Abstract
A five-component Al <sub>x</sub> CoCrFeNi high-entropy alloy (HEA) system with finely-divided Al contents (x in molar ratio, x = 0-2.0) was prepared by vacuum arc melting and casting method. The effects of Al addition on the crystal structure, microstructure and mechanical property were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Vickers hardness tester. The as-cast Al <sub>x</sub> CoCrFeNi alloys can possess face-centered cubic (FCC), body-centered cubic (BCC) or mixed crystal structure, depending on the aluminum content. The increase of aluminum content results in the formation of BCC structure which is a dominant factor of hardening. All the BCC phases in the as-cast alloys have a nano-scale two-phase structure formed by spinodal decomposition mechanism. The Al <sub>0.9</sub> CoCrFeNi alloy exhibits a finest spinodal structure consisting of alternating interconnected two-phase microstructure which explains its maximum hardness of Hv 527 among the alloys. The chemical composition analysis of FCC and BCC crystal structures, their lattice constants, overall hardness demonstrate that the formation of a single FCC solid solution should have Al addition <11 at.% and the formation of a single BCC solid solution requires Al addition at least 18.4 at.% in the Al <sub>x</sub> CoCrFeNi system. © 2012 Elsevier Ltd. All rights reserved.