Atomic-scale nanoindentation of irradiated CL-20: Defect mechanisms and anomalous mechanical response

Energetic materials are widely used in military, civilian, and aerospace applications and may experience long-term exposure to harsh environments such as ionizing radiation during service. Understanding how irradiation alters their mechanical properties is essential for reliable performance assessment. Here, we employ reactive molecular dynamics nanoindentation to systematically investigate the load-displacement response, the evolution of elastic modulus, and the atomistic mechanisms of defect structures in CL-20 crystals subjected to irradiation damage in the range of 0–1760 kGy. The results show that the mechanical response after irradiation does not degrade monotonically with dose. Instead, it is governed by specific defect morphologies and exhibits a competition between irradiation-induced hardening and softening. Decomposition products generated by irradiation form high-pressure gas-filled voids within lattice interstices, reaching pressures close to 2 GPa. These pressurized voids impose a prestress on the surrounding lattice and thereby induce anomalous hardening. With increasing indentation depth, the gas-filled voids progressively rupture and collapse under the applied load, and the ensuing local amorphization ultimately reduces the elastic modulus. Further analysis of energy dissipation indicates that this gas-phase defect structure induces a local viscoplastic temperature rise under external loading. Acting as an effective local heating mechanism, it is identified as a high-risk precursor site for hot spot formation. These findings establish a structure-oriented mechanistic framework of “gas pressurization strengthening-local amorphization softening”, elucidate how irradiation defects regulate the mechanical behavior and safety response of nitramine explosives, and provide an atomistic basis for evaluating the irradiation tolerance and safety design of CL-20 in radiation environments.