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This work concerns the fatigue crack growth behaviour of nickel base single crystal superalloys. The main industrial application of this class of materials is in gas turbine blades, where the ability to withstand severe mechanical loading in combination with high temperatures is required. In order to ensure the structural integrity of gas turbine blades, knowledge of the fatigue crack growth behaviour under service-like conditions is of utmost importance. The aim of the present work is both to improve the understanding of the crack growth behaviour of single crystal superalloys and to improve the testing and evaluation methodology for crack propagation under thermomechanical fatigue loading conditions. Single crystal superalloys have anisotropic mechanical properties and are prone to localization of inelastic deformation along the close packed planes of the crystal lattice. Under some conditions, crystallographic crack growth occurs along these planes, and this is a complicating factor throughout the whole chain of crack propagation life simulation; from material data generation to component calculation. Crack growth testing has been performed, both using conventional isothermal testing methods and using thermomechanical fatigue crack growth testing. Experimental observations regarding crystallographic crack growth have been made and its dependence on crystal orientation and testing temperature has been investigated. Quantitative crack growth data are presented for the case of Mode I crack growth under isothermal as well as thermomechanical fatigue conditions. Microstructural investigations have been undertaken to investigate the deformation mechanisms governing the crack growth behaviour. A compliance-based method for the evaluation of crack opening force under thermomechanical fatigue conditions was developed, to enable a detailed analysis of the test data. The crack opening force evaluation proved to be of key importance for the understanding of the crack driving, Detta arbete behandlar sprickpropageringsbeteendet hos monokristallina nickelbaserade superlegeringar. Den huvudsakliga industriella tillämpningen för denna materialgrupp är som skovelmaterial i gasturbiner, vilket kräver förmågan att motstå hög mekanisk belastning i kombination med höga temperaturer. För att säkerställa gasturbinskovlarnas hållfasthet är kunskap om sprickpropageringsbeteendet, under driftsliknande förhållanden, av yttersta vikt. Målet med detta arbete är både att förbättra förståelsen för monokristallina nickelbaslegeringars sprickpropageringsbeteende och att förbättra metodiken för sprickpropageringsprovning med termomekanisk utmattningsbelastning. Monokristallina superlegeringar har anisotropa mekaniska egenskaper och en benägenhet till lokalisering av inelastisk deformation längs kristallgittrets tätpackade plan. Under vissa förhållanden sker kristallografisk spricktillväxt längs dessa plan och detta är en komplicerande faktor genom hela kedjan av sprickpropageringssimulering; från materialdatagenerering till komponentberäkningar. Sprickpropageringsprovning har utförts både med konventionella isoterma provningsmetoder och med termomekanisk sprickpropageringsprovning. Kristallografisk sprickpropagering har studerats experimentellt för att klargöra dess beroende av kristallorientering och temperatur. Kvantitativa sprickpropageringsdata presenteras för sprickväxt i Modus I, under isoterm såväl som termomekanisk utmattningsbelastning. Mikrostrukturundersökningar har gjorts för att studera deformationsmekanismerna som styr sprickpropageringsbeteendet. En kompliansbaserad metod för att utvärdera spricköppningskraften vid termomekanisk utmattning har utvecklats, för att möjliggöra en djupgående analys av provdatat. Spricköppningsutvärderingen visade sig ha en nyckelroll för förståelsen av drivkraften för sprickpropagering vid olika provningsförhållanden. Inverkan av hålltid på sprickpropageringsbeteendet analyserades både med avseende på krypspricktill, Funding agencies: The Swedish Energy Agency and Siemens Energy

The crack growth behaviour of the alloy CM 247 LC is investigated for out-of-phase TMF and isothermal tests at the same temperature as the minimum temperature in the TMF tests. The results suggest that it is possible to characterise crack growth behaviour if experimental corrections for crack closure are accounted for. The repli -cation of these experimental tests using a numerical FE-solver results in similar crack growth behaviour, sug-gesting that the main mechanism in place is plasticity-induced crack closure. A pragmatic analytical model to characterise crack closure including hold time effects is proposed. The comparison of the response from this model with the experimental and numerical results suggests that the proposed analytical model is capable to approximate crack closure effects for cases where substantial creep deformation is to be expected., Funding Agencies|Link oping University; Siemens Energy

Crack closure effects are known to have a large impact on crack growth behaviour. In this work, tests were performed on Inconel 792 specimens under TMF loading conditions at 100–850 °C with extended hold times at tensile stress. The effective stress-intensity range was estimated experimentally using a compliance-based method leading to the conclusion that crack closure appears to have a primary impact on the crack growth behaviour for this material under the conditions studied. The crack closure behaviour for the tests was successfully modelled using numerical simulations, including creep., Ytterligare forskningsfinansiär: Siemens Industrial Turbomachinery AB through “Turbines for Future Energy Systems” (Turbiner för framtidens energisystem), Grant No. 44100-1

