Alloy Design and Properties Optimization of High-Entropy Alloys Y. ZHANG, 1,3 X. YANG, 1 and P. K. LIAW 2 1.—High-Entropy Theory Center, State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China. 2.—Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA. 3.—e-mail: yongzhangustb@gmail.com This article reviews the recent work on the high-entropy alloys (HEAs) in our group and others. HEAs usually contain five or more elements, and thus, the phase diagram of HEAs is often not available to be used to design the alloys. We have proposed that the parameters of d and X can be used to predict the phase formation of HEAs, namely X ‡ 1.1 and d £ 6.6%, which are required to form solid-solution phases. To test this criterion, alloys of TiZrNbMoV x and CoCr FeNiAlNb x were prepared. Their microstructures mainly consist of simple body- centered cubic solid solutions at low Nb contents. TiZrNbMoV x alloys possess excellent mechanical properties. Bridgman solidification was also used to control the microstructure of the CoCrFeNiAl alloy, and its plasticity was improved to be about 30%. To our surprise, the CoCrFeNiAl HEAs exhibit no apparent ductile-to- brittle transition even when the temperatures are lowered from 298 K to 77 K. INTRODUCTION With the fast development of the new technologies and theories for developing advanced materials, the number of constituent principal elements for metallic alloys is increased from one to three or more. For the conventional alloys, e.g., HT-9 steel and NUCu steel, they contain one dominant element (Fe), and the contents of other elements are very low, usually lower than 5 at.%. The intermetallic-based alloys, e.g., Fe 3 Al- or TiAl-based alloys, usually contain two dominant elements, and the contents of other ele- ments are very low. With more than three or four principal elements, the alloys were intuitively thought to be complex. The phase diagrams for com- plex systems are often not available. Theoretical simulation and modeling of HEAs are very challeng- ing and, thus, are lacking in the literature. As a result, most reports on HEAs were done by the traditional trial-and-error method. According to the regular solution approach, with an increasing of the number of principal elements in the system, the configura- tional entropy of mixing increases and reaches its maximum when the concentration of each element is equal. This feature forms the core concept of HEAs. Compared with the conventional metallic alloys based on one or two major elements, HEAs generally have five or more major metallic elements, and each has an atomic percentage between 5% and 35%. 1–9 Since HEAs possess a very high configurational entropy of mixing, solid-solution phases can be more stable than intermetallic compounds or other com- plex-ordered phases during solidification. 4,5,10–12 HEAs usually possess excellent mechanical proper- ties, thermal stability, and corrosion resistance to- gether with low fabricated costs. Thus, HEAs are considered as potential candidate materials for many challenging industrial applications. 7,9,13–20 However, how to design appropriate alloy compo- sitions with required properties theoretically re- mains a daunting task. So far, most of the existing HEAs are developed from trial-and-error experi- ments. Hence, establishing a reasonable phase for- mation rule for HEAs is essential to guide alloy design. In addition, the property optimization for the existing HEAs can widen their potential applica- tions. In this article, the solid-solution formation rule for multicomponent HEAs will be reviewed first. Then, TiZrNbMoV x and CoCrFeNiAlNb x alloys are prepared to verify the theory. The properties of the HEAs are optimized by compositions adjustment and Bridgman-solidification technique. In the end, the mechanical properties of body-centered cubic (bcc) AlCoCrFeNi HEA at 298 K and 77 K are presented. EXPERIMENTAL PROCEDURES Alloy ingots were prepared by arc melting the mixture of high-purity metals with the purity better JOM, Vol. 64, No. 7, 2012 DOI: 10.1007/s11837-012-0366-5 Ó 2012 TMS 830 (Published online July 10, 2012)