Individual layer thickness-dependent microstructures and mechanical properties of fcc/fcc Ni/Al nanolaminates and their strengthening mechanisms

Abstract

Individual layer thickness-dependent nanoindentation hardness (H) and reduced elastic modulus (E) were measured for face-centered cubic/face-centered cubic (fcc/fcc) nickel (Ni)/aluminum (Al) nanolaminates deposited via magnetron sputtering onto a single-crystal silicon (Si) substrate with a wide, equal individual layer thickness (h) ranging from 5 to 100 nm. The microstructures of the fabricated nanolaminates varied due to the variation in h and these microstructural changes affected the mechanical properties of the Ni/Al nanolaminates. The microstructural changes in the nanolaminates have been characterized using different microscopy analysis. H gradually increased to a plateau of ∼5.07 GPa with a reduction in h down to 10 nm, while the further reduction in h caused H to remain unchanged. Interestingly, E exhibited a monotonic increase to attain a maximum of ∼134.65 GPa with a reduction in h to a critical value of 20 nm, while it started to reduce with further reduction in h. This unusual reduction of E below the 20 nm individual layer thickness may be related to the formation of disordered amorphous layers as well as broken and intermixed interfaces in the Ni/Al nanolaminates. This individual layer thickness-dependent strengthening mechanism in Ni/Al nanolaminates suggests that the dislocation-based Hall–Petch (H–P) strengthening mechanism governs the strength of the Ni/Al nanolaminates from 100 nm down to 20 nm, while the confined layer slip (CLS) strengthening mechanism takes place below 20 nm, and the strengthening mechanism becomes independent of h when it decreases from 10 nm to a few nanometers. The results offer a method for design-engineering ductile Ni/Al nanolaminates with high strength and low reduced elastic modulus.