Turbine Disc Alloy Research Articles

2D and 3D characterization of oxidation‐fatigue mechanisms in an advanced Ni‐based superalloy: Effects of microstructure and elevated temperature

AbstractThe effects of varying γ′ size and grain boundary carbide distribution at elevated temperatures on the fatigue behavior of turbine disk alloy RR1000 were examined through combining 2D and 3D imaging characterization. The fatigue crack growth rate at 725°C is higher than at 650°C due to enhanced oxidation damage at higher temperature, dominated by an intergranular fracture mode. This is more marked with longer dwell time (linked to diffusion time) and when carbides have formed continuously at grain boundaries. The fatigue crack path was characterized by X‐ray CT scan to present the complex 3D crack propagation morphology. It is noted that a discontinuous crack path and uncracked ligaments can be discerned ahead of the crack tip particularly in the microstructure where carbides are continuously formed at grain boundaries. This is linked to embrittled zones ahead of the crack tip formed during longer dwell times at the maximum stress during cycling at elevated temperature.

Investigation of the precipitation behavior and its role on creep deformation and failure mechanisms at high temperatures for a turbine disk alloy

The creep deformation behavior of a turbine disk alloy at high temperatures was studied, and the role of the multi-scale precipitates on the creep deformation and failure mechanisms was revealed. The turbine disk alloy shows a typical microstructure with the micron-scale MC carbides, submicron delta phase, together with nanoscale γ″ and γ' precipitates distributed in the matrix after hot working and aging treatment. The MC carbides are mostly formed from the solidification process due to enrichment of Nb in the inter-dendritic areas. The creep life decreases from 763.5 h to 5.7 h when the creep temperature increases from 953 K to 1023 K at 500 MPa. The fracture surfaces exhibit a transgranular fracture pattern accompanied with the formation of substantial microvoids. Slipping occurs in both unidirectional and bidirectional modes during deformation. The microvoids along the grain boundaries are generated through decohesion of δ precipitates from the matrix or the interaction between slip traces and grain boundaries. When the intragranular MC carbides interact with the slip traces, microcracks can be formed by particle fracturing. The dense distribution of particles along the grain boundaries will result in a significantly higher critical stress required for microvoid nucleation, whereas the non-coherent particles inside the grains, particularly the large size MC carbides, can decrease the critical shear stress for microcrack formation to as low as 88.9 MPa, thereby contributing to alloy rupture. For the coherent precipitates, a comprehensive strengthening effect of 389.12 MPa is achievable by the synergistic precipitation of γ″ and γ', which primarily serve to enhance and coordinate intragranular deformation during creep.

LCF life prediction of turbine disc alloy using crystal plasticity and 3D representative volume element containing twin boundaries

The traditional LCF life prediction method based on macroscopic behavior is difficult to reveal the microscopic fatigue failure mechanism. Therefore, the crystal plasticity finite element method and the representative volume elements considering the real microstructure were used to simulate the cyclic deformation of a nickel-based superalloy FGH4098, and a fatigue failure criterion is established to predict the LCF life. The influence of twin boundary on the prediction accuracy is particularly discussed. Meanwhile, the hotspots which are potential sites for fatigue crack initiation were analyzed. The results show that the consideration of twin boundary can improve the accuracy of life prediction, with a scatter factor of ±1.3 only.

Fabrication of a turbine disk alloy by electron beam sheet lamination and investigation on the improved high temperature creep performance

Fabrication of a turbine disk alloy by electron beam sheet lamination and investigation on the improved high temperature creep performance

Methodology for Application of Damage Mechanics Approach to Model High Temperature Fatigue Damage Evolution in a Turbine Disc Superalloy

Aeroengine gas turbine components operate under complex loading environments. Turbine disc is one such component which experiences stresses at high temperature and accumulates life critical cyclic damage in the material during usage. This accumulation of cyclic damage results into significant deterioration in material strength which in turn may initiate the failure in these rotating components. This necessitates the need to develop an advanced lifing approach for fatigue life assessment of turbine disc. As there are no available standards for damage mechanics application, an attempt has been made in the present study to develop a methodology for application of damage mechanics approach to model high temperature fatigue damage evolution in a turbine disc Superalloy. High temperature (650oC) stress-controlled fatigue tests on turbine disc alloy have been performed to evaluate parameters for damage mechanics based models. Using this approach, damage evolution has been simulated at specimen level. A good correlation has been observed in the damage mechanics based model’s predicted damage and experimentally determined values.

