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STRESS RELAXATION OF MCrAlY BOND COAT ALLOYS AS A FUNCTION OF TEMPERATURE AND STRAIN
| Content Provider | Semantic Scholar |
|---|---|
| Author | Wereszczak Haynes, J. Allen |
| Copyright Year | 2012 |
| Abstract | The tensile stress relaxation behavior of two NiCoCrAlY bond coat alloys was examined at several temperatures between 25 and 899°C (1650°F) and at 0.1, 0.3, 0.5, and 0.8% strain. One alloy was made from Praxair's CO211 powder and served as the reference alloy, while the other was a Westinghouse-developed, oxidedispersion-strengthened alloy. The specimens were loaded to the desired tensile strain, at a constant strain rate, and the elastic modulus, yield strength, and yield strain were determined as a function of temperature for the two alloys using the stress/strain information from this loading segment. There was not a statistically significant difference in the high temperature elastic properties between the two alloys, although the oxide-dispersionstrengthened alloy tended to exhibit larger yield strengths. The relaxation data for both alloys were reduced into a form in which instantaneous stressing rate during relaxation was examined as a function of stress and temperature using an Arrhenius power-law model. The oxide-dispersion-strengthened alloy exhibited a larger stress exponent and activation energy than the reference alloy between 677-899°C (1250-1650 °F), and was generally more creep resistant. The results from this study demonstrate that bond coat relaxation should occur during engine operation. Bond coatings fabricated from the oxide-dispersion-strengthened alloy have the potential to reduce residual stresses in the TBC ceramic top coating. INTRODUCTION The operating temperatures of air-cooled superalloy hardware in land-based gas turbine engines can be effectively reduced by application of plasma-sprayed thermal barrier coatings (TBCs). These bi-layer coating systems consist of an oxidation-resistant metallic bond coating overlaid with a thermally-insulating ceramic top coating. A typical bond coat consists of an MCrAlY alloy (where M = Ni and/or Co) applied by plasma-spraying, with NiCoCrAlY being the most commonly used bond coat alloy due to its good ductility [Stringer and Viswanathan, •1994]. The bond coating is intended to provide oxidation protection, hot corrosion resistance and, for plasma-sprayed top coats, a rough surface to which the ceramic top coat can bond. The plasma-sprayed top coating increases the durability of superalloy components by reducing their operating temperature, but these ceramic layers are susceptible to failure by spallation (due to stresses resulting from oxidation and thermal cycling). Degradation of plasma-sprayed TBCs is a complex problem and the factors which dictate the stress state of the ceramic layer are not yet fully understood. It is generally agreed that stresses generated during engine operation are primarily due to (I) thermal expansion mismatches between the metallic and ceramic components during heating and cooling, and (2) oxidation along the metal-ceramic interface [Miller, 1987]. There are also indications that the high-temperature mechanical properties of the bond coat can play an important role in determining TBC durability [Wortman, et al., 1989, Brindley and Whittenberger, 1993, and Brindley, 1997]. Since the MCrAlY bond coat alloys become very ductile at temperatures above 700°C [Hillery, 1988], they typically relax thermally-induced stresses imposed by the superalloy substrate and ceramic top coat, which can alter the stress state of the coating system. Furthermore, it has been recognized that bond coat mechanical properties affect adhesion of the thermally grown oxide layer [Evans, 1995]. Several factors provided motivation for the present study. First, it appears that the stress relaxation behavior of the bond coat alloy exerts a strong influence on the residual stress state of the ceramic top coating. Relaxation of the bond coat at elevated temperatures results in an effective increase in the stress-free temperature, which subsequently induces larger compressive stresses in the ceramic top coat during cooling to ambient temperature. Increased stresses can accelerate fracture and spallation of the top coat. Second, the thermally-induced strains Presented at the International Gas Turbine & Aeroengine Congress & Exhibition Stockholm, Sweden — June 2—June 5, 1998 Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 12/01/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use |
| File Format | PDF HTM / HTML |
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| Language | English |
| Access Restriction | Open |
| Subject Keyword | BOND Ceramics Coating Excipient Computer cooling Cool - action Corrosion of Medical Device Material Durability (database systems) Elastic Modulus Elegant degradation Exponent Gases Heating Instruction creep Lagrangian relaxation Large Linear programming relaxation Metal Ceramic Alloys Modulus robot Moore's law Plasma Active Relaxation (approximation) Residual stress Specimen Stress ball Stress testing (software) TBC Thermal barrier coating Thermal spraying Westi anatomical layer ductile nickel oxidation physical hard work |
| Content Type | Text |
| Resource Type | Article |
