More than entropy in high-entropy alloys: Forming solid solutions or amorphous phase Sheng Guo a , Qiang Hu b , Chun Ng c , C.T. Liu a, * a Center of Advanced Structural Materials, College of Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong, China b School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China c Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China article info Article history: Received 14 March 2013 Accepted 2 May 2013 Available online Keywords: E. Phase stability, prediction B. Alloy design B. Glasses, metallic abstract Metastable solid solutions can form preferably over intermetallic compounds, in cast high-entropy alloys or multi-component alloys with equi- or nearly equi-atomic compositions, due to the entropy contri- bution at elevated temperatures. Meanwhile, the high mixing entropy also favors the amorphous phase formation. The phase selection between solid solutions and the amorphous phase upon alloying in high- entropy alloys is intriguing. A two-parameter physical scheme, utilizing the atomic size polydispersity and mixing enthalpy, is found to be capable of capturing this phase selection mechanism. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction High-entropy alloys (HEAs) emerge as a new type of advanced metallic materials [1e4], and have received increasing attentions from the materials community. HEAs possess some excellent me- chanical and physical properties [4], and they have great potential to be used as high temperature materials, coating materials requiring high hardness and high wear resistance, and corrosion resistant materials with high strength. Some unique applications include the diffusion barrier material between copper in- terconnects and the Si substrate [5]. The scientific understanding toward HEAs, however, lags much behind the technical exploration to them. The probably most outstanding scientific questions are the mechanism and condition for the solid solution phases (and not intermetallic compounds) to form in these highly concentrated multi-component alloys. The high entropy contribution at elevated temperatures lowering the Gibbs free energy of the solid solution phase [6], and the confusion principle [7] are apparent mecha- nisms, but meanwhile one is aware that these two mechanisms simultaneously favor the formation of the amorphous phase. The amorphous alloys, also known as metallic glasses (MGs), are another type of novel metallic materials and are at present the hot spot in the materials and condensed matter physics field [8e11]. The recent discovery of high-entropy bulk metallic glasses (HE- BMGs) [12e16] made the question more prominent: why (qualita- tively) and when (quantitatively) do solid solutions or the amor- phous phase form in multi-component alloys with high mixing entropies? A scientific scheme capable of predicting the formation of solid solutions or the amorphous phase in HEAs thus becomes both scientifically and technically significant, which constitutes the topic of this work. We will show that simply using two alloy composition dependent physical parameters, i.e., a topology natured atomic size polydispersity, and a chemistry natured mixing enthalpy, the phase selection mechanism in HEAs is effectively captured. 2. The high-entropy effect By definition, HEAs are multi-component alloys having at least 5 principle elements, and all the principle elements are mixed in equiatomic or close-to-equiatomic ratios [2]. HEAs differ with con- ventional multi-component alloys in that there are no dominant one or two principle elements in the former. This metallurgy concept is a breakthrough in the history of the alloy development, as in the past people thought such a high percentage of alloying elements would lead to the formation of many unwanted phases, particularly the hard intermetallic compounds, and hence embrittle the material. Sur- prisingly, in various cast HEAs only simple solid solutions with fcc and/or bcc structures are obtained, and no intermetallic compounds form at all. Intuitively, the preferred formation of solid solutions over intermetallic compounds can originate from the high mixing en- tropy, as thermodynamically a high entropy contributes significantly to decrease the Gibbs energy of the solid solutions at elevated * Corresponding author. Tel.: þ852 3442 7213; fax: þ852 3442 0172. E-mail addresses: csuguosheng@gmail.com (S. Guo), chainliu@cityu.edu.hk (C.T. Liu). Contents lists available at SciVerse ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.05.002 Intermetallics 41 (2013) 96e103