Secondary phases in Al CoCrFeNi high-entropy alloys: An in-situ TEM heating study and thermodynamic appraisal

Secondary phases, either introduced by alloying or heat treatment, are commonly present in most high entropy alloys (HEAs). Understanding the formation of secondary phases at high temperatures, and their effect on mechanical properties, is a critical issue that is undertaken in the present study, using the AlxCoCrFeNi (x = 0.3, 0.5, and 0.7) as a model alloy. The in-situ transmission-electron-microscopy (TEM) heating observation, an atom-probe-tomography (APT) study for the reference starting materials (Al-0.3 and Al-0.5 alloys), and thermodynamic calculations for all three alloys, are performed to investigate (1) the aluminum effect on the secondary-phase fractions, (2) the annealing-twinning formation in the face centered-cubic (FCC) matrix, (3) the strengthening effect of the secondary ordered body-centered cubic (B2) phase, and (4) the nucleation path of the a secondary phase thoroughly. The present work will substantially optimize the alloy design of HEAs and facilitate applications of HEAs to a wide temperature range. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.