In this study, a temperature dependent elastic-viscoplastic cohesive zone model is developed to predict the thermomechanical fatigue (TMF) behaviour of superalloys. The theory is based on rheological models enhanced with fatigue and creep damage variables which are micro-mechanically motivated. This cohesive law can capture monotonous, cyclic, thermomechanical, and rate dependent loads.The material model is used to simulate the deformation behaviour and to determine the lifetime for isothermal, in-phase- (IP) and out-of-phase- (OP) TMF loading. The capability of the model to capture phase difference between mechanical and thermal loading is demonstrated, and the effects are analysed.

The focus of this paper is the simulation of the thermal-mechanical fatigue behavior (TMF) of two single crystalline Nickel-based superalloys in a temperature range between 400°C and 980°C. The newly developed rhenium-free alloy Astra-3OptW and the rhenium-free alloy CMSX-6 are analyzed concerning the basic deformation mechanisms, i.e. elasticity, time-independent and time-dependent plasticity contributing to hardening. In detail, the relevant parameters for high temperature deformation are identified from isothermal creep experiments and used in a numerical model to simulate the deformation behavior under instationary thermal and mechanical loading. Special attention is focused on the determination of the hardening by the second phase (γ′-precipitates) and their influence on time-dependent deformation and relaxation mechanisms. Therefore, the parameters describing the stress and temperature dependence of the creep rate (i.e. stress exponent n and activation energy Q) are interpreted in terms of a threshold stress taking into account the hardening contribution of the γ′-phase. Thus, only a reduced effective stress is active for plastic deformation. Particular attention is focused on the accurate determination of the threshold stress as a function of temperature and applied stress from the Langeborg-Bergmann-plot. The comparison of the simulated TMF-deformation to the experimental TMF-data clearly indicates the accuracy of the model in predicting the resulting stresses induced by instationary thermal and mechanical loading.

The effect of the minor elements B, Zr, and C on the castability of a Nickel-based γ′-strengthened superalloy has been investigated. Tube-like specimens were prepared by directional solidification where the rigid ceramic core leads to hoop stresses and grain boundary cracking. It was found that an important improvement in castability can be achieved by adjusting the minor elemental composition. Too low C (≤0.15 pct) and too high B and Zr contents (≥0.05 pct) lead to material that is very prone to solidification cracking and should be avoided. The results cannot be rationalized on the basis of the current models for solidification cracking. Instead, pronounced hot tearing is observed to occur at high amounts of γ/γ′-eutectic and high Zr contents. The critical film stage where dendrites at the end of solidification do not touch and are separated by thin liquid films must be avoided. How Zr promotes the film stage will be discussed in the paper.

Using the predictions made by a microstructure sensitive creep model for high-gamma' -containing superalloys, this study will compare the predictions made for the dislocation activity during different stages of the microstructural evolution against experimental results in order to understand the role and influence of microstructural coarsening. Furthermore a composition simulation was undertaken to present optimization potential with respect to microstructural coarsening properties.

The creep and oxidation behavior of the second generation nickel based superalloy PWA1484 is investigated. Particular attention is given to the testing of thin samples with a maximum thickness of 1.0 mm. The specimen were uncoated or aluminized. The oxidation experiments were carried out stress free up to 1000 h at temperatures of 980 degrees C and 1100 degrees C in ambient air. The uncoated samples show three different oxide layers at both temperatures followed by a small altered region of the initial gamma/gamma'-microstructure. The thickness of each layer is growing with extended oxidation times. In contrast, the aluminized samples show a small oxide layer of about one micron, but after long exposition to high temperatures a formation of TCP-phases. These phases have a needle to plate-like structure and extend far into the original structure. In addition, the NiAl coating converts to a martensitic structure.Uncoated and aluminized specimens were tested in creep at temperatures of 980 degrees C and 1100 degrees C with stresses up to 170 MPa in air. Both, the uncoated and aluminized samples show a great scatter in rupture time and strain rate at a temperature of 980 degrees C regardless of the sample thickness. The thickness debit effect was not observable. However, at temperatures of 1100 degrees C the thickness debit effect was clearly measured with both sample types. The scatter in rupture time and minimum creep rate is small.

A framework for accelerated alloy development of Ni-base superalloy single crystals is presented. It combines CALPHAD simulations with fast experimental feedback. The hardening efficiency of refractory elements at 980 degrees C was assessed within this framework. The creep strength of modern two phase superalloys is dependent to a high degree by the strength of the matrix phase. It is shown that the combination of computer simulations, i.e. the alloys-by-design approach, with fast experimental feedback is crucial.

