Karl-Heinz Lang

Thermal-mechanical and isothermal fatigue of IN 792 CC

Abstract The cyclic deformation and lifetime behaviour of the cast Ni-base superalloy IN 792 CC was investigated both under thermal-mechanical fatigue (TMF) and isothermal fatigue (IF) conditions. During TMF the phase relations between temperature and mechanical strain were in-phase and out-of-phase, respectively. For both phase relations a similar cyclic deformation behaviour is observed. In all cases out-of-phase TMF causes tensile mean stresses, whereas in-phase TMF leads to compressive mean stresses. At T max below 800 °C out-of-phase cycling results in smaller lifetimes than in-phase loading. In spite of the rather high compressive mean stresses developing at T max above 800 °C, at these temperatures in-phase loading causes shorter lifetimes than out-of-phase TMF. This effect is due to the different damage mechanisms caused by in-phase and out-of-phase loadings: at higher T max considerable intergranular damage caused by in-phase loading reduces the lifetime below the respective values measured during out-of-phase TMF, after which no intergranular damage could be detected. A comparison of the TMF data with the cyclic deformation and lifetime behaviour under IF conditions shows that the materials reactions under TMF cannot be assessed satisfactorily by the results obtained from isothermal fatigue tests.

Effect of superimposed high cycle fatigue loadings on the out-of-phase thermal-mechanical fatigue behaviour of CoCr22Ni22W14

The thermal-mechanical fatigue (TMF) behaviour of CoCr22Ni22W14 was investigated under total strain controlled out-of-phase (OP) experiments without and with superimposed high cycle fatigue (HCF) loadings. The minimum temperature T min was 200 °C and the maximum temperature T max was varied between 750 and 1200 °C. The mechanical strain amplitude e me a, during pure TMF tests was kept equal to the thermal strain amplitude e th 1 and the superimposed HCF amplitude e HCF a,t was varied between 0.05 and 0.2%. In both loading conditions cyclic hardening is observed, which is the less pronounced the higher T max is. Only at T max = 1200 °C, cyclic softening appears after cyclic hardening in the first cycles as a result of creep damage accumulation. With increasing superimposed HCF amplitudes, the cyclic deformation behaviour is obviously more and more determined by the superimposed HCF loadings. Due to the dynamic relaxation processes at higher temperatures, tensile mean stresses develop during all TMF tests performed. Under TMF-OP conditions a significant lifetime reduction is observed as a result of superimposed HCF loadings. This lifetime reduction increases with growing HCF amplitudes and may approach 90% of the fatigue lifetime obtained from pure OP experiments. For each T max the dependence between the total strain amplitude (e a.t = e me a.t + e HCF a.t ) and the number of cycles to failure can be described as a potential function (e a.t = A × N B b ) with an exponent b which decreases with increasing T max and which depends on the material properties at different temperatures.

Thermal-Mechanical Fatigue Behaviour of NiCr22Co12Mo9

Every start up and shut down of a system operating at high temperatures causes in the components transient temperature gradients through which complex strain and stress fields occur and local damage may be produced. For example, the lifetime of combustion chambers in gas turbines is often limited by damage resulting from start-stop-cycles and changes of the operating temperature [1,2]. Frequently, data from isothermal strain controlled fatigue tests are used to predict the deformation behaviour and the lifetime of components exposed to thermal-mechanical fatigue (TMF) [3]. This procedure, however, is connected with large uncertainties, especially if the cyclic stress-strain response and the microstructural changes are different under isothermal and thermal-mechanical fatigue. In the present study, the TMF behaviour of the solid solution and carbide precipitation hardened nickel base superalloy NiCr22Col2Mo9, which is commonly used as sheet material in gas turbines, is investigated. The TMF life determined is compared to the lifetime in isothermal fatigue tests.

Lifetime, cyclic deformation and damage behaviour of MAR-M-247 CC Under in-Phase, out-of-Phase and phase-Shift TMF-Loadings

In-Phase-, Out-of-Phase- and Phase-shift-TMF tests were carried out on MAR-M-247 CC. A minimum temperature of 400°C, maximum temperatures from 800°C up to 1050°C, total mechanical strain amplitudes between 70% and 100% of the thermal strain amplitude and dwell times at maximum temperature from 0s up to 1800s were applied. Out-of-Phase-TMF leads to higher tensile mean stresses than Phase-Shift-TMF. In-Phase-TMF loadings induce compressive mean stresses. The amounts of the mean stress, the stress amplitude and the plastic strain amplitude generally increase with increasing maximum temperature and dwell time. Out-of-Phase- and Phase-Shift-TMF lead to higher numbers of cycles to failure than In-Phase-TMF. Under Out-of-Phase-TMF a transgranular fracture path is observed even at high maximum temperatures and dwell times. This loading condition leads to compressive stresses at high temperatures and tensile stresses at lower temperatures. Due to that, intergranular creep damage is suppressed. Under In-Phase-TMF the tensile stresses occurring at high temperatures lead to creep induced intergranular crack propagation even at loadings with a dwell time of zero and relatively low maximum temperatures. Under Phase-Shift-TMF loadings transgranular crack propagation occurs at low maximum temperatures, whereas higher maximum temperatures and dwell times lead to intergranular damage. Because the damage mechanisms depend on the phase relationship and the dwell time, the damage parameter POST which accounts for the plastic strain amplitude and the maximum stress as damage relevant quantities is not able to give a common description of the lifetime behaviour for all considered TMF cycle types.

