Supersonic shock tests reveal that metals weaken under extreme speeds

Researchers at Cornell University have overturned a long-standing principle in materials science known as the Hall–Petch law, which states that metals become stronger as their internal grain size decreases. Using laser-induced microprojectile impact testing to subject copper samples to supersonic speeds (over 761 mph), the team discovered that, contrary to expectations, metals with smaller grain sizes actually softened under these extreme deformation rates. Larger-grained copper samples exhibited greater hardness and energy dissipation, challenging the conventional understanding that grain boundaries always block dislocation movement and thus strengthen metals. The study attributes this reversal to the behavior of dislocations—microscopic defects responsible for deformation—at ultra-high strain rates. Under normal conditions, grain boundaries impede dislocation motion, strengthening the metal. However, at supersonic deformation speeds, dislocations accelerate enough to interact with vibrating atoms in the metal lattice, a phenomenon called dislocation–phonon drag, which alters the strengthening mechanism. Although the experiments focused on copper, preliminary tests on other