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

Abstract

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°C-8h, 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 Al 2 O 3 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.

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

“…Given the fact that oxidation kinetics for nickel alloys at 650C is extremely slow [25], there is insufficient time for oxide formation to happen ahead of the crack tip within the order of seconds. Direct observations of internal oxidation and associated damage were only available on smooth specimens, which have been exposed to oxidizing environment for a considerably long time, mostly greater than 20 hours [2,12,13,26]. The current work also showed that crack growth rates in air at high-temperature can be predicted well from the dynamic embrittlement hypothesis, though collecting experimental evidence of crack tip oxygen diffusion process and measurements of oxygen concentration near the crack tip are challenging tasks and yet to be accomplished.…”

“…Evidence of oxidation reaction with nickel alloys has been presented for smooth specimens with detectable oxide layers on the surface and at internal grain boundaries, a result of oxygen attack [2,12,13]. Oxidation has also been evidenced during crack propagation tests, with oxides clearly observed on crack surfaces [5,8,10,11,14].…”

High Temperature Intergranular Oxidation of Alloy 718

“…In order to assess the harmfulness of IOP, tensile tests were performed under imposed strain rate controlled mode (dε/dt = 10 -3 s -1 ) in air at room temperature after different pre-oxidation times at 1000°C. Figure 6: Tensile tests performed at room temperature on 0.3 mm thick specimens (Pre-oxidation at 1000°C in air) [13] The tensile curves are presented in Figure 6 [13] and focus on three main points: -A pre-oxidation treatment at 1000°C for 1 hour under vacuum results in a loss in term of mechanical properties of alloy 718 at room temperature (-40 MPa compared to the yield strength of the non pre-oxidized specimen).…”

“…For this purpose, specific specimens were prepared by mechanical polishing. The following procedure ( Figure 1) was applied [13,14]: the coupon was cut to fit SIMS specimen size. After that, its oxidized face was glued on a cylindrical rod (diameter: 30 mm, height: 20 mm) made of copper.…”

“…-650°C in air: similar to "in service" conditions of turbomachine disks. Under such conditions, the material is known to be sensitive to oxidation assisted crack propagation [3][4][5][6][7][8][9][10][11][12][13]. The core of the present study is to assess the harmfulness of intergranular oxides penetrations (IOP) on the mechanical properties of alloy 718.…”