M. Karunaratne

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

Abstract 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.

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.

MCrAlY creep behaviour modelling by means of finite-element unit cells and self-consistent constitutive equations

The MCrAlY bond coats (BCs) used in thermal barrier coatings (TBCs) undergo severe microstructural changes that affect their creep behaviour. One method to take into account the effect of the high-temperature degradation in the creep properties of these alloys is by means of unit cell calculations that reproduce the microstructure present in the alloy. However, this method is not suitable to be included in large-scale calculations and a self-consistent constitutive model, based on Eshelbys inclusion technique, is presented for that purpose, showing good numerical agreement. Results are compared with experimental data obtained for several MCrAlY BCs.

Finite element modelling of the development of stresses in thermal barrier coatings

Thermal barrier coatings (TBCs) are used to allow higher gas temperatures (and hence greater efficiencies) in power generation gas turbines and/or to lengthen blade lifetimes, by reducing the heat transfer from the combustion gases to the blade substrate materials. However, the lives of TBC-coated components tend to be limited by the growth of an oxide layer between the thermally-insulating top coat and the MCrAlY-coated superalloy substrate; this results in stresses which can lead to spallation (flaking-off) of the top coat. The present abstract gives an overview of a recent programme of modelling work undertaken to understand the development of stresses due to the growth of the oxide layer. Typical examples of the rough interface between top coat and bond coat are characterized in terms of their aspect ratios. Representative geometries are then studied using a series of 2D finite element models of the interface layer. Initial models assumed a simple parabolic growth law for the oxide layer; the models were then developed to consider the evolving properties of the substrate and bond coat, and a more rigorous model of the oxidation process was implemented. The resulting model takes as its input the results of a microstructure evolution model developed at Loughborough University, which provides phase proportions. These in turn are used in conjunction with a constitutive model based upon an analytical homogenisation (based on Eshelby approach) that allows the substrate and bond coat creep and elastic behaviour to be predicted as the microstructure evolves. The formation of the thermally-grown oxide (TGO) is modelled by considering the volume change due to oxidation. In turn, the model predicts the evolution of stresses at positions within the TGO layer. The influences of interface roughness, temperature and bond coat formulations are all explored by running the coupled model with different input parameters.

Modelling of microstructural evolution in multi-layered overlay coatings

Functionally graded, multi-layered coatings are designed to provide corrosion protection over a range of operating conditions typically found in industrial gas turbines. A model incorporating diffusion, equilibrium thermodynamics and oxidation has been developed to simulate the microstructural evolution within a multi-layered coating system. The phase and concentration profiles predicted by the model have been compared with an experimental multi-layered system containing an Al-rich outer layer, a Cr-enriched middle layer and an MCrAlY-type inner layer deposited on a superalloy substrate. The concentration distribution and many microstructural features observed experimentally can be predicted by the model. The model is expected to be useful for assessing the microstructural evolution of multilayer coated systems which can be potentially used on industrial gas turbine aerofoils.

Finite element modelling of development of stresses in thermal barrier coatings

Thermal barrier coatings (TBCs) are used to allow higher gas temperatures (and hence greater efficiencies) in power generation gas turbines and/or to lengthen blade lifetimes, by reducing the heat transfer from the combustion gases to the blade substrate materials. However, the lives of TBC-coated components tend to be limited by the growth of an oxide layer between the thermally-insulating top coat and the MCrAlY-coated superalloy substrate; this results in stresses which can lead to spallation (flaking-off) of the top coat. The present abstract gives an overview of a recent programme of modelling work undertaken to understand the development of stresses due to the growth of the oxide layer. Typical examples of the rough interface between top coat and bond coat are characterized in terms of their aspect ratios. Representative geometries are then studied using a series of 2D finite element models of the interface layer. Initial models assumed a simple parabolic growth law for the oxide layer; the models were then developed to consider the evolving properties of the substrate and bond coat, and a more rigorous model of the oxidation process was implemented. The resulting model takes as its input the results of a microstructure evolution model developed at Loughborough University, which provides phase proportions. These in turn are used in conjunction with a constitutive model based upon an analytical homogenisation (based on Eshelby approach) that allows the substrate and bond coat creep and elastic behaviour to be predicted as the microstructure evolves. The formation of the thermally-grown oxide (TGO) is modelled by considering the volume change due to oxidation. In turn, the model predicts the evolution of stresses at positions within the TGO layer. The influences of interface roughness, temperature and bond coat formulations are all explored by running the coupled model with different input parameters.

Microstructural evolution in coated superalloy systems

Abstract Ni based superalloy systems are used in gas turbine engines for power generation because of their ability to function at high temperature and pressure in oxidative and often corrosive environments for prolonged time periods. A superalloy system is usually composed of a substrate (either conventionally cast, directionally solidified or single crystal), a coating (either overlay or diffusion) to maximise either oxidation and/or corrosion resistance and a ceramic thermal barrier coating to reduce the temperature experienced by the superalloy. Under service conditions the microstructure of the superalloy system evolves, which is often accompanied by a gradual degradation in mechanical properties of the material. It is highly desirable to be able to predict how a particular system would evolve in a given environment so that the part can be replaced before failure. In order to understand the changes that occur in systems with such microstructural complexity, advanced characterisation techniques are required. In this paper examples of how these techniques have been used to quantitatively study changes in the grain size and phase distribution within the coating, unambiguously identify precipitated phases formed within the oxidation layer and at the coating-substrate interdiffusion zone, and provide a full three-dimensional tomographic description of microstructure, are described.