1. Introduction

Fusion energy is one of the most promising carbon-free energy in the future. One problem limiting the development and application of fusion energy is the radiation damage of materials. Irradiation resistance of a material is determined by the motion and recombination of point defects (vacancy and interstitial) [1]. The motion and recombination of point defects are governed by the vacancy mobility at material service temperature which is always higher than the vacancy mobile temperature [1, 2]. Thus, vacancy migration energy is a critical factor for the ability of nuclear materials to resist radiation damage. Moreover, it is becoming a popular and an effective method to simulating the irradiation-induced microstructure changes to predict the mechanical properties of materials during irradiation with the development of computational materials. Vacancy migration energy is also a fundamental parameter for modeling and should be experimentally measured in advance.

Kiritani et al. have firstly proposed one method as early as the 1970s by experimentally measuring growth speed of interstitial dislocation loops at different irradiation temperatures to measure the motion of vacancies in iron by using HVEM [2, 3]. High-energy electron irradiation produces pure Frenkel point defect pairs, which is very beneficial to the fundamental research of point defect mobility. In combination with in situ heating sample holder, HVEM has been proven to be a powerful apparatus to investigate microstructural evolution and measure the vacancy migration energy under in situ high-energy electron irradiation. Since then, the method proposed by Kiritani et al. was often used to measure the vacancy migration energy of materials. The effects of impurities, helium, and hydrogen on

vacancy migration energy in pure Fe-based alloys, F82H and electron-beam-welded F82H joint, have been investigated by Hashimoto et al. [4–6]. The effect of deuterium on vacancy migration energy in Fe-10Cr alloy has also been investigated [7]. Point defect behavior in pure vanadium and its alloys were also analyzed under HVEM irradiation by using the method [8–10]. However, it is important to note that the method did not consider the effect of sample surface on the measurement of the vacancy migration energy. And, it is always very difficult to ensure that the observed loops are deep in the specimen (located so far apart from specimen surface) by HVEM stereo observation operation [11]. Thus, Wan et al. derived a new formula for calculation vacancy migration energy by considering the surface effect in the 1990s [11]. In this brilliant derivation, the effect of specimen surface was treated as a constant, and three data of growth rate at different irradiation temperatures were used once to numerically calculate the vacancy migration energy by iterated operation. This method considered the surface effect intrinsically, but iterated solution by using a computer is needed. This operation would produce two roots, in which one should choose the right one according to physical meaning. It is a little bit difficult to calculate and choose the right root. Probably for this reason, reference of using this numerical method is not many. Moreover, the effects of other sinks such as irradiation-induced small void, fine precipitates, and grain boundary on the growth speed of dislocation loops also needed to be considered when using HVEM to measure the vacancy migration energy.

In this chapter, other sinks such as surface, small void, precipitates, and grain boundary were taken into consideration to develop a new method for calculating vacancy migration energy according to the relationship of the reciprocal of irradiation temperature and growth speed of dislocation loops. An influential factor relating to the effects of other sinks was also presented in this new method. In addition, the formula derived by Wan et al. has also been rediscussed to avoid one root and help to choose the right root, which may encourage a broader range of applications of this numerical method. The characteristic of these three methods is discussed to help choose the appropriate method to measure the vacancy migration energy when in need in different situations. In addition, the motion interstitial is also important and could be measured by using HVEM [12, 13]. In this chapter, however, we only discussed the mobility of vacancy because vacancy moves much slower than that of interstitial, and the recombination of vacancies and interstitial defects is governed by the vacancy mobility at material service temperature which is higher than vacancy mobile temperature.
