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Secondary phases, either introduced by alloying or heat treatment, are commonly 31 present in most high-entropy alloys (HEAs). Understanding the formation of secondary 32 phases at high temperatures, and their effect on mechanical properties, is a critical issue 33 that is undertaken in the present study, using the Al x CoCrFeNi (x = 0.3, 0.5, and 0.7) as 34 a model alloy. The in-situ transmission-electron-microscopy (TEM) heating observation, 35 an atom-probe-tomography (APT) study for the reference starting materials (Al 0.3 and 36 Al 0.5 alloys), and thermodynamic calculations for all three alloys, are performed to 37 investigate (1) the aluminum effect on the secondary-phase fractions, (2) the 38 annealing-twinning formation in the face-centered-cubic (FCC) matrix, (3) the 39 strengthening effect of the secondary ordered body-centered-cubic (B2) phase, and (4) 40 the nucleation path of the secondary phase thoroughly. The present work will 41 substantially optimize the alloy design of HEAs and facilitate applications of HEAs to a 42 wide temperature range.

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