Bulk nanostructured metals are often formed via severe plastic deformation ͑SPD͒. The dislocations generated during SPD evolve into boundaries to decompose the grains. Vacancies are also produced in large numbers during SPD, but have received much less attention. Using transmission electron microscopy, here we demonstrate a high density of unusually large vacancy Frank loops in SPD-processed Al. They are shown to impede moving dislocations and should be a contributor to strength. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2794416͔ Bulk ultrafine-grained ͑UFG͒ and nanostructured ͑NS͒ metals are often prepared via severe plastic deformation ͑SPD͒, 1,2 which refines the originally large grains. The highangle and low-angle grain boundaries ͑GBs͒, as well as the subgrain dislocation structures stored, are usually taken as the only microstructural features responsible for the macroscopic strength/ductility properties of the UFG/NS metal obtained. However, SPD also introduces into the material another nanoscale feature in large numbers. That is, the aggregates of point defects generated throughout the SPD process. 3,4 For example, the movement of a screw dislocation with a jog should result in the creation of a row of either interstitial atoms or vacancies. 3 The vacancies are dominant because the energy to form a vacancy is smaller than that to form an interstitial. A high density of vacancies is expected for SPD, because of the intense dislocation interactions and tremendous plastic strain characteristic of SPD. The vacancy population present during and after SPD depends on the deformation rate, SPD route, temperature, etc.Point defect clusters are known to be very important in controlling the strength, toughness, and stability ͑such as swelling͒ of irradiated ͑nuclear͒ metals and alloys. [5][6][7] For point defects produced via deformation, evidence for vacancy concentration of the order of 10 −4 , close to the equilibrium value at the melting temperature ͑T m ͒, has been provided by differential scanning calorimetry, electrical resistivity measurements, and x-ray Bragg profile analysis. 8,9 However, the nanoscale entities formed by vacancies and their effects on the properties of the SPD-processed UFG/NS metals have not drawn adequate attention. For example, the stacking faults ͑SFs͒ observed in UFG/NS metals have all been attributed to dislocation dissociation or partial dislocation emission. 10 Little consideration is given to the vacancy clusters that may form SFs. It is therefore the goal of this letter to call attention to these vacancy-type defects produced by SPD. Their size, density, thermal stability, and effects on strength will be evaluated in the following using Al as an example.High-purity Al ͑99.999%͒ specimens were subjected to equal channel angular pressing ͑ECAP͒ for four passes ͑true shear strain of ϳ4͒ via route B C at room temperature ͑RT͒. No storage of vacancy-type clusters was observed in the transmission electron microscopy ͑TEM͒ after such ECAP. This is because for a metal like A...