Design of ultrastrong but ductile medium-entropy alloy with controlled precipitations and heterogeneous grain structures

•Develop a Co2CrNi1.5Al0.25Ti0.25 alloy with heterogeneous grain structure and trimodal nano-precipitation.•High yield strength is caused by the synergistic effect of back-stress hardening, precipitation hardening and pre-existed dislocation hardening.•The variation of back stress plays a critical role in the three-stage strain-hardening behavior.•The increased geometrically necessary dislocation density and nano-spaced SF networks lead to the high strain-hardening rate and good ductility.