Scripta Materialia 188 (2020) 264–268 Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat Impact of interstitial carbon on self-diffusion in CoCrFeMnNi high entropy alloys O.A. Lukianova a,∗ , Z. Rao b , V. Kulitckii a , Z. Li b,c , G. Wilde a , S.V. Divinski a,∗ a Institute of Materials Physics, University of Münster, Germany b Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany c School of Materials Science and Engineering, Central South University, Changsha, China a r t i c l e i n f o Article history: Received 19 May 2020 Revised 19 July 2020 Accepted 20 July 2020 Keywords: Tracer diffusion High-entropy alloy Carbon solubility a b s t r a c t Tracer diffusion of the matrix elements in CoCrFeNiMn-based high-entropy alloys with 0.2, 0.5 and 0.8 at.% carbon is measured at 1373 K. The diffusion coefficients are found to depend non-monotonously on the carbon content. The diffusion retardation in low-C alloys is discussed in terms of the impact of interstitially-dissolved carbon on the vacancy concentration and the migration barriers in the alloy. In high-C alloys, formation of carbides alternates the observed trends, most probably by increasing the total vacancy concentration. © 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Due to its beneficial mechanical and physical properties, such as a high ductility and elevated fracture toughness [1], especially at cryogenic conditions [2], the equiatomic CoCrFeMnNi high-entropy alloy (HEA) is considered as a model material for anticipated tech- nological applications. However, a relatively low yield strength is a common drawback for all materials with the FCC structure. Al- loying by interstitial elements represents a general pathway to strengthen these alloys so that they can be used for structural ap- plications. High-purity high-entropy alloys with additional intersti- tial carbon element were found to reveal outstanding mechanical, physical and chemical properties [3]. In particular, a most success- ful compromise of a high yield strength and acceptable ductility was achieved due to the interstitial carbon [4] that was explained in terms of TWIP (Twinning Induced Plasticity) and TRIP (Transfor- mation Induced Plasticity) mechanisms. Wu et al. [5] have shown that the CoCrFeMnNi HEA with a carbon content of 0.5 at.% reveals an increased strength in comparison to the carbon-free alloy. Formation of nanosized carbides presents another strengthen- ing mechanism in such alloys. In these alloys, two main types of carbides are typically formed: M 23 C 6 [6] in the five-component CoCrFeMnNi alloy and M 7 C 3 in the four-component CoCrFeNi alloy [7]. Thus, a number of strengthening mechanisms work simultane- ously in these alloys (in addition to the solid solution strengthen- ing [4]). ∗ Corresponding authors. E-mail addresses: sokos100@mail.ru (O.A. Lukianova), divin@uni-muenster.de (S.V. Divinski). Guo et al. [8] explained in details the strengthening mecha- nisms clarifying the enhancement of the mechanical properties of the reported HEAs that contained carbon. These authors attributed the observed hardening to the refinement of the grains, lattice fric- tion stress and the precipitation of the M 23 C 6 carbides. The au- thors suggested that the observed carbides impede the migration of grain boundaries and thus refine the grain size [8]. Fan et al. also described HEAs with carbon and showed improved mechani- cal properties (microhardness, yield strength, fracture strength and strain) with an increase of the carbon content [9]. A fundamental change of the microstructure of carbon-containing AlFeCoNi high- entropy alloys has been reported [9], revealing their typical den- dritic morphologies with a single B2 phase as dendrites and a mix- ture of the FCC and E2 1 phases formed due to both ordering and spinodal decomposition in the interdendritic regions while the al- loy without carbon had a typical columnar structure with a single B2 phase [9]. Carbon could also be present in high-entropy alloys as a spu- rious element due to manufacturing conditions. Since diffusion properties are of key importance for predicting the life-time prop- erties and the phase stability, knowing the impact of interstitial carbon and carbide formation on the atomic transport is impera- tive for developing HEA for advanced applications. It is known that interstitial carbon enhances generally diffu- sion in the BCC lattice [10,11]. Similarly, diffusion enhancements in close-packed FCC iron [12], Co [13] and Ni [13,14] with an increas- ing carbon content were already reported. A smaller activation en- ergy of Ni diffusion than for Fe self-diffusion in BCC iron contain- ing C was observed in the temperature range 423–523 K, indicat- https://doi.org/10.1016/j.scriptamat.2020.07.044 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.