Microstructural and oxidation effects on fatigue crack initiation mechanisms in a turbine disc alloy

Effects of microstructure and oxidation on fatigue crack initiation and early propagation processes were investigated in RR1000 turbine disc alloy with different γ′ distributions and carbide distributions on the grain boundary. Fatigue tests were carried out under three-point bending and trapezoidal waveform loading (with a 90 s dwell) at 650 °C in air. The failure mode in both γ′ variants is clearly characterised by intergranular features. A number of fatigue cracks are seen to initiate at grain boundaries with bulged Co-rich oxides at the surface and/or interfaces between carbides and grain boundaries, resulting from oxidation damage assisted by applied loading. Reduced lifetime is closely linked to significant intergranular crack initiation and frequent consequent crack coalescence events, which results in enhanced fatigue crack growth (FCG) rates. The extent of intergranular features and enhanced FCG are more marked where more continuous carbides exist at the grain boundary.

Modelling and predictions of time-dependent local stress distributions around cracks under dwell loading in a nickel-based superalloy at high temperatures

Finite element (FE) analysis modelling of crack configurations was used to provide insights into the role of time-dependent deformation on dwell fatigue crack growth (DFCG) in turbine disc alloys. In particular, the potential influence of time-dependent deformation on fatigue crack growth rates observed during dwell holding periods and the potential crack growth retardation phenomena that can occur for overload-dwell cycles were investigated. The FE model evaluated the mechanical stress state evolution in a two-dimensional cracked geometry subjected to standard dwell and overload-dwell stress waveforms for a given microstructural condition (based on its γ’ particle distribution). Crack-opening stress distributions as a function of dwell time and distance ahead of the crack-tip were interrogated, with the aim to investigate the potential influence of local crack-tip stresses on crack growth during dwell holding periods and the potential for crack growth retardation following overload cycles. Stress distribution profiles predicted by dwell-only simulations showed how quickly local crack-tip stresses relax due to time-dependent plasticity. Characteristic effects of overloads of dwell crack growth resistance observed in experiments were also reasonably well captured. A numerically-derived incubation time was successfully applied in predicting the general trends of achieving more pronounced retardation effects with larger overload factors and extended periods of overloading prior to periods of dwell. FE modelling predicted a significant difference in behaviour between an overload for a given overload factor with a hold time at peak load of 1 s and 10 s. This study emphasises the importance of studying local crack-tip stresses in regards to characterising DFCG behaviour of turbine disc alloys.

Evolution of geometrically necessary dislocation at the γ-γ′ interface and its effect on tensile deformation behaviour of disk super alloy

Abstract The evolution of geometrically necessary dislocation density (GND) at the γ and primary γ′ interface during tensile deformation of turbine disk alloy was studied. Extensive electron microscopy analysis (i.e. EBSD and TEM) was carried out to explore the structure and GND evolution at the γ-γ′ interfaces. It is found that GND value is highest for tensile test specimens at 800 °C, whereas the lowest for tensile specimen tested at 350 °C, and this is attributed to γ/γ′ misfit (δ) value. At high-temperature tensile deformation (i.e. at 720 and 800 °C) specimen starts necking just after the yield point, compared to RT, 350 and 650 °C tensile tested specimens. It is evidenced from GND values and grain interior distortion that localized plastic deformation in γ-matrix near to γ-γ′ interfaces is the primary reason for necking just after yielding of material. Higher GND values and grain interior distortion are caused by the greater value of δ at high test temperature due to difference in thermal expansion between γ and γ′.

