Ultra-strong and ductile Ta/Co nanolaminates strengthened via grain-boundary expanding and interfacial sliding

Abstract

The deformation and strengthening behavior of body-centered cubic/hexagonal closed-pack Ta/Co nanolaminates with individual layer thickness h ranging from 5 nm to 100 nm were studied via nanoindentation and micropillar compression tests. The deformation behavior of the Ta/Co micropillars transitioned from dislocation-dominated co-deformation/deformation at columnar grain boundaries in constituent layers with partial/major shearing of the micropillars at larger h to homogeneous deformation with plastic barreling through expansion of grain boundaries and interfacial sliding of constituent layers at a few nanometers’ length-scale. Also, increased amorphous/intermixed interfaces of the Ta/Co micropillars with a few nanometers’ h may have worked as constraints and sinks for dislocation pileups. The 5 nm h Ta/Co micropillars showed ultrahigh strength with a proof strength (σ0.2%) of ~2.32 GPa, a flow strength (σmax) of ~2.54 GPa and a yield strength (σys) of ~2.67 GPa, respectively, with minimal deformation even at 30% strain. It is worth noting that the 5 nm h Ta/Co micropillar showed an extraordinarily high σys (~2.67 GPa), which is 1.2 times higher than the highest yield strength of nanolaminates with at least one hcp constituent reported in the literature, to date. The dislocation pileup–based Hall–Petch strengthening model operated well with h = 25–100 nm, while the strength at h = 10 nm followed the confined layer slip strengthening mechanism. Surprisingly, the strength of the Ta/Co micropillars at h = 5 nm followed none of the strengthening mechanisms; rather, it became independent of h and mostly followed the plasticity at the grain boundaries and interfaces. The exceptionally high flow strength and ductility of the Ta/Co micropillars at h = 5 nm suggests the development of ultra-strong, yet ductile Ta/Co nanolaminates.