Scripta Materialia 178 (2020) 391–397 Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat Yield strength increase of a CoCrNi medium entropy alloy by interstitial nitrogen doping at maintained ductility Igor Moravcik a,b,∗ , Hynek Hadraba c , Linlin Li a , Ivo Dlouhy b,c , Dierk Raabe a , Zhiming Li a,d a Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany b NETME Centre, Institute of Materials Science and Engineering, Brno University of Technology, Technicka 2896/2, Brno, Czechia c Institute of Physics of Materials CAS, Zizkova 22, 61662 Brno, Czechia d School of Materials Science and Engineering, Central South University, Changsha 410083, China a r t i c l e i n f o Article history: Received 23 October 2019 Revised 27 November 2019 Accepted 4 December 2019 Available online 17 December 2019 Keywords: Interstitials Strengthening Alloying Tension test Concentrated solid solution a b s t r a c t We show that interstitial nitrogen doping improves the tensile properties of a CoCrNi alloy. A mate- rial with 0.5 at% interstitial nitrogen was compared to a nitrogen-free CoCrNi alloy. The nitrogen-doped variant has a stable, single-phase face-centered cubic (FCC) lattice structure without nitrides, also after different annealing treatments (800–900 °C, 10 and 30 min). The nitrogen caused an increase in yield strength by 24–33% at identical ductility compared to the nitrogen-free material with similar grain size. The strengthening effect of nitrogen was explained in terms of the simultaneous increase of the lattice friction stress and of the Hall-Petch coefficient. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. High entropy alloys (HEAs) and medium entropy alloys (MEAs) are metallic materials without a prevalent base element [1,2]. MEAs are composed of 3–4 elements in equimolar ratios [3], while HEAs from 5 or more of such elements. MEAs and HEAs are of interest as some variants have very good cryogenic fracture toughness, strength-ductility combination and high-temperature stability [3,4]. Among the vast number of MEA compositions studied so far, the equimolar CoCrNi alloy with single face-centered cubic (FCC) solid solution microstructure exhibited the best synergy of strength and ductility [5,6]. The material also showed excellent resistance to corrosion and to oxidation at high temperatures [7,8]. The CoCrNi MEA deforms by dislocation plasticity and massive de- formation twinning, providing high strain hardening. Additionally, deformation-driven phase transformation from the FCC matrix to the hexagonal close-packed (HCP) phase in the vicinity of twin boundaries has been reported [9]. While many studies of MEAs and HEAs have been concerned with improving the ultimate tensile strength (UTS) – ductility synergy [4,10], the yield strength is an even more important material property from a standpoint of struc- tural design [11]. However, most FCC single-solid solution alloys have relatively low yield strength [10], comparable to other FCC ∗ Corresponding author at: NETME Centre, Institute of Materials Science and En- gineering, Brno University of Technology, Technicka 2896/2, Brno, Czechia. E-mail address: igor.moravcik@vutbr.cz (I. Moravcik). alloys, e.g. Co–Cr [12,13]. In fact, most of the strongest FCC MEAs and HEAs have even lower lattice friction stresses than common commercial FCC steels such as AISI 316L [14–16]. Yield strength can be improved by several means, however, usually at the expense of ductility such as precipitation strengthening or cold-working [17–19]. Recently, interstitial strengthening was shown to pro- foundly affect the properties of HEAs [20–22]. Most HEAs have been so far interstitially doped with C, often causing formation of undesired carbides [23,24]. Very few reports presented N- doping in that context [25,26]. However, in some steels interstitial N-doping has been even favored over C-doping [27] due to its usually higher solubility and slower kinetics in the formation of nitrides compared to the formation of carbides [28,29]. Here, we therefore study the effect of interstitial N-doping on the room temperature tensile properties of a CoCrNi model MEA. Two variants are probed, one with 0.5 at% interstitial N and one without N. Both were produced by standard liquid metallurgy methods and tensile tested. We found that the dissolved N signif- icantly improves yield strength by increasing lattice friction stress and grain size sensitivity in the MEA, without reducing tensile ductility. Ingots of equimolar CoCrNi MEA and the interstitially doped CoCrNi+0.5 at% N (~Co 33.2 Cr 33.2 Ni 33.2 N 0.5 ) with dimensions of 25×60×60 mm 3 were prepared by vacuum induction melting (VIM) from pure metals of >99.8% purity and FeCrN 2 as N-source. These two MEAs are referred to as CoCrNi and CoCrNiN alloys, https://doi.org/10.1016/j.scriptamat.2019.12.007 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.