Stress-strain calculation and fatigue life assessment of V-shaped notches of turbine disk alloys

For local stress-strain behavior modeling at the notch root, both the Neuber's rule and equivalent strain energy density (ESED) method are commonly utilized in engineering practice, in which the former overestimates notch elasto-plastic responses while the latter gives conservative predictions. In this work, a correction coefficient related to the effective Poisson's ratio veff is proposed to quantify the weight of dissipated heat energy term with the modified ESED model. Notch-root strains predicted by the proposed model are compared with nonlinear FE calculations of GH4169 and TC4 V-shaped notched-bars, in which the proposed solution provides good agreement with FE results for both cases. Then, consulting the idea of local stress-strain approaches, a fatigue lifing model is derived assuming that the notched specimens fail once reach the critical total strain energy as same as that of smooth ones. Results indicated that prediction accuracy of local equivalent strains at notch roots is improved, and yields better life predictions than the Ellyin's model for the two alloys.

A low cycle fatigue life model for a shot peened gas turbine disc alloy

Turbine disks in gas turbines are subjected to cyclic load at high temperature, making, especially the fir tree type blade attachments, susceptible to fatigue. Shot peening of the fir tree attachments may be used to increase the fatigue life by introducing compressive residual stresses. In the current study, both polished and shot peened notched specimens made from alloy 718 were subjected to low cycle fatigue at 450–550 °C. The shot peening generally increased the fatigue life, although the effect diminished for high loads. It was shown that the effect of shot peening could be handled as mean stress effects in a life model based on a Smith–Watson–Topper (SWT) type parameter, σmaxΔε/2. A material model which captured the mean stress was set up to get the SWT parameter at the notch root. It was shown that thermal relaxation of residual stresses and initial strain hardening from cold work could be excluded from the finite element analysis used to establish the mean stress; this since the plasticity in the first cycle dominated the plastic deformation of the specimen. Overall, the SWT-based life model worked satisfactorily. However, the prediction of correct mean stresses at 550 °C proved somewhat difficult as the degree of mean stress relaxation at this temperature varies widely in available literature data.

Critical plane–based multiaxial fatigue life prediction of turbine disk alloys by refining normal stress sensitivity

For engineering components subjected to complex multiaxial loadings, critical plane approaches like Fatemi–Socie criterion have been commonly utilized for life prediction of these components. Within the Fatemi–Socie criterion, the normal stress sensitivity parameter k is usually fitted from additional experimental data, which introduces inconvenience for practice especially under limited testing data conditions. In this regard, a simple critical plane–based damage parameter is put forward with no additional material constants, which attempts to provide a robust method for multiaxial fatigue analysis of turbine disk alloys. Using experimental datasets of TC4 and GH4169 alloys under different loadings, the proposed model provides better correlations with fatigue life of the two alloys than the models of Smith–Watson–Topper and Wang–Brown.

Fatigue reliability analysis of crack growth life using maximum entropy method

In this article, a new method for fatigue reliability analysis of crack growth life based on the maximum entropy theory and a long crack propagation model is proposed. A modified generalized passivation-lancet model for long fatigue crack propagation rate is presented with explicit physical meaning. Experimental results for turbine disk alloy ZSGH4169 under different strain ratios and temperatures (at 650°C and room temperature) are used to verify the applicability of the new model. Results show that predictions by the proposed model are almost identical to the experimental data. The presented model is better than the other three models to reflect the rapid propagation characteristics of the crack. In order to perform fatigue reliability estimation, the probabilities of failure are calculated using the maximum entropy theory based on the fatigue crack growth life that derived from the proposed modified crack propagation model and the above existing three models. Results have shown that maximum entropy theory is very apt for fatigue reliability analysis of turbine disk under different loading conditions with a limited number of samples because it does not need any distribution assumptions for random variables. The effectiveness and accuracy of the combination of fatigue crack propagation models and maximum entropy method for fatigue reliability analysis are demonstrated with examples.