The solid-solution hardening potential of the refractory elements rhenium, tungsten and molybdenum in the matrix of single-crystal nickel-based superalloys was experimentally quantified. Single-phase alloys with the composition of the nickel solid-solution matrix of superalloys were cast as single crystals, and tested in creep at 980°C and 30–75MPa. The use of single-phase single-crystalline material ensures very clean data because no grain boundary or particle strengthening effects interfere with the solid-solution hardening. This makes it possible to quantify the amount of rhenium, tungsten and molybdenum necessary to reduce the creep rate by a factor of 10. Rhenium is more than two times more effective for matrix strengthening than either tungsten or molybdenum. The existence of rhenium clusters as a possible reason for the strong strengthening effect is excluded as a result of atom probe tomography measurements. If the partitioning coefficient of rhenium, tungsten and molybdenum between the γ matrix and the γ′ precipitates is taken into account, the effectiveness of the alloying elements in two-phase superalloys can be calculated and the rhenium effect can be explained.

Improving the creep resistance of the matrix by alloying with refractory elements is a major strengthening effect in nickel-based superalloy with rhenium as one of the most effective elements. In this work, the influence of rhenium on creep properties of single-phase single crystals with varying rhenium content and matrix-near composition is investigated. The use of single-crystalline material leads to very distinct results which are not deteriorated by grain boundary effects. So the strengthening effect can be solely attributed to the alloying element rhenium and is quantified for the first time. By comparing the creep strength of two matrix compositions with the corresponding single-crystal superalloys using the threshold stress concept, the potential of creep strengthening of the matrix in two-phase single-crystal alloys is quantified.

The nickel-based alloy Haynes 230 was investigated under low-cycle fatigue (LCF) and thermo-mechanical fatigue loading at different temperatures and with different phase shift. Whereas the yield strength is low, causing high plastic strains in every cycle, appreciable cyclic hardening up to double the yield strength was observed. The crack propagation rates were evaluated at different temperatures, with different mean stress and a standard waveform. A dwell signal was also tested. The crack propagation was modelled assuming a plasticity controlled fatigue part, which was correlated to the crack tip opening displacement by modified cyclic J-integral. At higher temperatures, an additional thermally activated oxide embrittlement increased the crack growth rate. Both mechanisms were considered separately in the model as they work independently. This model enabled us to describe the experimental growth rate, but cannot account for the threshold effects. As the cyclic life was found to be controlled by crack propagation starting from carbides or oxide spikes, life was predicted based on crack propagation. The results compare well for LCF within a scatterband of factor 2 at the lower cycles to failure. For higher cycle numbers, the measured lives were higher, which was assumed to be caused by the lack of a crack growth threshold in the model.

Due to high temperatures and mechanical loads, cracks are initiated in aero engine turbine blades which limit the cyclic life of these components. The materials used for components which underlie high thermal and mechanical load are single crystalline (SX) nickel based super alloys that in most cases contain a certain amount of rhenium. Dramatically increasing Re prices lead to the development of Re-free alloys.In this work, low-cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests were carried out on the Re-free single crystal M-247LC SX. The test results are shown and a model based on crack propagation was used to predict LCF and TMF life. It was shown, that the modeling results fit properly for out-of-phase TMF and LCF life while for in-phase TMF differences between calculated life and experiments occur due to a different mechanism of fracture.

The influence of specimen thickness on creep behavior of coated and uncoated specimens was investigated. Creep experiments were carried out on the single crystal Ni-base superalloy M247LC SX at 980 degrees C and 1100 degrees C. Tests were performed at different stress levels for 0.3 mm and 1.0 mm flat specimens in vacuum and ambient air.A decrease in creep strength with decreasing thickness was observed. Aluminized specimens showed less scatter in creep behavior in terms of both, rupture times and minimum creep rates. If only the initial two-phase area is considered (without interdiffusion zone and coating layer) the influence of specimen thickness on creep behavior were found to be negligible.The results show, that for turbine blade design the influence of wall thickness has to be considered. Thin-walled structures (below 1.0 mm) should be aluminized in order to reduce scatter in material behavior and to minimize the influence of oxidation on matrix/gamma'-microstructure in order to form gamma' reduced or gamma' depleted zones. (C) 2012 Elsevier B.V. All rights reserved.

Microstructure evolution in single-crystal superalloys is modelled as the interplay between oxide growth and substrate response. This is of particular importance for specimens with reduced wall thicknesses, where the affected substrate constitutes a significant fraction of the material. A model based on thermodynamic and kinetic data only is presented in order to predict the growth kinetics of oxides and the resulting influence on microstructure evolution of the substrate. The present work focuses on alumina (Al2O3) growth as it is the most important oxide regarding long-term behaviour. Al2O3 growth is described using a dynamic growth parameter which is derived using thermodynamic and kinetic principles. The substrate response model calculates the distribution of the alloying elements as well as the evolution of the phase fractions as a function of depth and oxidation time. The model has been applied on the strong alumina-forming alloy RenéN5 and the weak alumina-forming alloySCA425+. Since γ′fraction is one of the most relevant factors for high temperature creep properties, the present work concentrates on the calculation of the time- and space-dependent γ′precipitate fraction profile. The model predictions were verified with very good agreement with respect to Al2O3 growth, element distribution and γ′fraction distribution.

