Fig.1 SRIM calculation of irradiation damage and He concentration profile in Cu under 30 keV helium irradiation

Fig.2 Cavities formed after helium ion irradiation at room temperature

Fig.3 Cavities formed in the interior of the grain after 30 keV He ions irradiation at room temperature: a multifaceted shape with size smaller than 5 nm (measured by the distance shown in the micrograph); b hexagonal cavity with its facets aligned with the matrix. The FFT was superimposed in the inset in b

Fig.4 Cavities near general grain boundary a, twin boundary b in Cu after helium irradiation

Fig.5 a Cavity formation near the grain boundary triple-junction; b two general grain boundaries marked as GB1 and GB2, with SAED patterns from the three grains superimposed in the micrograph; c zoom-in image of micrograph b; d cavity distribution along GB1 after being tilted a small angle (α = 8°, β = 7°)

Fig.6 a Cavities near the twin boundary with its corresponding SAED pattern; b high-magnification micrograph of the cavities near the twin boundary

Fig.7 Statistical data of the cavities number adjacent to two types of the boundaries