Single-crystal nickel-base superalloys frequently experience two distinct fatigue crack growth modes. It has been observed that, under certain conditions, cracks transition from a path perpendicular to the loading direction to a crystallographic slip plane. As crystallographic cracking is associated with an increased fatigue crack growth rate, it is important to be able to predict when this transition occurs. In this work three different criteria for crystallographic cracking based on resolved anisotropic stress intensity factors are evaluated in a three-dimensional finite element context. The criteria were calibrated and evaluated using isothermal fatigue experiments on two different specimen geometries. It is suggested by the results, that a threshold value of a resolved shear stress intensity factor can act as a conservative criterion indicating cracking mode transition. Further, a trend hinting towards a loading frequency dependency could be observed., Funding Agencies|Linkoping University; Siemens Industrial Turbomachinery AB

Cracks in single-crystal nickel-base superalloys have been observed to switch cracking mode from Mode I to crystallographic cracking. The crack propagation rate is usually higher on the crystallographic planes compared to Mode I, which is important to account for in crack growth life predictions. In this paper, a method to evaluate the crystallographic fatigue crack growth rate, based on a previously developed crystallographic crack driving force parameter, is presented. The crystallographic crack growth rate was determined by evaluating heat tints on the fracture surfaces of the test specimens from the experiments. Complicated crack geometries including multiple crystallographic crack fronts were modelled in a three dimensional finite element context, The data points of the crystallographic fatigue crack growth rate collapse on a narrow scatter band for the crystallographic cracks indicating a correlation with the previously developed crystallographic crack driving force., Funding Agencies|Linkoping University; Siemens Industrial Turbomachinery AB

The inherent anisotropy of single-crystal nickel-base superalloys brings many difficulties in terms of modelling, evaluation and prediction of fatigue crack growth. Two models to predict on which crystallographic plane cracking will occur is presented. The models are based on anisotropic stress intensity factors resolved on crystallographic slip planes calculated in a three-dimensional finite-element context. The developed models have been compared to experiments on two different test specimen geometries. The results show that a correct prediction of the crystallographic cracking plane can be achieved. This knowledge is of great interest for the industry and academia to better understand and predict crack growth in single-crystal materials., Funding Agencies|Swedish Energy Agency; Siemens Industrial Turbomachinery AB through the Research Consortium of Materials Technology for Thermal Energy Processes [KME-702]

A higher gas turbine efficiency can be achieved by increasing the operating temperature in hot sections. AD730™ is a recently-developed wrought/cast nickel-based superalloy which can maintain excellent mechanical properties above 700 ℃. However, machining of AD730™ could be a difficult task like other nickel-based superalloys. Therefore, studies are needed with respect to the machinability of this new alloy. In this paper, high-speed turning was performed on AD730™ using polycrystalline cubic boron nitride (PCBN) tools and coated tungsten carbide (WC) tools at varied cutting speeds. The surface integrity was assessed in two important aspects, i.e., surface and sub-surface plastic deformation and residual stresses. The PCBN tools generally showed better performance compared with the WC tools since it led to reduced machining time without largely compromising the surface integrity achieved. The optimal cutting speed was identified in the range of 200-250 m/min when using the PCBN tools, which gives rise to a good combination of machining efficiency and surface integrity. The further increase of the cutting speed to 300 m/min resulted in severe and deep plastic deformation. Meanwhile, a continuous white layer was formed at the machined surface. When turning with the WC tools, the increased cutting speed from 80 m/min to 100 m/min showed very little effect with respect to the plastic deformation on the machined surface. It was found that tensile residual stresses were developed on all machined surfaces no matter when the PCBN or WC tools were used, and the surface tension was generally increased with increasing cutting speed. The tensile layer might need to be modified by e.g., post-machining surface treatments such as shot peening, if taking good fatigue performance into consideration.

Single-crystal nickel-base superalloys are often used in the hot sections of gas turbines due to their good mechanical properties at high temperatures such as enhanced creep resistance. However, the anisotropic material properties of these materials bring many difficulties in terms of modelling and crack growth prediction. Cracks tend to switch cracking mode from Mode I cracking to crystallographic cracking. Crystallographic crack growth is often associated with a decrease in crack propagation life compared to Mode I cracking and this must be taken into account for reliable component lifing. In this paper a method to evaluate the crystallographic crack propagation rate related to a crystallographic crack driving force parameter is presented. The crystallographic crack growth rate is determined by an evaluation of heat tints on the fracture surface of a specimen subjected to fatigue loading. The complicated crack geometry including two crystallographic crack fronts is modelled in a three dimensional finite element context. The crack driving force parameter is determined by calculating anisotropic stress intensity factors along the two crystallographic crack fronts by finite-element simulations and post-processing the data in a fracture mechanics tool that resolves the stress intensity factors on the crystallographic slip planes in the slip directions. The evaluated crack propagation rate shows a good correlation for both considered crystallographic cracks fronts., Funding Agencies|Swedish Energy Agency; Siemens industrial Turbomachinery AB through Research Consortium of Materials Technology for Thermal Energy Processes [KME-702]

The elastic and plastic anisotropy of the single-crystal materials bring many difficulties in terms of modeling, evaluation and prediction of fatigue crack growth. In this paper a single-crystal material model has been adopted to a finite element-environment, which is paired with a crack growth tool. All simulations are performed in a three-dimensional context. This methodology makes it possible to analyze complex finite element-models, which are more application-near than traditional two-dimensional models. The influence of the crystal orientation, as well as the influence of misalignments of the crystal orientation due to the casting process are investigated. It is shown that both the crystal orientation and the misalignment from the ideal crystal orientation are important for the crack driving force. The realistic maximum limit of 10 degrees misalignment is considered. It can be seen that crack growth behavior is highly influenced by the misalignment. This knowledge is of great interest for the industry in order to evaluate the crack growth in single-crystal components more accurately.

