Kinetic and Metallography Study of the Oxidation at 1250 °C of {Co+Ni}-Based Superalloys Containing Ti to Form MC Carbides

Patrice Berthod,Erwan Etienne,Ghouti Medjahdi,Lionel Aranda,Synthia Annick Ozouaki Wora

doi:10.3390/met12010010

Patrice Berthod, Erwan Etienne + Show 3 more

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https://doi.org/10.3390/met12010010

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Abstract

Six conventionally cast chromium-rich titanium-containing alloys based on cobalt and nickel with various Co/Ni ratios were considered. They were tested in oxidation in air at 1250 °C for 70 h in a thermo-balance. The mass gain curves were exploited to specify different types of kinetic constants as well as several parameters characterizing the oxide spallation occurring during cooling. The obtained results show that, the higher the Ni content, the slower the mass gain and the better the quality of the protective external chromia scale. Secondly, no dependence of the oxide spallation characteristics on the Co content was clearly noted. Globally, the isothermal oxidation behavior becomes better when Ni is more and more present at the expense of Co. Titanium seems to be playing a particular role in the process of oxidation. It notably leads to the presence of an external thin TiO2 continuous scale beyond the chromia scale. The thermogravimetry records were numerically treated to determine the parabolic constant and the chromia volatilization constant. The values of these constants evidenced a double tendency: chromia growth acceleration and chromia volatilization slow-down. These trends are to be confirmed and further investigated.

Highlights

Many applications—industrial or transportation engines, for instance—involve, in service, local temperatures that can reach very high levels
The SEM observations, in BSE mode, of the metallographically prepared samples showed that the microstructures of the alloys are similar to the ones of the samples previously studied in oxidation at 1200 ◦ C [15]: all the initial microstructures are dendritically structured and contain more or less dark particles present in the interdendritic zones (Figure 1)
Energy Dispersion Spectrometry (EDS) spot analyses were carried out with thin electrons beam on the coarsest carbides found in the metallographic samples

Summary

Introduction

Many applications—industrial or transportation engines, for instance—involve, in service, local temperatures that can reach very high levels. Among the superalloys usually considered for such applications, there are the nickel-based {gamma prime}-reinforced single-crystalline superalloys These ones are still the best performing metallic materials for service at a temperature around 1000 ◦ C under severe stress conditions. They cannot be used at 1100 ◦ C and even a little above. Refractory alloys based on metals with high melting points, such as niobium, molybdenum or tantalum, still need to be improved before industrialization. Their densities will inevitably weigh down the aircrafts or increase the centrifugal stresses affecting the pieces subjected to high-speed rotation (e.g., turbine blades). Their densities will inevitably weigh down the aircrafts or increase the centrifugal stresses affecting the pieces subjected to high-speed rotation (e.g., turbine blades). (Refractory)

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