Thermomechanical fatigue crack initiation in disc alloys using a damage approach

A fatigue crack initiation model based on damage accumulation via a fatigue memory surface in conjunction with a plastic strain energy parameter was evaluated for thermomechanical fatigue loading in a gas turbine disc alloy. The accumulated damage in each hysteresis loop was summed up, and it was assumed that the damage at the stable state is repeated until failure occurs. Crack initiation occurs when enough fatigue damage has been obtained, and the number of cycles can thus be directly determined. The fatigue damage is highly coupled to the constitutive behaviour of the material, where the constitutive behaviour was modelled using a non-linear hardening description. Based on this, a stable state was achieved and the obtained damage could be extracted. A user-defined material subroutine was implemented, incorporating both the constitutive description and the fatigue damage accumulation. The framework was adopted in a finite element context to evaluate the thermomechanical fatigue crack initiation life of the disc alloy RR1000. From the evaluation it could be seen that a good prediction of the thermomechanical fatigue life was achieved compared to performed experiments.

Hot corrosion and low cycle fatigue of a Cr2AlC-coated superalloy

Low temperature Type II hot corrosion is a serious problem for low cycle fatigue (LCF) failure of advanced turbine disk alloys operating at increased temperatures. Accordingly, the present effort studied 15–20µm corrosion resistant Cr2AlC sputter coatings on Low Solvus High Refractory (LSHR) disk alloy LCF test specimens. These were cycled to failure at 840/−430MPa and 0.33Hz, after 500h oxidation and 50h of Mg-Na2SO4 hot salt corrosion, all at 760°C. The coating successfully prevented hot corrosion pitting that was responsible for a 90% decrease in uncoated LCF specimens. However, fractography identified unintentional 15–30µm deep defects produced by grit blast surface preparation of coated samples. These acted as failure origins and introduced anomalous life reduction for some coated test specimens. Furthermore, the presence and growth of an undesirable Cr7C3 second phase diminished protectiveness by promoting internal oxidation and embrittlement of the coating.

Probabilistic fatigue life prediction and reliability assessment of a high pressure turbine disc considering load variations

In the present work, a probabilistic framework for fatigue life prediction and reliability assessment of an engine high pressure turbine disc is proposed to incorporate the effects of load variations and mean stress, which provides a reference for engine structural design under a given target failure probability. Within this framework, a new probabilistic fatigue damage accumulation model under random loadings is elaborated based on a ductility exhaustion model, and probabilistic [Formula: see text] curves for the high pressure turbine disc under different flight missions are derived based on experimental data of turbine disc alloy GH4169. The influence of random load variations on fatigue reliability of the high pressure turbine disc has been investigated and quantified by combining the engine load spectrum with finite element analysis.

Enabling Large Superalloy Parts Using Compact Coprecipitation of γ′ and γ′′

Next-generation gas turbines will require disk materials capable of operating at 923 K (650 °C) and above to achieve efficiencies well beyond today’s 62 pct benchmark. This temperature requirement marks a critical turning point in materials selection. Current turbine disk alloys, such as 706 and 718, are limited by the stability of their major strengthening phase, γ′′, which coarsens rapidly beyond 923 K (650 °C) resulting in significant degradation in properties. More capable γ′ strengthened superalloys, such as those used in jet engine disks, are also limited due to the sheer size of gas turbine hardware; the γ′ phase overages during the slow cooling rates inherent in processing thick-section parts. In the present work, we address this fundamental gap in available superalloy materials. Through careful control of Al, Ti, and Nb levels, we show that fine (<100 nm) γ′ and compact γ′/γ′′ coprecipitate structures can be formed even under extremely slow cooling rates from high temperature. The presence of Ti is shown to have a dominant effect on phase formation, dictating whether γ′, γ′/γ′′ coprecipitates, or other less desirable acicular phases form on cooling. Sensitivity to cooling rate and aging heat treatment is also explored. A custom phase field model along with commercial precipitation kinetics software is used to better understand the phase evolution and stability of compact coprecipitates. The alloying strategies discussed here enable a new class of superalloys suitable for applications requiring large parts operating at high temperature.

