Entropy-driven phase stability and slow diffusion kinetics in an Al 0.5 CoCrCuFeNi high entropy alloy Chun Ng a , Sheng Guo b , Junhua Luan b, c , Sanqiang Shi a, * , C.T. Liu b, ** a Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China b Center of Advanced Structural Materials, Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong, China c School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China article info Article history: Received 25 March 2012 Received in revised form 30 April 2012 Accepted 1 July 2012 Available online 24 July 2012 Keywords: B. Thermal stability C. Thermomechanical treatment E. Phase stability, prediction abstract Previous work on the stability of the solid solution phases in the high entropy alloys is inconclusive. We used a series of thermo-mechanical treatments to study the stability of the solid solution phases in a high-entropy Al 0.5 CoCrCuFeNi alloy. The solid solution phases were found to be stable, against the intermetallic compounds, at high temperatures >850  C and at low temperatures <300  C. At inter- mediate temperatures, however, the intermetallic s-phase co-existed with the solid solution phases. The experimental observations were verified by the thermodynamic calculation results. The mechanisms for the phase stability, both at equilibrium and after quenching-equivalent annealing treatments, were discussed, and the roles of high entropy and slow diffusion kinetics were highlighted. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction High entropy alloys (HEAs), are defined as alloys made of multiple (normally 5) principle elements and these principle elements are normally mixed in an equal or nearly-equal atomic ratio [1]. This is a new alloying concept, differentiating HEAs from those traditional metallic alloys like steels, Al alloys, or NiAl- and TiAl-based alloys, where one or at most two elements dominate. They are termed as high entropy alloys because the equal atomic ratio means that the configuration entropy is high, according to Boltzmann’s definition of the entropy of mixing [2]. One interesting characteristics of HEAs is their relatively simple phase constitution: quite often only single-phase or dual-phase solid solution phases (fcc and/or bcc type) form, without the formation of intermetallic compounds, as would otherwise be expected from the traditional physical metallurgy point of view. HEAs can possess some unique mechanical and functional properties, like high hardness, high wear resistance, high temperature softening resistance, good oxidation and corrosion resistance, and low thermal conductivity [3,4]. As a result, HEAs are attracting ever-increasing interests from materials scientists and engineers. A great number of HEAs have been developed so far, however, the scientific understanding toward HEAs is still at a preliminary stage, and particularly there lack scientific or even empirical prin- ciples guiding the design of HEAs, to achieve the desired phase constitution and hence the mechanical properties. For example, although simple solid solution phases can form in HEAs, amor- phous phase and intermetallic compounds can also appear (note that all the phase constitutions are refereed to the as-cast state here and afterward, if not otherwise specified) [5]. It is thus important for us to predict the stability of phases (solid solution, amorphous phase or intermetallic compound) from a given HEA composition. On the other hand, assuming solid solution phases are formed, can we predict whether fcc-type solid solution or bcc-type solid solu- tion will form, also for a given HEA composition? As we already know that, fully fcc-typed HEAs are soft and ductile [6], while the bcc phase containing HEAs are generally hard but tend to be brittle [7], the answer to this question is critical to design the mechanical properties of HEAs for structural applications. We have recently addressed to these important issues [5,8], based on considerations of the fundamental properties of the constituent elements, including the atomic size mismatch, mixing entropy, mixing enthalpy, electronegativity and electron concentration. Some useful information, although still not definitive, has been extracted from our analyses. For example, we have revealed that the atomic size mismatch plays a decisive role in forming solid solutions or bulk amorphous alloys [5], and the valence electron concentration can critically separate the fcc or bcc solid solution formation [8]. * Corresponding author. Tel.: þ852 2766 7821; fax: þ852 2365 4703. ** Corresponding author. Tel.: þ852 3442 7213; fax: þ852 3442 0172. E-mail addresses: mmsqshi@polyu.edu.hk (S. Shi), 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 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2012.07.001 Intermetallics 31 (2012) 165e172