Superalloys 2021: Tuesday Part IV - Environmental Behavior
Program Organizers: Sammy Tin, University of Arizona; Christopher O'Brien, ATI Specialty Materials; Justin Clews, Pratt & Whitney; Jonathan Cormier, ENSMA - Institut Pprime - UPR CNRS 3346; Qiang Feng, University of Science and Technology Beijing; Mark Hardy, Rolls-Royce Plc; John Marcin, Collins Aerospace; Akane Suzuki, GE Aerospace Research

Tuesday 4:10 PM
September 14, 2021
Room: Live Session Room
Location: Virtual Event

Session Chair: Bruce Pint, Oak Ridge National Laboratory; Venkat Seetharaman, Raytheon Technologies Corporation (Rtx)

4:10 PM
Investigation into the Effects of Salt Chemistry and SO₂ on the Crack Initiation of CMSX-4 in Static-loading Conditions: Fabian Duarte Martinez¹; Grant Gibson²; Jonathan Leggett²; Julian Mason-Flucke²; Nicolau Morar¹; Gustavo Castelluccio¹; John Nicholls¹; Simon Gray¹; ¹Cranfield University; ²Rolls-Royce Plc.
    Although evidence exists of the potential impact of stress, co-incident with corrosive environments at high temperature, for single crystal turbine blades, the mechanism responsible is not fully understood. This work explores the effect of CaSO4, Na2SO4 and sea salt on the scale formation and crack initiation of CMSX-4 at 550°C in 50 ppm of SO2 and synthetic air under a static stress of 800 MPa. The fracture surface analysis showed that the CaSO4 and the Na2SO4 salted specimens did not undergo a significant degree of corrosion degradation and no cracks were detected after 400 hours of exposure. However, sea salt caused significant degradation to the scale and cracks were detected by X-ray CT scanning after 400 hours of exposure. The findings from this study suggests that the sulfation of chlorine containing species in sea salt led to the formation, vaporisation and re-oxidation of metal chlorides and this mechanism was found to play a key role in the formation of a non-protective scale. An active oxidation mechanism has been proposed to interpret the results. In conclusion, it is hypothesized that due to the synergistic effect of stress and the formation of a non-protective scale, fast diffusion paths for sulfur, oxygen and chlorine ingress were formed. Further work is currently being undertaken to understand the effect of these species on the local embrittlement of CMSX-4 that ultimately led to the initiation of cracks in the specimen.

4:35 PM
Characterization of the Benefit of APS Flash Coatings in Improving TBC Lifetime: Bruce Pint¹; Michael Lance¹; Ercan Cakmak¹; Kenneth Kane¹; James Haynes¹; Edward Gildersleeve²; Sanjay Sampath²; ¹Oak Ridge National Laboratory; ²Stony Brook University
    The addition of an air plasma sprayed (APS) “flash” layer on top of a high velocity oxygen fuel (HVOF) bond coating has been shown to extend the lifetime of thermal barrier coatings. A series of furnace cycle tests (FCTs) has been conducted at 1100 °C in air + 10 % H2O to study the benefit of flash coatings on rod and disk alloy 247 specimens and provide a better mechanistic understanding of their benefit. Flash coatings of NiCoCrAlY and NiCoCrAlYHfSi both improved the FCT lifetime of rod specimens tested in 100-hr cycles and disk specimens tested in 1-hr cycles. In 1-hr cycles, the NiCoCrAlY flash coating significantly outperformed an HVOF-only NiCoCrAlYHfSi bond coating and a NiCoCrAlYHfSi flash coating. Both flash coatings increased the bond coating roughness compared to HVOF. During exposure, the flash layer formed an intermixed alumina-metal layer that appeared to inhibit crack formation. Using a time series of observations, the lower Y+Hf content in the Y-only flash coating appeared to reduce Al consumption. The HVOF layer acted as a source of Al for the adjacent mixed zone. A second series of specimens included a fully APS bond coating where oxide had penetrated through the entire coating to the substrate after only 100, 1-hr cycles and lifetime was similar to an HVOF-only bond coating. The inner HVOF layer with the outer APS flash coating prevented this complete penetration from occurring.

5:00 PM
Using Rapid Thermal Annealing for Studying Early Stages of High Temperature Oxidation of Superalloys: Dorota Kubacka¹; Yolita Eggeler¹; Nicklas Volz¹; Steffen Neumeier¹; Erdmann Spiecker¹; ¹FAU Erlangen-Nuremberg
    The great potential of Rapid Thermal Annealing (RTA), a widespread technique in semiconductor technology, for studying high-temperature properties of superalloys is demonstrated. As an application example, early stages of high-temperature oxidation of Co/Ni-base superalloys are investigated. Model alloys with different Co/Ni ratio are oxidized in synthetic dry air at 900 °C to understand the differences between the oxidation behavior of Ni- and Co-base alloys. Upon short exposure times (below 64 s), the two-phase ă/ă′ microstructure has a pronounced influence on both the morphology and growth kinetics of Al2O3 precipitates. Depending on the base element, Al2O3 nucleates in either the ă′ phase in the form of needles (Ni-base superalloy) or in the ă matrix as lath- and plate-like precipitates (Co-base superalloy) with the latter showing much slower formation kinetics. Observed differences in oxidation behavior can be directly correlated to changes in the partitioning of W, which acts as a ă former in Ni- and a ă′ former in Co-base superalloys. We propose that a significant amount of W in ă′ has an inhibitory effect on the diffusion of Al, suppressing the formation of Al2O3 in the ă′ phase of Co-base superalloys. Our approach proved to be very successful for oxidation studies and opens up new opportunities for superalloys research.

5:25 PM Question and Answer Period