High-Temperature Oxidation in Dry and Humid Atmospheres of the Equiatomic CrMnFeCoNi and CrCoNi High- and Medium-Entropy
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High‑Temperature Oxidation in Dry and Humid Atmospheres of the Equiatomic CrMnFeCoNi and CrCoNi High‑ and Medium‑Entropy Alloys Christiane Stephan‑Scherb, et al. [full author details at the end of the article] Received: 22 April 2020 / Revised: 17 September 2020 / Accepted: 10 November 2020 © The Author(s) 2020
Abstract Surface degradation phenomena of two model equiatomic alloys from the CrMnFeCoNi alloy system were investigated in 2% O2 and 10% H2O (pO2 = 0.02 and 10−7 atm, respectively) at 800 °C for times up to 96 h. The crystallographic structures, morphologies, and chemical compositions of the corrosion layers developing on CrMnFeCoNi and CrCoNi were comparatively analyzed by mass gain analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The oxidation resistance of CrMnFeCoNi is relatively poor due to the fast growth of porous Mnoxide(s). CrCoNi forms an external chromia layer that is dense and continuous in a dry 2% O2 atmosphere. This layer buckles and spalls off after exposure to 10% H2O atmosphere. Beneath the chromia layer, a Cr-depleted zone forms in the CrCoNi alloy in both environments. As the oxide scale spalls off in the H 2O-containing atmosphere, a secondary chromia layer was observed and correspondingly enlarges the Cr-depleted zone. In contrast, as the chromia layer remains without significant spallation when CrCoNi is exposed to a dry oxidizing atmosphere, the region depleted in Cr is narrower. Graphic Abstract
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Vol.:(0123456789)
Oxidation of Metals
Keywords High-temperature oxidation · High- and medium-entropy alloys · CoCrFeMnNi · NiCoCr · Chromia · Mn-oxides
Introduction High-entropy alloys (HEAs) are, according to the original taxonomy of Yeh et al. [1, 2] single-phase and disordered solid solutions, which are consisting of at least five elements in near-equiatomic proportions, while those containing 2–4 elements have been defined as medium-entropy alloys (MEAs). One of the material systems, which has been intensively studied for more than 1 decade and a half from experimental and theoretical points of view, is the equiatomic face-centered cubic (fcc) CrMnFeCoNi alloy (also referred to as Cantor alloy [3]) and its equiatomic subsystems. These alloys possess excellent mechanical properties including high ductility, work hardening rate, ultimate tensile strength, and fracture toughness [4–8]. For instance, Gali and George [4] investigated the tensile properties of the CrMnFeCoNi HEA with a mean grain size of 32 µm and showed that its yield strength, ultimate tensile strength, and ductility all decrease from 450 to 200 MPa, 1100 to 500 MPa, and ~ 65 to ~ 35%, respectively, when the temperature increases from −196 to 200 °C. While the yield stress and ductility remain roughly constant between 200 and 1000 °C, the ultimate tensile strength further decreases from 500 to 100 MPa. Later on, Wu et al. [6] reported that among the equiatomic subsystems of the CrMnFeCoNi alloy that are
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