Failure behaviour of the superalloy MAR-M247 LC under LCF, HCF and combined LCF/HCF loading

Abstract Materials for turbine blades experience in service a combined loading of low and high cycle fatigue at high temperatures. In order to understand the failure behaviour under these loading conditions, systematic investigations were carried out. Low cycle fatigue, high cycle fatigue and combined low and high cycle fatigue tests were realised on MAR-M247 LC at 650 °C in an air environment under total strain control. Surface damage and fracture surfaces were analysed. Under combined low and high cycle fatigue, the lifetime is reduced if the low cycle fatigue leads to a degradation of the high cycle fatigue strength caused by crack initiation and crack growth. By analysing the fracture surface, the crack growth rate under combined cycle fatigue loading could be determined and it was significantly higher than under pure low cycle fatigue loading. The accelerated crack growth mainly causes the lifetime reduction.

The influence of superimposed creep loadings on the thermal-mechanical fatigue behaviour of the Ni-base superalloy IN 792 CC

The effect of superimposed creep loadings on the cyclic deformation andthe lifetime behaviour of the Ni-base superalloy IN 792 CC underout-of-phase TMF loadings is presented and discussed. The mean stressand the stress amplitude are not affected significantly by the creeploading. A slight cyclic hardening is observed even for cycles withsuperimposed creep loadings at a maximum temperature of 920°C whichmeans that the work hardening processes occurring under TMF loading arenot fully compensated by recovery during the creep phase. The plasticstrain amplitude increases with increasing creep stress. Using theManson–Coffin relationship, it is possible to calculate the influence ofthe superimposed creep loadings on the lifetime behaviour in goodaccordance with the experimental results.

Influence of the Mechanical Strain Amplitude on the In-Phase and Out-of-Phase Thermo-mechanical Fatigue Behaviour of NiCr22Co12Mo9

In total strain controlled in-phase and out-of-phase thermo-mechanical fatigue (TMF) tests on NiCr22Col2Mo9 (Inconel 617, Nicrofer 5520 Co) with a maximum temperature of 1123 K, a minimum temperature of 473 K and different mechanical strain amplitudes the cyclic stress-strain response, the change of the microstructure and the development of damage were analysed. The initial values of the induced stress amplitudes and plastic strain amplitudes, and the amount of cyclic hardening increase with the total mechanical strain amplitude. The observed cyclic hardening results from strong dislocation-dislocation and dislocation-particle interactions during plastic deformation at lower temperatures the latter being enhanced by the precipitation of small semi-coherent carbides at elevated temperatures of the TMF cycles. For each type of TMF tests the lifetime behaviour can be adequately described by the combination of the relationships of Basquin and Coffin-Manson. At equal total mechanical strain amplitudes in-phase tests always yield smaller lifetimes than out-of-phase tests. The difference between the two types of TMF-tests rises with increasing lifetime. This behaviour is caused by different accumulation of creep damage which is favoured by tensile stresses at high temperatures.

Fatigue Behaviour of Ni-base Alloys up to 1273 K

ABSTRACT The cyclic deformation behaviour of two Ni-base alloys was investigated in stress and total strain controlled push-pull tests in the temperature range 295 K ≤ T ≤ 1273 K. The cyclic deformation curves and the fatigue life were determined as a function of the different loading conditions. Characteristic experimental results are presented and discussed with respect to dynamic strain aging effects.

Assessment of the influence of interdendritic shrinkage cavities on the thermo-mechanical fatigue behaviour of the nickel-base superalloy MAR-M247LC

Abstract Precision cast high-temperature components often contain specific imperfections like micropores or microcavities. The influence of the position, size and distribution of such micro cavities as well as of the parameters of the thermomechanical fatigue loading on the lifetime behaviour of the superalloy MAR-M247LC was investigated using two specimen series, containing a high and a low amount of microcavities and a state from which any microcavity was removed by hot isostatic pressing. Thermo-mechanical fatigue (TMF) tests with zero (“in phase”) and −135° phase shift between temperature and mechanical strain were performed. The different damage behaviour at in-phase and −135° TMF requires different methods of the assessment of microcavities. For the in-phase loading, they are regarded as cracks and their effect on the local strain distribution is determined by the crack opening displacement. This leads to very good results for both specimen states investigated. For the −135° cycle, it is taken into ...

Cyclic deformation and life time behaviour of NICR22CO12MO9 at isothermal and thermal-mechanical fatigue

Abstract In the present study, the materials reaction and the microstructural changes during isothermal and thermal-mechanical fatigue are presented. In total strain controlled isothermal fatigue tests at temperatures between 1123 and 1473K and a frequency of 10 2 Hz the cyclic deformation behaviour is influenced by thermally activated recovery and a neutral cyclic deformation behaviour is found. At this condition the life time behaviour is determined by creep-fatigue interactions. In total strain controlled in-phase and out-of-phase thermal-mechanical fatigue tests the initial values of the induced stress amplitudes and plastic strain amplitudes are the higher and the cyclic hardening is the more pronounced, the higher the total mechanical strain amplitude is. The observed cyclic hardening is on the one hand caused by the development of high dislocation densities due to plastic deformation at lower temperatures, and on the other hand by the precipitation of small semi-coherent carbides at higher temperatures. At high total mechanical strain amplitudes with the same magnitude, in-phase tests yield smaller lifetimes than out-of-phase tests. At low total mechanical strain amplitudes the contrary is true. This is the result of competitive processes: creep damage favoured by high tensile stresses at high temperatures under in-phase loading and tensile mean stresses developing during out-of-phase loading.