Length-scale dependent deformation, strengthening, and ductility of fcc/fcc Ni/Al nanolaminates using micropillar compression testing

Abstract

The deformation and strengthening behaviors of face-centered cubic/face-centered cubic (fcc/fcc) nickel (Ni)/aluminum (Al) nanolaminates were systematically investigated in uniaxial compression tests using micropillars with a varied range of individual layer thickness h from 5 nm to 100 nm. The deformation behavior of micropillar samples of Ni/Al nanolaminates was strongly dependent on the individual layer thickness of Ni and Al, where at larger h the deformation of micropillars took place first by plastic deformation of the Al layers and proceeded to dislocation pileup–based Hall–Petch (H-P) strengthening, which further nucleated in the Ni layers. The deformation behavior of the Ni/Al nanolaminates transitioned from dislocation-dominated homogeneous co-deformation with multiple shearing in Ni and Al layers and grain boundary–mediated massive fractures in the Ni layers at larger h to stress-concentrated broken interfaces and valleys with plastic barreling due to layer shearing and smaller fractures in the Ni and Al layers at smaller h. With decreasing h from 100 nm down to 5 nm, the Ni/Al micropillars exhibited a gradual transition from lower to higher strain hardening, and the strain-hardening rate reached a peak at h = 20 nm; while it started declining with further decreasing of h. The yield strength (σ0.2%) and flow strength (σ8%) of the Ni/Al micropillars reached the peak of ~1.93 GPa and ~2.35 GPa, respectively, with decreasing h to 10 nm. The confined layer slip (CLS) strengthening mechanism operated well at h = 10 nm, whereas H-P strengthening model was applicable for h = 20–100 nm, and surprisingly the strengthening at h = 5 nm was independent of h. Interestingly, significant strain softening was observed at h = 5 nm due to the severe plastic deformation at the broken interfaces, interfacial sliding of layers, and defects related to the presence of intermixed Ni and Al layers at the interface, and valleys between the islands of stacked constituent layers.