Size effects on the mechanical properties of nanocrystalline NbMoTaW refractory high entropy alloy thin films

The newly-surged high-entropy alloys (HEAs), in particular the refractory HEAs, manifest excellent properties for practice applications. In this work, we systematically investigated the plastic deformation characteristics (including the hardness, strain rate sensitivity (SRS), and activation volume) of nanocrystalline (NC) equiatomic NbMoTaW body-centered cubic (BCC) high-entropy alloy films (HEAFs) with thicknesses spanning from 100 to 2000 nm at room temperature. The BCC NbMoTaW HEAFs exhibited the strongest grain size ∼10 nm with the hardness of ∼16.0 GPa, above which the hardness decreases with increasing the grain size or film thickness, while below this critical size the material softening occurs. Given the friction stress and the source activation stress cannot be additive because they don't appear under the same circumstances, the size-dependent ultra-high strength in BCC HEAFs was discussed in terms of the solid solution hardening, the boundary dislocation density and the source strength. It was unveiled that above a critical grain size of ∼40 nm, the solid solution hardening becomes the predominant strengthening mechanism, while below this critical size the source strengthening becomes the predominant one in BCC NbMoTaW HEAs. Also, the strong size-dependence of SRS is emerged, which monotonically increased with decreasing thicknesses (just opposite to the activation volume). A thermally activated model based on the depinning process of bowed-out dislocations was employed to quantitatively account for the size-dependent SRS of the NbMoTaW HEAFs. Our findings not only provide deep insight into understanding the mechanical behavior of the NC BCC NbMoTaW HEAFs, at least in the studied size-range, also offer some clues for the metastability engineering strategy to design high performance HEAFs at small-scales.