Interplay between single phase solid solution strengthening and multi-phase strengthening in the same high entropy alloy

High entropy alloys (HEAs) offer the opportunity to achieve an unprecedented balance of properties by accessing novel multi-scale microstructural combinations. Despite the large range of combinations of strength and ductility reported in HEAs, the complex interplay between multiple strengthening mechanisms has not been addressed. The single-phase fcc solid solution state of the Al0.3CoCrFeNi alloy exhibits a strong Hall-Petch hardening effect with reducing grain size. While the same alloy can be strengthened by a composite-reinforcement effect of hard intermetallic B2 and sigma precipitates, within a fine-grained fcc matrix. Such precipitation leads to solute depletion within the parent fcc matrix, resulting in a substantially reduced Hall-Petch hardening effect. Additional formation of nano-clusters within the fcc matrix can strengthen the same alloy to 1.85 GPa at room temperature, via an Orowan strengthening mechanism. This paper presents the complex interplay between strengthening mechanisms operative at different length scales.