High-Temperature Oxidation in Dry and Humid Atmospheres of the Equiatomic CrMnFeCoNi and CrCoNi High- and Medium-Entropy Alloys

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 Mn-oxide(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 H2O-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

Early Material Damage in Equimolar CrMnFeCoNi in Mixed Oxidizing/Sulfiding Hot Gas Atmosphere

“…The differences in type and structure of the manganese oxide observed can mainly be explained by the low oxygen partial pressure p O2. When comparing different past studies, it can be concluded that the observed phases Mn 3 O 4 and Mn 2 O 3 are stable at much higher p O2 , e.g., p O2 = 0.02 atm and 10–7 atm in the study by Stephan‐Scherb et al, [ 16 ] compared to the p O2 = 2 × 10 −36 atm used in this study. However, the high diffusivity of manganese and its affinity for oxidation dominates the oxidation and corrosion process.…”

“…After exposure to 0.5% SO 2 for 6 h at 600 °C, the fcc grain structure is still clearly visible, but the entire CrMnFeCoNi surface appears oxidized (Figure 1a). The grains, sized on average 50 μm in diameter, [ 16 ] reveal a gray oxide layer with dark gray precipitates (region 1). Along the grain boundaries, mainly dark gray (region 2, about 10 % in area) and partly white precipitates have formed.…”

“…The alloys were finally recrystallized to an average grain size of %50 μm by heat treatment for 1 h at 1020 C. More details about alloy preparation can be found elsewhere. [41,42] The alloy composition has been verified by electron probe microanalyzer, as has been reported in the study by Stephan-Scherb et al [16] Samples for corrosion tests were cut to 3 mm-thick disks and then ground with abrasive paper down to a grit size of 1200.…”

“…[ 10–13 ] It shows excellent mechanical properties in terms of high ductility work hardening rate, ultimate tensile strength, and fracture toughness, especially at low temperatures, but it has also proven to show interesting oxidation behavior at higher temperatures around 600 °C. [ 14–16 ] Even though it will probably not be used in that temperature range, it can serve as a reference for many face‐centered cubic (fcc) structured, Co–Cr–Fe–Ni‐based HEAs that are designed for high‐temperature applications such as high‐entropy superalloys. [ 17 ] Until now, the focus of corrosion studies in the range of 500–800 °C has been on material behavior in laboratory or synthetic air, [ 14,15 ] and dry (2%O 2 ) and humid (10% H 2 O) atmospheres [ 16 ] for exposure times of up to 100 h. All of these studies described fast growing porous Mn‐rich oxides, leading to accelerated material degradation by corrosion.…”

Effects of Cr/Ni ratio on physical properties of Cr-Mn-Fe-Co-Ni high-entropy alloys

“…The equiatomic face-centered cubic (FCC) CrMnFeCoNi alloy, the so-called Cantor alloy [1] , is a model high-entropy alloy (HEA) that has received considerable interest in, e.g. , materials science and engineering [2][3][4][5] , diffusion [6,7] , corrosion/oxidation [8][9][10][11] , and electrocatalysis [12] . Due to its good malleability [13][14][15] , which made it the pioneering HEA to be thermomechanically processed and investigated in bulk form, and interesting mechanical properties at cryogenic temperatures, e.g.…”