Size-dependent microstructure evolution and hardness of He irradiated Nb/Zr multilayers under different ion doses

The microstructural evolution and hardness of He-irradiated Nb/Zr crystalline/crystalline multilayers with a wide range of layer thickness h from 5 to 100 nm were investigated under three different ion dose Φ of 1.0 × 10¹⁶, 1.0 × 10¹⁷ and 5.0 × 10¹⁷ ions·cm⁻² at room temperature. Careful microstructural examinations found the occurrence of a radiation-induced amorphization phenomenon near the interface. Besides, He bubbles were distributed within the Nb layers and at the interface. Both the amorphization and He bubble distribution were highly dependent on h and Φ, which caused the mechanical properties sensitive to h and Φ. With reducing h, the hardness of as-deposited Nb/Zr multilayers showed a maximum at h = 10 nm. While the He-irradiated samples manifested a hardening behavior at Φ = 1.0 × 10¹⁶ ions·cm⁻², a softening behavior at Φ = 5.0 × 10¹⁷ ions·cm⁻² and a transition from hardening to softening at Φ = 1.0 × 10¹⁷ ions·cm⁻² with reducing h. These strengthening behaviors were rationalized by a coupling effect of dislocation-amorphous and dislocation-bubble interactions. Modified mechanistic models based on interactions of dislocations with obstacles were employed to quantitatively describe the underlying strengthening mechanisms.