A Study on Void Nucleation Mechanisms with Realtime Detection of Nanovoids by Positron Beam

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Vacancy defects produced during ion irradiation were studied by using an irradiation system combined with a slow-positron apparatus and a high-energy ion accelerator. For two fcc metals of Ni and Al, in situ positron annihilation Doppler broadening measurements were performed during irradiation and non-irradiation. We found that an increase in the S parameter occurs during irradiation for Ni, but not for Al. The result for Ni strongly suggests that vacancy concentration during irradiation is high compared to that survived after irradiation, i. e., it includes transient or non-survivable vacancies. The formation of such a transient vacancy will be closely related to processes of defect accumulation or defect clustering. The present in situ observation technique is demonstrated to serve as a powerful tool for the investigation of vacancies produced in materials under an ion irradiation environment. The Doppler broadening line-shape of positron annihilation radiation is known to provide comparative information but less information about defect characteristics such as the density and the open volume size. In contrast, positron lifetime measurements allow obtaining quantitative information. We plan to do further experiments on in situ positron lifetime measurement during ion irradiation of materials in order to obtain more accurate insights into the dynamics of irradiation effects, in particular a quantitative understanding of vacancy characteristics induced transiently during irradiation.