Etude de l'oxydation à 850°C de l'alliage NiCoCrAl(Y)Ta coulé et projeté au chalumeau plasma sous pression réduite

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The alloy studied (Amdry 997: 22–24 wt.% Co, 18–21 wt.% Cr, 7.5–9.5 wt.% Al, 0.7–0.8 wt.% Y, 3.5–4.5 wt.% Ta and balance nickel) is used as a powder to form coatings by the low pressure plasma-spraying process. The choice of a temperature of 850°C was dictated by a complementary study on the hot corrosion of the same alloy by Na 2SO 4. First the nature and composition of the equilibrium phases were determined. γ-, γ′ and β-aluminides and a σ phase are present in the two types of alloy (cast and sprayed) as well as an intermetallic compound M 5 Y ( M  Ni + Co + Cr + Al). In the sprayed alloy, yttrium is also detected as Y 2O 3. An oxidation study of these alloys at 850 °C in air (isothermal and cyclic conditions) has shown the important influence of their structure. In the cast alloy the large size of the grains of the main constituents (γ and β) promotes the heterogeneity of the oxide scale. In this case it mainly consists of Cr 2O 3 in the γ phase, and Al 2O 3 in the β phase. In contrast, the finer cyrstallization of the sprayed alloy favours the growth of a homogeneous and compact Al 2O 3 layer partially modified by Cr 2O 3. Satisfactory behaviour is observed in the cyclic oxidation of both alloys. Indeed, spallation of the protective scales is very limited. Finally, their oxidation behaviour has been compared with that of an yttrium-free cast alloy. At the same time the different phases constituting the NiCoCrAlYTa alloy have been examined in oxidizing conditions, at the same temperature, to determine their respective influence on the oxidation mechanisms of the complex alloy.