The microstructural evolution of uncoated single crystal superalloys is modeled taking into account the interplay between oxide growth and substrate response. Experimental investigations demonstrate that gamma' fraction of specimens with thicknesses less than 1 mm are strongly affected by oxidation. A model based on thermodynamic and kinetic data only, is presented calculating the growth kinetics of oxide scales and the resulting influence on microstructure evolution of the substrate. The model combines models for oxide growth and substrate response. Currently the main focus is on alumina (Al2O3) scale growth as it is the most important oxide for long term behavior. A dynamic growth parameter is used to describe the growth rate of the alumina scale. The model predicts the distribution of the alloying elements as well as the evolution of the generated phases as functions of depth and oxidation time. The model has been applied to three different alloys: the strong alumina forming alloy Rene N5, the moderate alumina forming alloy M247LC SX and the weak alumina forming alloy SCA425+. Since the gamma' fraction is one of the most relevant factors for high temperature creep properties, the present work concentrates on the calculation of the time and space dependent gamma' precipitate profile, which is most important for thin wall specimens. The predictions have been verified with very good agreement at an oxidation temperature of 980 degrees C with respect to alumina scale growth and gamma' fraction distribution. Predicted and measured alumina scale growth and gamma' fraction distribution for oxidation at 980 degrees C are in very good agreement.

The elastic properties of thermal barrier coatings (TBCs) are important for modelling the lifetime of these coatings. A new test setup has been developed to measure the system modulus of electron-beam enhanced physical vapour deposited (EB-PVD) TBC coatings by miniaturized bend tests. Due to the brittleness, low stiffness and small thickness of the top coat and its complex microstructure, it is difficult to measure its Young's modulus by standard mechanical testing. For this reason, a special sample material has been prepared which consists of a 1 mm thick layer of EB-PVD TBC. This material was isothermally heat treated for different times at 950 degrees C, 1100 degrees C and 1200 degrees C and then tested in a specially developed miniaturized bend test. The bend test setup permits mechanical tests with a high resolution in stress and strain, where the strain is measured by digital image correlation. So the stiffness of the freestanding TBC samples could be measured with a high accuracy and the sintering behaviour of the EB-PVD TBC and the consequent rise of Young's modulus could be determined. The results show a significant increase of the system modulus with heat treatment time and temperature caused by sintering of the coating. An activation energy of 220 kJ/mol for the process has been determined.In addition, the material was tested by nanoindentation in order to measure Young's modulus on a local scale, and the porosity of the samples was determined by quantitative image analysis. (C) 2010 Elsevier B.V. All rights reserved.

In this paper, a thermal barrier coating system is studied numerically using finite elements. If the system is isothermally compressed, large stresses at the coating interface may develop due to the differences in Young’s modulus. Since the coefficient of thermal expansion of the substrate differs from those of the coating materials, axial loading of the coating system is also relevant upon cooling. By artificially changing the coefficient of thermal expansion of the coating materials, the effect of axial loading and of stresses due to thermal differential strains in radial direction can be evaluated. Consequences for mechanical loading are discussed, showing that out-of-phase loading of a TBC system unloads critical regions of the TBC on the microscale.

Axial stress components caused by radial temperature gradients in TMF specimens lead to local control deviations in temperature and mechanical strain. Measurements and simulations have been carried out in order to estimate the level of control deviation in non-isothermal heated TMF specimens.Thermal cycle tests were carried out with a variety of specimen coil configurations which cause different radial temperature profiles. Temperature gradients were measured by means of instrumented specimens under different heating and cooling rates.In order to calculate the resulting stresses, model parameters of a thermal–mechanical FE-analysis (e.g., coefficient of convection, emissivity and heat transfer) were fitted on the basis of the measured temperature distribution.TMF tests have been carried out on Ni90 specimens with two induction coil configurations and with two different heating and cooling rates. The experimental results were reported and discussed on the basis of the measured and simulated results.

Brazing is a well established repair technique for high temperature components in both industrial gas turbines and aero engines. Conventional nickel base braze alloys contain boron or silicon as melting point depressing elements. The major benefit of boron and silicon compared to other melting point depressants is its large effect on the melting point and its high diffusion coefficient in nickel base superalloys. However these elements promote precipitation of undesired brittle phases during the brazing process. To avoid these phases, transient liquid phase bonding in combination with boron and silicon free brazing alloys will be examined in this work. The influence of the brazing temperature on solidification and diffusion behaviour during transient liquid phase bonding for a single crystalline first generation and a second generation superalloy will be reported. Our experiments show that isothermal solidification without precipitation of brittle phases in the braze joint or the base material can be achieved. The brazed joint consists of fine γ/γ´ microstructure. EBSD measurements demonstrated that the single crystalline orientation of the base material was maintained throughout the joint. Electron probe micro analysis is used to characterize the diffusion behaviour. Solidification velocity will be compared with the theory of transient liquid phase bonding established by Tuah-Poku [1].