Microparticle impact as a window on phonon drag-limited plasticity

Laser-induced microparticle impact testing (LIPIT) has the unique capability of producing extreme strain rates (up to 10⁸ s^-1) while being able to span from high velocities and pressures (where shock physics is dominant), to low pressures (where materials physics dominates). In the latter regime, this has permitted a new wave of quantitative measurements of the phonon drag-limited regime of plasticity in a variety of metals. This talk will review work in this regime specifically, with emphasis on the extreme rate dependence and counterintuitive temperature dependence of phonon drag. In pure metals, we are able to directly document the “hotter-is-stronger” effect: thermal activation produces more pinning points and higher strength in the drag regime. With some innovations in LIPIT methodology to span a range of strain rates, we can measure the drag coefficient directly from experiments. In alloyed metals, the competition between intrinsic pinning points (from solutes) and thermally-activated ones (from phonons) can be explored systematically, and used together to control the strength and its functional dependencies. Finally, at high temperatures, the mechanisms by which phonon drag breaks down are also addressable in a quantitative sense through the use of LIPIT.