Effect of He-irradiation fluence on the size-dependent hardening and deformation of nanostructured Mo/Zr multilayers

Previous studies demonstrated that the He ion irradiation damage tolerance of nanostructured metallic multilayers (NMMs) was closely dependent on the layer thickness (h): smaller h led to lower irradiation hardening. Here in Mo/Zr NMMs, we uncovered that the h-dependent irradiation hardening and corresponding plastic deformation characteristics are also sensitive to the He⁺ irradiation fluences. At a low irradiation fluence of 1.0 × 10¹⁶ He⁺·cm^-2, the irradiation hardening decreased monotonically with reducing h. While at a high irradiation fluence of 1.0 × 10¹⁷ He⁺·cm^-2, a non-monotonic h-dependence was unexpectedly displayed with the minimum irradiation hardening at h of ∼25 nm. The irradiation fluence also affects the strain rate sensitivity (m) remarkably. Irradiation of 1.0 × 10¹⁶ He⁺·cm^-2 induced a transition in SRS m from positive in the as-deposited Mo/Zr NMMs to negative SRS in their irradiated counterparts. This transition was rationalized in terms of dynamic strain aging that considered dislocation-bubble interactions. Irradiation of 1.0 × 10¹⁷ He⁺·cm^-2, however, resulted in a non-monotonic h-dependence of m, with the bottom located at a turning point of h ∼50 nm. Coupling effect of the layer thickness and the applied fluence on irradiation hardening and plastic deformation characteristics was quantitatively elucidated by employing a strengthening model and a thermally activated model, respectively, where parameters of the characteristic microstructural features, i.e., the layer thickness and irradiation (He) defects, were both included.