Oxidation at 1200°C of titanium-containing equi-axed polycrystalline cast nickel-based alloys reinforced by chromium carbides

摘要

Titanium is often encountered in the chemical compositions of superalloys, as base (Ti-based alloys, titanium aluminides) or as gamma prime forming element, for instance. Titanium is a particularly oxidable element at high temperature and it may play a specific role in the high temperature process of superalloys containing it. In this work this is in the case of cast chromia-forming carbides-reinforced nickel-based alloys that the effect of the presence of titanium was investigated. Three alloys of the {Ni(bal.)-25Cr-0.25 or 0.50C-1 or 2Ti}-type were synthesi2ed by conventional casting then tested in oxidation by hot air, in comparison with similar but Ti-free reference alloys. Isothermal oxidation in thermo-balance was realized at 1200°C for 46 hours in a dry synthetic air flow. The mass variation was also continuously recorded during the post-isothermal stage cooling performed at -5K/min. The isothermal mass variation files were exploited to visualize the mass gain type and extract the values of the different types of kinetic constants. Oxide spallation was characterized by analysing the mass variation during cooling and by observation and XRD analysis of the external scale before cutting. External scales, internal oxidation and subsurface alloy deterioration were characterized on cross sections by SEM/BSE observations and SEM/EDS analysis. In the microstructural field the Ti-containing and the Ti-free alloys are quite similar, with a dendritic matrix containing almost all the titanium atoms, and an interdendritic network of eutectic chromium carbides, despite the presence of Ti which is known as a rather strong carbide-former element. The mass gain kinetic are parabolic-type in all cases, whatever the presence of Tior not in the considered alloy. The parabolic and volatilization constants are all typical of a chromia-forming behaviour but it seems that the Ti-containing alloys tend to oxidize faster than the Ti-free ones. This was later confirmed during the cross sectional observations by the comparison of the chromia thicknesses and the chromium depletions. The oxidized Ti-containing alloys are covered by a double layer of external oxide: a continuous thick chromia scale close to the alloy surface and a continuous thin TiO_2 scale isolating chromia and air. This was earlier detected by X-ray diffraction, before cross-section preparation. Concerning the behaviour of the oxide scales during post isothermal oxidation cooling the Ti-containing alloys showed an obvious better resistance against oxide spallation. This is precisely on this later property that the presence of Ti was really beneficial in the oxidation behaviour field. Since it did not influence the type and morphology of the reinforcing network of primary carbides, one can thus think that adding a little titanium to such cast {Ni-Cr-}-based high temperature alloys is a neutral mean to improve their global hot oxidation behaviour.