Experimental study on solid solution strengthening in nanocrystalline alloys using multilayered films

In nanocrystalline alloys, some microstructural factors are usually tied to grain interior composition, including grain size, grain boundary composition, and grain boundary width. Due to the coupling of these factors, it is challenging to reveal the individual effect of solid solution strengthening. In this paper, sputtered Cu–Ti alloy multilayered films were designed to isolate the effect of solid solution strengthening in nanocrystalline alloys. Two series of nano-multilayered films were synthesized consisting of different low-alloy-content crystalline layers (6–188 nm thick) and same high-alloy-content amorphous layers (~5 nm thick). The columnar crystals in crystalline layers with two different compositions both have the same diameters. The comparison of the two series of multilayered films show that the increase of Ti content from 0.3 to 5.3 at% results in a constant hardness enhancement of 1.5±0.2 GPa, as the height of columnar crystals in diameter of 10 nm decreases from 188 nm to 6 nm. This indicates that the solid solution strengthening effect of nanocrystalline alloys is independent of grain size not only when the deformation behavior is dominated by intragranular dislocation mechanism as coarse-grained ones, but also, more importantly, when it becomes dominated by grain boundaries.