Size-dependent plasticity in an Nb25Mo25Ta25W25 refractory high-entropy alloy

High-entropy alloys (HEAs) are evolving multi-component intermetallic systems, wherein multiple principal elements tend to form single solid-solution-like phases with a strong tendency to solid solution strengthening. In this study, an Nb25Mo25Ta25W25 refractory HEA was synthesized by arc melting and well homogenized at 1800 °C. Single-crystalline HEA pillars in two orientations ([0 0 1] and [3 1 6]) and with diameters ranging from 2 μm to 200 nm were produced by focused ion beam milling and compressed using a flat-punch tip in a nanoindenter. The HEA pillar samples can reach extraordinarily high strength levels of ∼4–4.5 GPa, which is ∼3–3.5 times higher than that of the bulk HEA; meanwhile the ductility is significantly improved. Compared to pure Nb, Mo, Ta and W pillars, the HEA pillars exhibit higher strengths than any of them in both absolute and normalized values, and the HEA pillars also show relatively low compressive size effects, as evaluated by the log–log slope of strength vs. pillar diameter. The higher strength levels and lower size dependence for the HEA could be attributed to the increased lattice resistance caused by localized distortion at atomic length scales. The correlation between normalized strengths, length scales and temperatures for body-centered cubic structured pillars is illustrated, and the relevance of a size-effect slope as well as the additivity of strengthening mechanisms is critically discussed.