NiCoCrAlYHf coating evolution through multiple refurbishment processing on a single crystal nickel superalloy

A combination of creep tests, ex-service blade samples, thermodynamic equilibrium calculations, combined thermodynamic and kinetic calculations, image analysis, chemical composition mapping and heat treatments have been conducted on PWA1483 to determine if microstructural rejuvenation can be achieved when taking the presence of oxidation coatings into account as part of a blade refurbishment strategy. The work has shown that the γˊ morphology changes during creep testing, and that through subsequent heat treatments the γˊ microstructure can be altered to achieve a similar γˊ size and distribution to the original creep test starting condition. Thermodynamic equilibrium calculations have been shown to be helpful in determining the optimum temperatures to be used for the refurbishment heat treatments. The interaction of oxidation resistant coatings with the alloy substrate and refurbishment process have been explored with both experimental measurements and coupled thermodynamic and kinetic calculations. The predictive nature of the coupled thermodynamic and kinetic calculations was evaluated against an ex-service blade sample which had undergone refurbishment and further ageing. In general there was good agreement between the experimental observations and model predictions, and the modelling indicated that there were limited differences expected as a result of two different refurbishment methodologies. However, on closer inspection, there were some discrepancies occurring near the interface location between the coating and the base alloy. This comparison with experimental data provided an opportunity to refine the compositional predictions as a result of both processing methodologies and longer term exposure. The improved model has also been used to consider multiple processing cycles on a sample, and to evaluate the coating degradation between component service intervals and the consequences of rejuvenation of the blade with repeated engine exposure. The results from the experimental work and modelling studies potentially offer an assessment tool when considering a component for refurbishment

A multicomponent diffusion model for prediction of microstructural evolution in coated Ni based superalloy systems

A multicomponent model which can simulate the microstructural evolution of a coated Ni based superalloy system has been developed. The model consists of a one-dimensional finite difference diffusion solver to calculate the component distribution, a power law based model for predicting surface oxidation and a thermodynamic calculation routine for determining the phase evolution. Apart from forecasting concentration and phase profiles after a given thermal history, the model can estimate the losses due to oxidation and the remaining life of a coating based on a concentration and/or phase fraction dependent failure criteria. The phase constitution and concentration profiles predicted by the model have been compared with an experimental NiCoCrAlY coated CMSX-4 system, aged for times up to 10 000 h between 850 and 1050°C, and many experimental features can be predicted successfully by the model. The model is expected to be useful for assessing microstructural evolution of coated turbine blade systems

Modelling the coefficient of thermal expansion in Ni based superalloys and bond coatings

The coefficient of thermal expansion (CTE) of nickel based superalloys and bond coat layers was modelled by considering contributions from their constituent phases. The equilibrium phase composition of the examined materials was determined using thermodynamic equilibrium software with an appropriate database for Ni-based alloys, whereas the CTE and elastic properties of the principal phases were modelled using published data. The CTEs of individual phases were combined using a number of approaches to determine the CTE of the phase aggregate. As part of this work, the expansion coefficients of the superalloy IN-738LC and bond coat Amdry-995 were measured as a function of temperature and compared with the model predictions. The predicted values were also validated with the published data for the single-crystal superalloy CMSX-4 and a number of other Ni based alloy compositions at 1000 K. Very good agreement between experiment and model output was found, especially up to 800°C. The modelling approaches discussed in this paper have the potential to be an extremely useful tool for the industry and for the designers of new coating systems

Modelling the high temperature behaviour of TBCs using sequentially coupled microstructural-mechanical FE analyses

Thermal barrier coatings provide a means of thermal insulation of gas turbine components exposed to elevated temperatures. They undergo severe microstructural changes and material degradation, which have been implemented in this work by means of a sequentially coupled microstructural mechanical calculation that made use of a self-consistent constitutive model within finite element calculations. Analyses for different temperatures and bond coat compositions were run, which reproduced the trends reported in previous research and identified the accumulation of high out-of-plane tensile stresses within the alumina layer as an additional phenomenon that could drive high temperature crack nucleation

Characterization and Modelling of Ni Based Superalloy Materials with a Dual Layered MCrALY Coating System

Multilayered MCrAlY type coatings have the potential to be optimised to protect superalloy components against both corrosion and oxidation simultaneously. In this paper, a dual layered coating (NiCrAlY on top of a CoNiCrAlY coating) has been aged and the microstructural evolution has been observed and quantified using various electron microscopy based techniques. In addition, the microstructural evolution of the coating has been modelled using a diffusion based model capable of modelling the ageing of multilayer coatings. It is demonstrated that the model is reasonably accurate in predicting both the phase and chemical distributions through the coating system during ageing, and therefore has the potential to be used as a design tool for the improvement of future layered coating systems

Characterization and modelling of Ni based superalloy materials with a dual layered MCrAlY coating system

Multilayered MCrAlY type coatings have the potential to be optimised to protect superalloy components against both corrosion and oxidation simultaneously. In this paper, a dual layered coating (NiCrAlY on top of a CoNiCrAlY coating) has been aged and the microstructural evolution has been observed and quantified using various electron microscopy based techniques. In addition, the microstructural evolution of the coating has been modelled using a diffusion based model capable of modelling the ageing of multilayer coatings. It is demonstrated that the model is reasonably accurate in predicting both the phase and chemical distributions through the coating system during ageing, and therefore has the potential to be used as a design tool for the improvement of future layered coating systems

Modeling of microstructural evolution in an MCrAlY overlay coating on different superalloy substrates

A multicomponent, one-dimensional diffusion model that was developed for simulating microstructure evolution in coated gas turbine blade systems has been used to compare the phase structures of three MCrAlY coated superalloy systems. The model is based on finite differences and incorporates oxidation and equilibrium thermodynamic computations. The superalloy substrates considered were the nickel-based superalloy CMSX-4, a high-Cr single-crystal superalloy, and a cobalt-based MAR-M509, and these were all coated with an MCrAlY bond coat of similar composition. The results predicted by the model have been compared with similar experimental systems. The model can predict many features observed experimentally and therefore can be expected to be a useful tool in lifetime prediction and microstructural assessment of turbine blade systems based on superalloys. The work also highlighted the fact that for a given coating, the phase evolution within system is dependent on the substrate material