Journal
ACTA MATERIALIA
Volume 60, Issue 10, Pages 4218-4228Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.actamat.2012.04.003
Keywords
Equal-channel angular pressing (ECAP); Low-temperature deformation; Vacancies; Positron annihilation (PAL); Hardness
Funding
- China Scholarship Council (CSC)
- Australian Research Council Centre of Excellence Program in the Centre for Antimatter-Matter Studies
- Fundamental Research Funds for the Central Universities [N100409002]
- Chinese Post-doctorate Science Fund [20100471455]
- State Basic Research Development Program of China [2012CB723307]
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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.
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