Designing ultra-strong nanocrystalline Cu–Me (Me = Mg, Al) thin films: Manipulation of nanotwin patterns

How to design ultra-strong solid solution-type Cu alloys is a long-term pursuit in materials community, which is technically superior and cost-effective for their promising energy efficient applications. In this work, we prepared Cu–Me (Me = Mg, Al) alloyed thin films via non-equilibrium magnetron sputtering disposition to study light element Me alloying effects on the microstructure and mechanical properties of nanocrystalline (NC) Cu thin films. In the studied solute Me content-range spanning from 1.0 at% to 17.0 at%, Mg alloying renders a peak number fraction (~80%) of columnar(-like) grains with only containing single nanotwins, while Al alloying renders monotonically increased fraction (to ~55%) of columnar grains with containing single and/or multiple-fold twins. Compared with single nanotwins, multiple-fold twins can more significantly enhance the hardness of NC Cu–Al alloys. A combined strengthening model was employed to quantify the Me concentration-dependent hardness of NC Cu–Me thin films, which can reach at least about twice that of NC Cu matrix (~1.8 GPa). Importantly, the present NC Cu–Me alloys with nanotwinned microstructure manifest excellent specific strength compared with reported Cu alloys. These findings convey the valuable information for designing ultra-strong NC Cu-based alloys by tuning internal morphologies of nanotwins, e.g. penta-twins and parallelogram twins.