Veronique Garat

Comparing Different Methods to Determine the Intergranular Oxidation Damage on a Nickel Based Superalloy

Thin tensile specimens of alloy 718 were oxidized under synthetic air, at 1000°C for different durations, then heat treated according to an aeronautical heat treatment (720°C8h, 620°C-8h) before being tested on a tensile machine at room temperature. Mechanical tests were performed on sufficiently thin specimens (thickness less than 0.3 mm and a gage length equal to 200 mm) in order to highlight the influence of the damaged zone upon the global tensile behavior. It was shown that the mechanical behavior remained unaffected in terms of the flow law but a significant drop in flow stress was seen in the case of oxidized specimens. This drop was at first attributed to a reduction of the loaded section, but this reduction should then be 30 to 40% greater than the intergranular oxidation area measured by SEM or SIMS. To explain this discrepancy, the hypothesis of oxygen diffusion ahead of the intergranular oxidation front was proposed. In order to check this hypothesis, a new method based on SIMS analysis was used. Elemental concentration profiles together with imaging were obtained following a specifically designed method that allowed the sputtering from the metal to the oxide. This analysis showed that Al2O3 forms at the intergranular oxidation front and that no measurable dissolved oxygen is found in the alloy ahead of this front, thus invalidating the proposed hypothesis. Tensile tests on a specimen treated under vacuum at 1000°C suggest that high-temperature heat treatment is responsible for half of the drop in mechanical properties. Finally, comparing the tensile curves of thin specimens of alloy 718 heat treated at high temperature to a standard curve is an accurate method to quantify the damages on this alloy due to the treatment.