Effect of periodic thickness on the helium bubble evolution and irradiation hardening in Cu/W(Re) multi-layered films under helium ion irradiation

Based on the basic understanding of prominent helium (He) bubble storage and defect annihilation in the dual-phase heterogeneous interfaces designed in nano-meter materials, the Cu/W(5.31 at.% Re) multilayered films with nominal periodic thickness ( h) ranging from 2 nm to 50 nm were irradiated by 60 keV He ⁺ at irradiation temperature of 725 K. At the fluence of 2×10 ¹⁷ cm ^-2, the lamellated interfaces in the Cu/W(Re) multilayered film of h = 2 nm were broken down due to the irradiation-induced intermixing between the Cu-W(Re) neighbouring interfaces, resulting in a stable distribution of bubble in grains or at grain boundaries. However, at He concentration peak region, the bubble clusters inside the Cu layers and the blurred interfaces were formed in the Cu/W(Re) multilayered film of h = 10 nm. The W phase structure was changed by the aggregated bubbles in lattices, and the Cu layers were damaged due to the distribution scope of aggregated bubble-cluster approximately increased to a critical periodic thickness (10 nm). The lamellated interfaces still kept stable at low He concentration region. The calculated bubble pressure and He density inside bubbles can effectively explain the lattice distortion induced partial phase transformation from α-W to γ-W in the W(Re) layers. Moreover, the effects of interfaces, He bubbles and dislocations on the increase of nano-hardness in the irradiated multilayers were discussed in detail.