For more than seventy years, metallurgists have relied on the Hall-Petch effect: shrink a metal's grains and it gets stronger, because grain boundaries block the movement of the crystal defects called dislocations. Cornell research published in Physical Review Letters shows the rule breaks down spectacularly at extreme speeds. In tests where microscopic particles were fired at copper faster than the speed of sound, fine-grained samples became softer while larger-grained copper proved harder.
The team, co-led by Mostafa Hassani of Cornell's materials science and mechanical engineering departments and doctoral student Laura Wu, used laser-induced microprojectile impact testing on copper samples with grain sizes spanning 1 to 100 micrometres. Larger grains left shallower indentations and absorbed more kinetic energy, and the reversal held as the researchers repeated experiments and added data points. The proposed mechanism is dislocation-phonon drag: at ultra-high strain rates, dislocations move fast enough to interact with vibrating atoms, strengthening the metal in a way that offsets the usual grain-size effect.
Though the study focused on copper, the group reports a similar trend emerging in other metals and alloys under test, suggesting the behaviour is general. The implications reach wherever materials must survive violent impacts: lightweight armour, spacecraft shielding against orbital debris, and additively manufactured metal parts. The work was funded by the National Science Foundation and the Army Research Office.