Research progress of hydrogen/helium effects in metal materials by positron annihilation spectroscopy

An important feature of the irradiation process in nuclear system is the formation of large displacement cascades, in which primary knock-on atoms and secondary particles formed by nuclear reactions generate a considerable number of defects such as dislocations, vacancies and transmutation gases. Predicting and mitigating the adverse effects of damage defect and transmutation hydrogen/helium produced by high-dose neutron irradiation on the mechanical properties of structural materials is the most significant challenge facing the current development of nuclear energy. To solve this problem, understanding the interaction mechanism between hydrogen/helium atoms and micro-defects is a very important breakthrough. Precursors of helium/ hydrogen bubble, small helium/hydrogen-filled vacancy complexes, may play an important role in realizing bubble nucleation, and the formation of these complexes is affected by many factors. However, only a little information about helium/hydrogen-vacancy clusters’ behavior has been obtained in metal/alloy materials. This is mainly limited by the characterization methods, such as the limited resolution of transmission electron microscope (TEM). Helium/hydrogen-vacancy clusters cannot be observed by TEM before the formation of helium bubbles. Applications of positron annihilation to the study of crystal lattice defects started around 1970s, when it was realized that positron annihilation is particularly sensitive to vacancy-type defects and that annihilation properties manifest the nature of each specific type of defect. In recent years, with the continuous development of slow positron beam and the improvement of various experimental testing methods based on slow positron beam, the application of positron annihilation technology has been extended to the research field of hydrogen/helium behavior in metal materials, which plays an important role in studying the hydrogen/helium radiation damage to metal materials. In this review, the basic principles of positron annihilation spectroscopy are briefly discussed and the three most important measurement methods used for hydrogen/helium effect studies are described (i.e. positron annihilation lifetime spectroscopy (PALS), Doppler broadening spectroscopy (DBS), coincidence Doppler broadening spectroscopy (CDBS)). In this paper, the application of positron annihilation spectroscopy to the study of hydrogen/helium behavior in metal materials is reviewed in combination with the reported relevant developments (including our research group’s achieve-ments). The advantages of three commonly used measurement methods in the following specific studies are highlighted: 1) The estimation of bubble size and concentration; 2) irradiation damage induced by hydrogen/helium; 3) the evolution behavior of irradiation-induced defects in the heat treatment process; 4) sy-nergistic effect of hydrogen and helium.