Effect of Co on phase stability and mechanical behavior of CoxCrFeNiMnAl0.3 high entropy alloys with micro/nano hierarchical structure

•Reducing Co content in CoCrFeNiMnAl0.3 high entropy alloys (HEAs) can effectively reduce the stability of the face-centered cubic (FCC) solid solution phase and promote the formation of the BCC (body-centered cubic) phase.•The ratio of Ni equivalent and Cr equivalent (Creq/Nieq) has been confirmed to be used to characterize the total BCC/FCC phase forming ability in CoxCrFeNiMnAl0.3 HEAs system.•Reasonable control of Co content and thermo-mechanical treatment technology can result in the formation of hierarchical structure composed of micro-FCC and BCC solid solution and nano-B2 precipitation.•The formed BCC phase can effectively suppress the grain growth of FCC solid solution during the recrystallization and improve the tensile strength of the alloy.