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

利用慢正电子仪和高能离子加速器相结合的辐照系统研究了离子辐照过程中产生的空位缺陷。对Ni和Al两种面心立方金属，在辐照和非辐照条件下进行了正电子湮没多普勒展宽的原位测量。我们发现，在辐照过程中，Ni的S参数增加，而Al的不增加。对Ni的研究结果强烈地表明，辐照过程中的空位浓度高于辐照后存活的空位浓度，即包括瞬时空位和非存活空位。这种瞬时空位的形成将与缺陷聚集或缺陷聚集过程密切相关。目前的原位观察技术被证明是研究离子辐照环境下材料中产生的空位的有力工具。正电子湮没辐射的多普勒展宽线形提供了比较信息，但较少提供有关缺陷特征的信息，如密度和开口体积大小。相比之下，正电子寿命测量可以获得定量信息。我们计划在离子辐照材料的过程中进行更多的正电子寿命原位测量实验，以便更准确地了解辐照效应的动力学，特别是对辐照过程中瞬时诱导的空位特性有一个定量的了解。

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.