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

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 (cr 0.2% ) of-2.32 GPa, a flow strength (crmax) of-2.54 GPa and a yield strength (crys) 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 crys (-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. (c) 2021 Elsevier Ltd. All rights reserved. Superscript/Subscript Available