Impact of solution heat treatment on microstructure and creep behavior of a novel cast & wrought FGH4096 turbine disk alloy

By means of microstructural observation and creep tests, a study has been made into the effects of solution heat treatments on microstructure and creep resistance of a novel cast & wrought FGH4096 superalloy. The results show that grain size increases and the volume fraction of primary γ´ phase decreases with the increase of solution heat treatment temperature. The super-solvus solution heat treatment leads to an increase of the total volume fraction of secondary and tertiary γ´ phases, and superior creep property can be obtained. Various cooling rates from sub-solvus solution heat treatment are applied to gain cooling secondary γ´ phase with different sizes and morphologies. It is found that the solution cooling rate determines secondary γ´ morphology and particle distribution that develops. The alloy samples with fine γ´ particles obtained from the fast solution cooling rate possess narrow matrix channels, which can effectively enhance the Orowan resistance and hinder dislocation movement within the matrix and thus improve the creep resistance at 704°C/690MPa. The creep tests for three samples with different distribution of γ´ particles are interrupted when entering the third stage to investigate the corresponding dominant deformation mechanisms by transmission electron microscopy. It is indicated that partial dislocations shearing precipitates leaving abundant stacking faults behind is the dominant creep failure mechanisms for samples with coarser secondary γ´ particles and wide γ matrix channels. The main creep mechanism for the samples with fine secondary γ´ particles is microtwinning shearing both γ´ particles and matrix.

Impact of compressive hold on out‐of‐phase thermomechanical fatigue crack growth in IN 718

AbstractAccurate characterization and understanding of the fatigue crack growth behaviour of components in jet turbine engines is critical for successfully using a damage tolerant design method to maximise safety and efficiency. The hot section components experience changing loads and temperatures, and hence, fatigue crack growth rates are typically studied under thermomechanical loading. One question that remains unclear is the role of the compressive holds that are often part of an aircraft loading‐temperature spectrum. This experimental study was undertaken to investigate a turbine disk alloy, Inconel 718, subjected to different cycling and temperature profiles considering different lengths of hot compressive holds to determine its effect on the fatigue crack growth rate. It was found that the addition of a compressive hold at temperatures from 650 to 725 °C has no significant impact on the fatigue crack growth rate when compared with a cycle without a compressive hold. Fractographic analysis shows that crack growth is primarily transgranular in all cases studied suggesting that grain boundary oxidation, often observed during hot tensile holds, is insignificant.

Thermal and mechanical effect during rapid heating of astroloy for improving structural integrity

The behaviour of γ′ phase to thermal and mechanical effects during rapid heating of Astroloy(Turbine Disc alloy) a Powder metallurgy (PM) nickel base superalloy has been investigated. The thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ) microstructure of an inertia friction welded Astroloy were simulated using a Gleeble thermo-mechanical simulation system. Detailed microstructural examination of the simulated TMAZ and HAZ and those present in actual inertial friction welded specimens showed that γ′ particles persisted during rapid heating up to a temperature where the formation of liquid is thermodynamically favoured, and subsequently re-solidified eutectically. The result obtained showed that forging during the thermo-mechanical simulation significantly enhanced resistance to weld liquation cracking of the alloy. This is attributable to strain-induced rapid isothermal dissolution of the constitutional liquation products within 150 μm from the centre of the forged sample. This was not observed in purely thermally simulated samples. The microstructure within the TMAZ of the as-welded alloy is similar to the microstructure in the forged Gleeble specimens.

Probabilistic Fatigue Life Prediction of Turbine Disc Considering Model Parameter Uncertainty

AbstractAiming to improve the predictive ability of Walker model for fatigue life prediction and taking the turbine disc alloy GH4133 as the application example, this paper investigates a new approach for probabilistic fatigue life prediction when considering parameter uncertainty inherent in the life prediction model. Firstly, experimental data are used to update the model parameters using Bayes’ theorem, so as to obtain the posterior probability distribution functions of two parameters of the Walker model, as well to achieve the probabilistic life prediction model for turbine disc. During the updating process, Markov Chain Monte Carlo (MCMC) technique is used to generate samples of the given distribution and estimating the parameters distinctly. After that, the turbine disc life is predicted using the probabilistic Walker model based on Monte Carlo simulation technique. The experimental results indicate that: (1) after using the small sample test data obtained from turbine disc, parameter uncertainty of the Walker model can be quantified and the corresponding probabilistic model for fatigue life prediction can be established using Bayes’ theorem; (2) there exists obvious dispersion of life data for turbine disc when predicting fatigue life in practical engineering application.