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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.

Experiments and thermodynamic calculations were performed on five cobalt-base superalloys containing tantalum carbide and chromium carbides in order to evaluate the accuracy of thermodynamic calculation for this alloy family. The studied quantities were the solidus temperature and the phase fractions of the different carbides observed after a long time exposure at different high temperatures. The predicted and observed phases are in good agreement. However the measured phase-fractions and melting temperatures were sometimes higher than the calculated values. The disagreement in the phase fractions can be attributed to the fact that surface fractions and volume-fractions are not necessarily equal.

a b s t r a c tFive cast cobalt alloys based on Co-8Ni-30Cr-0.4/0.45C and containing Ta, Nb, Hf and/or Zr were studied by metallography in the as-cast condition and after treatments at 1300 • C. The obtained MC carbides were all interdendritic with a eutectic script-like morphology. For similar carbon contents, the HfC carbides are the most developed in the as-cast microstructure and the most stable at 1300 • C. As-cast, the TaC carbides are less developed than the former and they tend to become more fractioned and less present in microstructure at 1300 • C. The NbC carbides, which have initially the same morphology and the same fraction as TaC, rapidly dissolve at 1300 • C. The cobalt alloys containing HfC or TaC are chromia -forming at 1300 • C. The NbC-containing alloy catastrophically oxides after only few hours at 1300 • C. The average hardness is the highest for the HfC-containing alloy and the lowest for the NbC-containing alloy.

The corrosion of pure chromium was studied in four molten glasses, using both scanning electron microscopy and electrochemical methods to characterize the metal/glass interactions. It is shown that direct immersion of chromium into glass does not allow obtaining a protective oxide scale, even if the glass contains oxidizing species such as Fe III , Zn II , etc. In these conditions, the corrosion mechanisms vary with temperature and glass composition. When the metal is oxidized in hot air preliminary to glass diving, the passive state is reached and related to the presence of a Cr 2 O 3 continuous layer at the metal/glass interface. It is maintained up to a temperature called "depassivation temperature." This temperature is close to 1160°C in a borosilicate glass and is shifted to the higher values when the glass is enriched into oxidizing species ͑Fe III ͒. Coupling both SEM and electrochemical data shows that the Cr 2 O 3 layer continuity is ensured by microgalvanic couples occurring between parts where the oxide scale is in contact with the metal substrate and other parts where the oxide scale is peeled off. When the glass temperature is higher than the depassivation temperature, a chromium boride CrB layer develops at the metal/glass interface. Most of the alloys and superalloys used in the glass-making industry are chromia-forming materials.1-3 Generally, when a given material is used for a specific application, the process is never changed because it is known that a variation of one parameter can drastically change the metal behavior in molten glass, particularly its corrosion resistance. 4 One of the most important parameters is the glass temperature. Increasing the glass temperature increases its fluidity and the making of the glass becomes easier. On the contrary, the mechanical properties of the alloy are decreased as well as its corrosion resistance. 5-7Many works concerning the study of the pure metals behavior in glass melts have been realized since the late 1950s. Ni, 2,21 Fe, Co, and Cr, 2,22 which are proposed to be used in the glass-making industry as raw materials or coatings on ceramics. The results of these studies indicate that the behavior of pure metals immersed in molten glasses ͑in terms of corrosion rates and corrosion layers͒ varies with glass composition, melt temperature, and the metal potential ͑in the case of use of the material as a fusion electrode͒. Di Martino et al. 2 have shown that the glass working temperature increase can induce important changes in the metal behavior versus glass corrosion. As an example, these authors have shown that when pure chromium rods are dived in a C-type borosilicate molten glass, there is a corrosion mechanism modification when the glass temperature is increased.Most of the alloys in contact with molten glasses that are used in the glass industry are chromia-forming materials. As a consequence, the knowledge of pure chromium corrosion in glass melts will provide useful information concerning the comprehension of chromiaforming alloys in molten gla...

Experiments and thermodynamic calculations were performed on three nickel-base alloys containing chromium, carbon and tantalum. Solidus and liquidus temperatures, natures and surface fractions of the carbides after an exposure for 100 hours at 1000°C, 1100°C and 1200°C, were determined for each alloy. These results are compared with calculated results, using a thermodynamic database. A good agreement was generally found for the solidus temperatures but less for the liquidus ones. For alloys containing chromium carbides alone, carbides fractions and matrix compositions correspond to calculation results. But the presence of tantalum carbides in the third alloy was not predicted by calculations.