Increasing demand for energy and reduction of carbon dioxide emissions has revived interest in nuclear energy. Designing materials for radiation environments necessitates a fundamental understanding of how radiation-induced defects alter mechanical properties. Ion beams create radiation damage efficiently without material activation, but their limited penetration depth requires small-scale testing. However, strength measurements of nanoscale irradiated specimens have not been previously performed. Here we show that yield strengths approaching macroscopic values are measured from irradiated ~400 nm-diameter copper specimens. Quantitative in situ nanocompression testing in a transmission electron microscope reveals that the strength of larger samples is controlled by dislocation–irradiation defect interactions, yielding size-independent strengths. Below ~400 nm, size-dependent strength results from dislocation source limitation. This transition length-scale should be universal, but depends on material and irradiation conditions. We conclude that for irradiated copper, and presumably related materials, nanoscale in situ testing can determine bulk-like yield strengths and simultaneously identify deformation mechanisms.

A unique method for quantitative in situ nanotensile testing in a transmission electron microscope employing focused ion beam fabricated specimens was developed. Experiments were performed on copper samples with minimum dimensions in the 100–200 nm regime oriented for either single slip or multiple slip, respectively. We observe that both frequently discussed mechanisms, truncation of spiral dislocation sources and exhaustion of defects available within the specimen, contribute to high strengths and related size-effects in small volumes. This suggests that in the submicrometer range these mechanisms should be considered simultaneously rather than exclusively.