Microstructures and nanomechanical properties of nanolaminated Ta/Co composites and their strengthening mechanisms

In this study, nanolaminated tantalum (Ta)/cobalt (Co) composites (NTCCs) with an individual layer thickness (h) ranging from 5 nm to 100 nm were fabricated via magnetron sputtering. The microstructures and nanomechanical properties of the NTCCs were affected by variation in h. The NTCCs showed nanograin structures in Ta and Co layers, with Ta and Co textures that were randomly oriented. Nanohardness (H) and corresponding yield strength, ${\sigma }_{\mathrm{ys}}=H/2.7$, of the NTCCs gradually increased from 5.75 ± 0.15 GPa to 7.20 ± 0.13 GPa and from 2.12 ± 0.06 GPa to 2.67 ± 0.05 GPa, respectively, with reducing h from 100 nm to 5 nm. NTCC showed an extraordinarily high yield strength (∼ 2.67 GPa) at h = 5 nm due to its reduced individual layer thickness and non-defective microstructures, which is well above the maximum yield strength of studied nanolaminated materials (comprised of at least one hcp constituent), to date. The creep depth of 5 nm NTCC was lower than that of the 100 nm NTCC, and the creep deformation of 5 nm NTCC is related to the bending, breaking, and intermixing of Ta and Co layers, whereas the 100 nm NTCC exhibited bending and thinning of Ta and Co layers with more deformation. Strain rate sensitivity (m) of NTCC increased from 0.0666 to 0.2076 with increasing h from 5 nm to 100 nm. The Hall–Petch and Confined Layer Slip strengthening mechanisms governed the strength of the NTCCs for h = 25–100 nm and h = 10 nm, respectively. It is worth noting that the 5 nm NTCC did not follow any of the strengthening mechanisms and became independent of h; rather, the strength at this length scale was greatly influenced by grains, layer thickness, and microstructural variations at the interfaces. The increased σ_ys and E at h = 5 nm may facilitate tailoring the mechanical properties of NTCCs with high strength and ductility.