5 Werkstofftechnik
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The high-entropy alloy (HEA) CrMnFeCoNi and the medium-entropy alloy (MEA) CrCoNi with a face-centered cubic (fcc) structure have excellent mechanical properties and are considered for high-temperature applications. Both materials were exposed under several oxidized atmospheres in a wide temperature range/regime. Mn-oxide (Mn3O4, Mn2O3) was mainly formed on the HEA and Cr2O3 on the MEA. The fast diffusion of manganese prevents the formation of a continuous chromia layer on CrMnFeCoNi in Ar-O2, Ar-H2O and Ar-SO2 at 800°C.
The Cantor alloy and his ternary subsystem CrCoNi were exposed in Ar-O2-H2O-SO2 at 800°C up to 96 h to clarify their oxidation behavior in a mixed atmosphere. The oxidized samples were analyzed by weight measurement, scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD). It was found that mass gain of all samples increased with increasing exposure time. The oxidation rate of the Cantor alloy is significantly higher than that of the ternary alloy system.
The surface corrosion behaviour is a key issue which determines whether the material is applicable at a given atmosphere. Medium-entropy alloy FeCrNi alloy was exposed to synthetic air at 1000°C, 1050°C, and 1100 °C for up to 1000 h using a thermobalance. The oxidation rate was parabolic at 1000 and 1050°C, but breakaway occurred at 1100°C after 5 h of aging time. The whole oxide scales formed under the isothermal oxidation tests spalled off and additional oxidation tests were carried out at 1000 °C and 1050°C for 24 h and up to 100 h at 1000°C in a tubular furnace. The corrosion behaviour of the MEA was analysed by scanning electron microscope, energy-dispersive X-ray spectroscopy, and X-ray diffraction and compared to the behaviour of 316 L. The experimental results showed that under all conditions chromium is the main diffusion element resulting in the formation of a Cr2O3 layer at the MEA surface. Spallation of the layer induces the formation of additional oxidation products under the surface of the (spalled off) chromia layer.