystal lattice.

Purpose of the Study:

• To investigate the effect of compositional variation on the mechanical response of metals at extreme strain rates.
• To determine if the transition from antithermal hardening to thermal softening can be controlled by alloy composition.

Main Methods:

• Optically driven microballistic impact testing was employed to measure dynamic strength and hardness.
• Systematic compositional variation was performed on nickel, titanium, and gold.

Main Results:

• The study observed a transition in mechanical response from phonon drag to thermally activated pinning as purity decreased.
• This transition, from 'hotter-is-stronger' to 'hotter-is-softer', was mapped for nickel, titanium, and gold at high strain rates.
• The boundary between these deformation mechanisms was successfully identified.

Conclusions:

• Alloying provides a mechanism to tune the temperature dependence of metal deformation at extreme strain rates.
• This control over deformation mechanisms opens new avenues for designing alloys optimized for extreme environments.
• The findings challenge the universal applicability of antithermal hardening in pure metals under extreme conditions.