In this study the effect of SO2+water vapour on strain controlled low cycle fatigue resistance of three different nickel based superalloys has been studied at 450 °C and 550 °C. A negative effect was found on both the crack initiation and crack propagation process. The effect increases with increasing temperature and is likely to be influenced by both the chemical composition and the grain size of the material. In general the negative effect decreases with decreasing strain range even if this means that the total exposure time increases. This is explained by the importance of the protective oxide scale on the specimen surface, which is more likely to crack when the strain range increases. When the oxide scale cracks, preferably at the grain boundaries, oxidation can proceed into the material, causing preferable crack initiation sites and reduced fatigue resistance.

The applicability of fine grain polycrystalline nickel base superalloys in gas or steam turbine applications is often limited by their susceptibility to fast intergranular cracking during fatigue in combination with extended hold times at high temperatures and high tensile stresses. This effect is further enhanced in corrosive environments even at moderate temperatures such as 400-600°C. In this study the negative effect of SO2 + water vapour on the low cycle fatigue resistance of three different nickel based superalloys (Nimonic 901, Inconel 718 and 718plus) has been studied at 450°C and 550°C. A negative effect was found on both the crack initiation and crack propagation process. The effect increases with increasing temperature and is likely to be influenced by both the chemical composition and the grain size of the material. The negative effect of water vapour + SO2 is manifested by a decreased resistance to cyclic plastic deformation and a transition from transcrystalline to intercrystalline fracture behaviour. In Nimonic 901 this negative effect increases with the degree of plastic deformation. For lower mechanical strain amplitudes where the number of cycles and the total exposure time is increased the environmental impact is reduced. Similar trends can also be noticed for the other alloys. The surface corrosion in air and in SO2 + water vapour are found to be rather similar in this study and it is therefore concluded that the surface scale can remain adherent and protective if the strains on the oxide scale are low. However, for LCF tests with higher strain ranges, the oxide scale will rupture preferably at the grain boundaries and intergranular microcracks will initiate which promotes inward diffusion of embritteling elements such as oxygen and sulphur. Inconel 718 seems to be more sensitive to SO2 + water vapour than Nimonic 901 when tested at 450°C. The overall resistance to LCF is however still higher in Inconel 718 for the test conditions investigated in, Att det finns en negativ inverkan från korrosiva ämnen på utmattningsegenskaperna hos Nickelbas-material även vid förhållandevis måttliga temperaturer (400-600ºC) är ett faktum som är allmänt vedertaget men som sällan har studerats i noggranna laboratorieförsök. Interaktion mellan t.ex. syre och mekaniska spänningar i en sprickspets kan ge upphov till försprödningsmekanismer som kan vara förödande för materialets egenskaper. I denna studie har inverkan från SO2 (i gasfas) och vattenånga på lågcykelutmattningsegenskaperna (LCF) undersökts vid temperaturer kring 500ºC för 3 st olika smidda nickelbaslegeringar (Nimonic 901, Inconel 718 och Alloy 718plus). Syftet har varit att undersöka känsligheten för miljöinverkan på egenskaperna hos turbinskivor vid relativt långa driftstider. Laboratorieförsöken har dock accelererats genom förhöjda temperaturer och högre halter av korrosiva ämnen. Lågcykelutmattningsförsök har utförts vid olika temperaturer och i olika miljöer. Brottbeteendet hos dessa prover har studerats med hjälp av SEM. Från studien har följande slutsatser dragits: • Det finns en negativ effekt av SO2 + vattenånga redan vid korta exponeringstider och så måttliga temperaturer som 450°C. Den negativ effekten ökar med temperaturen. Det är även troligt att det finns en negativ effekt vid ännu lägre temperaturer om drifttiderna är tillräckligt långa. • Den negativa effekten av SO2 + vattenånga ger upphov till minskad motståndskraft mot cyklisk plastisk deformation samtidigt som brottbeteendet övergår från transkristallint till interkristallint. • Inconel 718 verkar vara mest känslig för miljöinverkan. Denna legering har från början ett mycket bra motstånd mot lågcykelutmattning i luft och klarar sig i en jämförelse även bra i de försök som har gjorts i miljö i denna studie, även om miljöfaktorn var störst för denna legering. • Den största negativa effekt av SO2 + vattenånga ses för höga töjningsomfång och få antal cykler till brott, även om detta innebär den kortast