Refractory high-entropy alloys O.N. Senkov a, b, * , G.B. Wilks a, c , D.B. Miracle a , C.P. Chuang d , P.K. Liaw d a Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433, USA b UES, Inc., 4401 Dayton-Xenia Rd., Dayton, OH 45432, USA c General Dynamics, Corp., Dayton, OH 45431, USA d Department of Material Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA article info Article history: Received 20 April 2010 Received in revised form 26 May 2010 Accepted 31 May 2010 Available online 23 June 2010 Keywords: B. Alloy design B. Crystallography C. Casting D. Microstructure F. Electron microscopy, scanning abstract Two refractory high-entropy alloys with near-equiatomic concentrations, WeNbeMoeTa and WeNbeMoeTaeV, were produced by vacuum arc melting. Despite containing many constituents both alloys have a single-phase body-centered cubic (BCC) structure. The lattice parameters a ¼ 3.2134(3)  A for the quaternary alloy and a ¼ 3.1832(2)  A for the quinternary alloy were determined with high-energy X-ray diffraction using a scattering vector length range from 0.7 to 20  A 1 . The alloy density and Vickers micro- hardness were r ¼ 13.75 g/cm 3 and H v ¼ 4455 MPa for the WeNbeMoeTa alloy and r ¼ 12.36 g/cm 3 and H v ¼ 5250 MPa for the WeNbeMoeTaeV alloy. The exceptional microhardness in these alloys is greater than any individual constituent, suggesting the operation of a solid-solution-like strengthening mechanism. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Metallic alloys with superior mechanical and functional prop- erties remain in high demand for the aerospace industry. Conven- tional alloys, especially for structural applications, are sometimes considered a relatively mature technology. Typically, such alloys have a base element that dominates the chemistry, accounting for w80% (by weight) or more of the total formulation. Even superal- loys, with as many as 12 elements in a single alloy, often contain over 50% of the base element. Only in comparatively rare cases, have superalloys with roughly 20% each of up to three transition metal elements (Fe, Ni, Co, and/or Cr) been developed. This limited scope of alloying strategy mainly stems from the fact that ternary or higher-order intermetallic compounds form unexpectedly in multi- component alloys, generally after long-term exposure at elevated operating temperatures. These new phases often have complex crystal structures that do not support plasticity, and scavenge desirable elements from the host phase. Formation of these complex intermetallics typically heralds a reduction in mechanical properties, corrosion resistance, and microstructure stability. This long-held experience provides a strong disincentive for unneces- sarily complex alloy formulations. Within the past several years, a fundamentally new alloying concept has been proposed [1e4]. Termed high-entropy alloys (HEAs), these new materials are formed by combining n elements of roughly equimolar concentrations. When n is large (typically n  5), the high entropy of mixing can stabilize solid-solution-like phases with relatively simple crystal structures rather than forming the conventionally expected complex intermetallic phases. Thus, while these alloys may be compositionally complex, they are microstructurally simple. Clearly, this concept offers a vast oppor- tunity to explore, discover, and develop fundamentally new classes of alloys for structural and functional applications. Alloying element combinations previously perceived as objectionable due to microstructure instability may now be a possibility, suggesting completely new families of light metal alloys, high strength metals, and high-temperature metals. To date, high-entropy alloy research seems to emphasize alloys based on the late transition metals such as Fe, Ni, Co and Cu. To the authors’ knowledge, there have been no systematic efforts to explore HEAs based primarily on refractory element constituents. Since metallic alloys for high-temperature load-bearing structures and thermal protection remain in high demand for the aerospace industry, there is a clear rationale for exploring HEAs composed of constituents with high melting temperatures. Therefore, this work describes the development and characterization of two obvious compositions of HEAs based on refractory elements. The first alloy was chosen to contain equal concentrations of W, Nb, Mo, and Ta, * Corresponding author. UES, Inc., 4401 Dayton-Xenia Rd., Dayton, OH 45432, USA. Tel.: þ1 937 2551320. E-mail address: oleg.senkov@wpafb.af.mil (O.N. Senkov). Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.05.014 Intermetallics 18 (2010) 1758e1765