Modulating hierarchical microstructural components to enhance strength-ductility synergy of a high-entropy alloy containing brittle sigma phase

Although brittle intermetallic phases are usually unwanted in the design of strong and ductile alloys, high strength and good ductility can be achievable in high-entropy alloys (HEAs) containing brittle phases. Here, we present an investigation on modulating the hierarchical microstructural components in a prototype metastable HEA (Fe30Cr25Co25Mn10Ni10, at. %) containing sigma (σ) phase to optimize the mechanical properties. An excellent combination of tensile strength (999.3 MPa) and ductility (57.7%) is achieved in alloy sample containing recrystallized grains of various sizes, which is mainly attributed to the continuous transformation-induced plasticity (TRIP) effect and the back stress effect caused by the pile up of geometrically necessary dislocations at various interfaces in the hierarchical microstructures. The decrease in fraction of hard domains (σ particles, non-recrystallized and small-sized grains) can lead to the decrease of tensile strength due to the weakened back stress effect. However, an excessively high fraction of hard domains (particularly with small-sized grains and σ particles) suppresses the TRIP effect, facilitating crack propagation and reducing ductility. The back stress effect can be rationally controlled through tuning the fraction and size of small-sized grains (accompanied with fine σ particles) by thermo-mechanical treatments, which can also enable the activation of TRIP effect in grains of various sizes to render high strain- hardening rate and good strength-ductility synergy. This provides a strategy for achieving an optimum combination of strength and ductility for metastable HEAs containing fine brittle phases via alleviating the embrittling effect of brittle phases and retaining the precipitation strengthening effect.