Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea Mechanical and microstructural characterization of powder metallurgy CoCrNi medium entropy alloy Igor Moravcik a, ⁎ , Jan Cizek b , Zuzana Kovacova c , Jitka Nejezchlebova d , Michael Kitzmantel c , Erich Neubauer c , Ivo Kubena e , Vit Hornik e , Ivo Dlouhy a a NETME Centre, Institute of Materials Science and Engineering, Brno University of Technology, Technicka 2896/2, Brno, Czech Republic b Institute of Plasma Physics, Czech Academy of Sciences, Za Slovankou 1782/3, 182 00 Prague 8, Czech Republic c RHP-Technology GmbH, Forschungs, und Technologiezentrum, 2444 Seibersdorf, Austria d Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Trojanova 13, 18200 Prague, Czech Republic e Institute of Physics of Materials CAS, Zizkova 22, 61662 Brno, Czech Republic ARTICLE INFO Keywords: Tensile test Mechanical alloying Plasticity Mechanical characterization Powder metallurgy ABSTRACT The present study is focused on synthesis and mechanical properties characterization of equiatomic CoCrNi medium entropy alloy (MEA). Powder metallurgy processes of mechanical alloying (MA) with subsequent spark plasma sintering (SPS) for bulk alloy densification have been utilized. As opposed to the single-phase alloys of identical composition fabricated via casting routes, the alloy after SPS compaction consisted of a major FCC solid solution phase (94.4%), minor fraction of secondary BCC phase (5.6%, precipitated at the FCC grains bound- aries), and negligible amount of oxide inclusions. The alloy exhibited high ultimate tensile strength of 1024 MPa and a elongation to fracture of 26%. Elastic modulus of the alloy reached 222 GPa and the thermal expansion coefficient (CTE) was measured as 17.4 × 10 -6 K -1 The plastic deformation in the alloy is carried out by a combination of dislocation glide and mechanical nano-twinning at room temperature. 1. Introduction Yeh et al. [1] came up with an idea of an equiatomic alloy com- prising of five elements displaying simple solid solution microstructure. Cantor et al. [2,3] simultaneously developed alloys following the same concept. This new class of metallic materials was denoted as high en- tropy alloys (HEA). They generated considerable interest in the scien- tific community, deriving their properties not from a single dominant element, but from a multiple elements arranged in a single cubic lattice. Over the time, a number of various alloy systems have been examined, exhibiting many intriguing properties, such as extremely good combi- nation of strength and ductility [4–7], high temperature strength [8,9], wear resistance [10–12], creep [13] etc. Considering these, it is not surprising to learn that high expectations are being put into the development of these new materials. However, most of the alloys in the presented studies derive from the original one solid solution concept (i.e., the currently acknowledged basic HEA principle) as they exhibit multiphase structures. In fact, it has been proposed by Wu et al. [14] that the idea of stabilization of a simple solid solution by the increase of configurational entropy by the addition of more elements to the alloy may be wrong. They highlighted a more prominent role of the elements selection, also for the improvement of the extent of solid solution strengthening. Gludovatz et al. followed this idea in their work [15]. In their at- tempt to improve low temperature fracture toughness of a (well-known) CrMnFeCoNi high entropy alloy, they have examined its variant using three elements only: Co, Ni, and Cr. The alloy was denoted as medium entropy alloy (MEA). Surprisingly, the CoCrNi alloy with pure face centered cubic (FCC) solid solution microstructure exhibited fracture toughness unrivalled by any other known HEA and competing with the best modern materials [15]. On top of that, strength and ductility fur- ther increased with decreasing temperatures. Interestingly, the CoCrNi composition was previously studied by Wu et al. [14,16], too. In their work, a wide range of FCC single phase equiatomic alloys composed of three to five elements was compared. Again, the alloy exhibited the best combination of tensile strength and ductility over a wide range of tested temperatures (-196 °C to 400 °C). These properties are believed to come from the observed deformation mechanism of a dislocation slip coupled with deformation nano-twin- ning, resulting in the suppression of plastic instability occurrence. The CoCrNi alloy surely exhibits an incredible ductility and fracture resistance. However, its strength may be insufficient in certain http://dx.doi.org/10.1016/j.msea.2017.06.086 Received 20 March 2017; Received in revised form 9 June 2017; Accepted 22 June 2017 ⁎ Corresponding author. E-mail address: moravcik@fme.vutbr.cz (I. Moravcik). Materials Science & Engineering A 701 (2017) 370–380 Available online 23 June 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved. MARK