Helium nano-bubbles in copper thin films slows down creep under ion irradiation

Neutron irradiation of structural materials in nuclear applications is accompanied with a variety of microstructural changes including the generation of helium nano-voids at high doses. When operating under stress, irradiation creep is taking place, leading to change of dimensions of components, as well as relaxation of the springs or screw torque. These distortions may have important implications on the performance of the nuclear reactor. Irradiation creep is particularly complicated to investigate and the link between creep rate and microstructure remains partly elusive. Ultra-miniaturized testing of thin films makes possible the use of ion irradiation and implantation methods to emulate the effect of in-reactor neutron irradiation. Here, on-chip tests on freestanding copper films reveal much slower irradiation creep of helium implanted layers compared with non-implanted copper, with or without ion irradiation hardening prior to testing. This enhanced resistance to creep is due to the presence of the high density of helium bubbles, as evidenced by transmission electron microscopy, that impede dislocation glide and climb. A unified elementary model for moderate to high stress creep under irradiation is proposed showing that the creep law for the studied range of stress primarily depends on the yield strength which increases with helium bubbles.