Study of vacancy-type defects by positron annihilation in ultrafine-grained aluminum severely deformed at room and cryogenic temperatures

"Commercial-purity aluminum was processed by equal-channel angular pressing (ECAP) at room temperature (RT-ECAP) and cryogenic temperature (CT-ECAP) with liquid nitrogen cooling between two successive passes.It was found that the RT-ECAPed samples showed equiaxed microstructure after 4 and 8 ECAP passes, while the CT-ECAPed samples displayed slightly elongated microstructure and slightly smaller grain size.Moreover, the CT-ECAPed samples had higher hardness values than the RT-ECAPed samples subjected to the same amount of deformation.Positron annihilation lifetime spectroscopy (PALS) was used to investigate the evolution of vacancy-type defects during the ECAP deformation process.The results showed that three types of defects existed in the ECAPed samples: vacancies associated with dislocations, bulk monovacancies and bulk divacancies.The CT-ECAPed samples had a higher fraction of monovacancies and divacancies.These two types of defects are the major vacancy-type defects that can work as dislocation pinning centers and induce hardening, resulting in higher hardness values in the CT-ECAPed samples.A quantitative relationship between material hardness and the defect concentration and defect diffusion coefficient has been established.(c) 2012 Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved."