Strength softening in copper upon compression: Competition between Hall–Petch and dislocation density effect

The strength enhancement of materials under modest loading was first documented by Bridgman, nearly a century ago. However, specific materials present anomalous strength softening at high pressures, including copper, which is a prototype fcc metal. To reveal the underlying mechanism of this anomalous phenomenon, we employed a combined Modified Williamson–Hall and Modified Warren–Averbach analytical approach to conduct a systematic and comprehensive analysis on the macroscopic to microscopic scales. The in situ crystallite size evolution, dislocation density variation, and flow stress dependence on pressure in copper is reported for the first time. Our results demonstrate an unconventional strength softening regime in copper between 3.6 and 9.5 GPa. This anomalous weakening originates from the competition between the pressure-induced reduction in dislocation density and the decrease in grain size (Hall–Petch effect). This work elucidates universal mechanisms governing strength evolution in materials under extreme loading conditions, as well as offers a protocol to determine the crystallite size evolution, dislocation density variation, and flow stress of materials as a function of pressure.