Study of Carbide Transformations during High-Temperature Oxidation of Nickel-Base Superalloys

Abstract

Microstructure phenomena resulting from high-temperature oxidation of three nickel-base superalloys were studied by microstructure examinations. Disappearance, nature modification, volume fraction evolution or precipitation of carbides were observed in the alloys near the external surface, depending on the temperature and the chemical composition of the alloys. Thermodynamic calculations allowed to better know what happened to carbon and to quantify its new distribution. The alloys studied lost a more or less great part of their sub-surface carbon content at 1200 • C while carbon seemingly diffused deeper in the alloy at 1100 • C and 1000 • C. The latter part of carbon promoted the coarsening of the pre-existing carbides, some modifications of their natures or the precipitation of new carbides in the matrix, then the occurrence of a new-carbides zone.

Influence of Chromium Carbides on the High Temperature Oxidation Behavior and on Chromium Diffusion in Nickel-Base Alloys

“…During oxidation, the chromium carbides disappear from the oxidation front at a depth which increases with time, and also with temperature. The depth of the carbide-free zone also depends of the density of the initial carbides since it is deeper for the Ni-30Cr-0.2C alloy than for the Ni-30Cr-0.8C one (except for 1,0008C where a part of the carbon obviously remained in the first alloy between the carbidefree zone and the bulk), a phenomena which was already encountered in previous works [12]. Even if the contribution of the chromium carbides to the Cr quantity supplied on the oxidation front is lower for the Ni-30Cr-0.2C alloy than for the Ni30Cr-0.8C alloy (Table 5), the carbides of the first alloy necessarily dissolve deeper than for the second alloy, because of its less-dense, initial-carbide network.…”

“…This zone is characterized by the precipitation of small, acicular new carbides in matrix. As demonstrated and quantified in a previous work [12], this is due to an inward diffusion of the carbon released by the dissolving carbides which promotes the intergranular solid-state formation of new carbides involving chromium present in solid solution in matrix. Other phenomena can be a growth or a change of stoichiometry of the interdendritic carbides which already existed here.…”

Effect of a Preliminary Aging Treatment on the Oxidation Kinetic at High Temperature for a Cobalt - Based Alloy Strengthened by Tantalum Carbides

“…6), the chromia layer was relatively thin (about 5 m thick) and in some places on a sample one can see precipitation of new carbides (acicular chromium carbides) in a zone separating the outer carbide-free zone (in which primary carbides disappeared because of oxidation) and the inner bulk, while no such precipitated carbides are visible in other locations on the same sample. This phenomenon was earlier explained by an inward diffusion of carbon atoms quitting the carbides disappearing because of oxidation [10]. The phenomenon of precipitation of new carbides did not occur at 1100°C (Fig.…”

Measurement and Discussion of Low-Temperature Hot Corrosion Damage Accumulation upon Nickel-Based Superalloy Rene 104

“…Furthermore, under such high-temperature exposure, the notion of socalled ''transient'' oxidation has been previously seen for nickel-based superalloys, [16] while very recently, Berthod and co-workers have noted explicitly the disappearance of secondary carbides in model nickel-based superalloys. [17,18] In that work, temperatures were again >1000°C; however, carbide loss was noted as occurring up to many tens of microns into the surface of the material. We suggest, however, that carbides on the surface alone may be oxidized at temperatures investigated herein.…”