Oxidation Behaviour of Model Cobalt-Rhenium Alloys During Short-Term Exposure to Laboratory Air at Elevated Temperature

Abstract

The alloys being used in high-temperature systems such as stationary gas turbines and aircraft engines are iron-, cobalt-and nickel-based superalloys, amongst which the latter is the most widely used for highest temperatures. However, the use of Ni-based alloys is limited to temperatures below 1,100°C. The experimental Co-Re-based alloys are promising for high-temperature applications for service temperatures beyond 1,200°C. The purpose of the present investigations, at this still early stage of the alloy development, is to gain a first insight into the oxidation mechanisms and to find ways to improve oxidation resistance of this class of materials. Thermogravimetric studies in combination with microstructural examinations of six model Co-Re alloys with different compositions showed the negative influence of rhenium on the oxidation resistance of Co-based alloys due to evaporation of rhenium oxide(s). Oxidation at 1,000°C in air yielded an oxide scale, that consists of a Co-oxide outer layer on a thick and porous Co-Cr oxide and a semicontinuous and therefore non-protective Cr-oxide film on the base metal substrate. This allowed for the vaporization of rhenium oxide formed during oxidation and hence led to a loss of Re. Computer-aided thermodynamic calculations were carried out to supplement the experimental analyses and were found to reasonably predict the stability ranges of the various oxide phases observed.

High temperature test rig for inert atmosphere miniature specimen creep testing

“…In this stage of alloy development, emphasis is placed on basic deformation mechanisms which can be observed using transmission electron microscopy (TEM) [26,30]. The preliminary alloy cannot be tested in air at very high temperatures because severe oxidation would result in a complete disintegration of the whole sample [31,32]. Inert gas miniature creep specimen testing of experimental Co-Re-Cr alloys is also attractive because small amounts of materials are sufficient to study the creep behavior.…”

High‐temperature oxidation behaviour of model CoReCr alloys at low oxygen partial pressure

“…It was shown in the previous study [1] that the s-phase is preferentially attacked during high-temperature oxidation in air as compared to the metallic matrix. This is not surprising because the s-phase is rich in the highly reactive element Cr (see Fig.…”

“…The first investigations of this class of alloys showed rather poor oxidation resistance during exposure to laboratory air [1,2]. The aim of the study at this stage of the Co-Re alloy development is to derive options for an improvement of the oxidation resistance at very high temperatures.…”

The Effect of Pre-Oxidation Treatment on the High-Temperature Oxidation of Co–Re–Cr Model Alloys at Laboratory Air

“…As soon as the Cr 2 O 3 scale does not prevent the contact of oxygen from the atmosphere with the metal substrate, oxidation of other elements, such as cobalt and rhenium takes place. Earlier investigations [3][4][5] on Co-Re-based alloys have shown that mass loss occurs during exposure to laboratory air because of evaporation of Re-oxide(s). It should be mentioned that the loss of rhenium occurs mainly by the formation of the gaseous rhenium oxides of type Re 2 O 7 [3].…”

“…Earlier investigations of Co-Re-Cr model alloys have shown rather poor oxidation resistance during exposure to laboratory air [3,4]. Therefore, an important aim of the Co-Re alloy development must be to derive options for an improvement of the oxidation resistance at very high temperatures.…”