Thermomechanical Fatigue Behaviour of a Nickel Base Superalloy

The isothermal low cycle fatigue (LCF)and thermomechanical fatigue (TMF) behaviourof a Ni-base superalloy was investigated. Theresults show that temperature plays an importantrole in both LCF and TMF. The alloy shows thelowest LCF fatigue resistance in the intermediatetemperature range (～760℃). For strain-controlledTMF, in-phase (IP) cycling is more damagingthan out-phase (OP) cycling. The high tempera-ture exposure in the TMF cycling influencesthe deformation behaviour at the low temperature.LCF lives at different temperatures, and IPand OP TMF lives are successfully correlatedby using the hysteresis parameter Δσ·Δε_p.     

Effect of Phosphorus on Microstructure and High Temperature Properties of a Cast Ni—base Superalloy

Effect of phosphorus on the microstructure and high temperature properties of a cast Ni-base superalloy M963 has been investigated.SEM observation and EDS analysis showed that P was mostly enriched in the interdendritic region,and the P-rich phase was formed in the front position of finally solidified eutectics in high P doped alloys.It was found that the P-rich phase,as preferred initiation and propagation site of cracks,could aggravate the fracture process at high temperature in high P doped alloys.Consequently,high P addition would reduce remarkably the ductility and creep life of M963 superalloy at high temperature.     

Microstructure characterization and HCF fracture mode transition for modified 9Cr-1Mo dissimilarly welded joint at different elevated temperatures

The high cycle fatigue(HCF) tests of modified 9 Cr-1 Mo dissimilarly welded joint were carried out at different elevated temperatures and the fracture mechanism was systematically revealed. The fatigue strength at 108 cycles based on S–N curve can be estimated as a half of weld joint's yield strength for all conducted temperatures, which can be a reliable criterion in predicting the fatigue life. The results show that the inter-critical heat affected zones(IC-HAZs) of both sides are the weak zones due to their low hardness and inferior fatigue resistance property. HAZ of COST-FB2(BM2) is the weakest zone at room temperature due to the existence of numerously distributed defects and the initiation of cracks, either in the surface or interior zone, impacting a crucial effect on the fatigue life of the joint. While at elevated temperatures, fatigue life was controlled mostly by the intrusion–extrusion mechanism at the specimen surface under high stress level and subsurface non-defect fatigue crack origin(SNDFCO) from the interior material under low stress amplitude. With increasing temperature, more and more fatigue failures began to occur at the HAZ of COST-E(BM1) due to its higher susceptibility of temperature. Besides, it is found that the-ferrite in the BM1 has no harm to the HCF behavior of the joint at the conducted temperatures.     

Structural Dependence of Creep/Fatigue Behaviour of Single Crystal Ni-base Superalloy

The effect of different initial microstructures deftned by γ' precipitate morphology has been investigated at the creep/fatigue conditions of 900℃ and 500 MPa. The wave form of stress as a function of time for cyclic load was of trapezoidal shape with a hold time of 10s at the upper stress level. The TEM was employed to examine the deformation process in strengthened γ' matrix in dependence of γ' precipitate morphology. The fracture lifetime and cycle number up to fracture were the criteria to evaluate the additional cyclic component efFect on the course of deformation     

Experimental Study on the Uniaxial Cyclic Deformation of 25CDV4.11 Steel

The strain cyclic characteristics and ratcheting behavior of 25CDV4.11 steel were studied by the experiments under uniaxial cyclic loading with relatively high cyclic number and at room temperature. The cyclic hardening/softening feature of the material was first observed under the uniaxial strain cycling with various strain amplitudes. Then, the ratcheting behavior of the material was researched in detail, and the effects of stress amplitude and mean stress on the ratcheting were discussed under uniaxial asymmetrical stress cycling. Comparing with the experimental results of SS316L stainless steel, it is concluded that the material exhibits remarkable cyclic softening feature, and then a special ratcheting behavior is caused. Some conclusions useful to establish corresponding constitutive model are obtained.     

Fatigue Behavior of HDPE Composite Reinforced with Silane Modified TiO2

The composite of high density polyethylene reinforced with silane-modified TiO2 particles (silane-TiO2/HDPE) is a potential bone substitute biomaterial. The structure, bioactivity, and mechanical properties of silane-TiO2/HDPE are analogous to those of natural bone, correspondingly. In order to investigate the effect of silane connection and saline solution on fatigue behaviors, flexural fatigue tests with this composite were carried out in both air and saline solution. Saline solution was found to have different effect on fatigue life. In saline solution, the fatigue life could be improved at stress levels lower than 30 MPa, while the fatigue life could be reduced at stress levels higher than 30 MPa. After analyzing the fracture morphologies, different failure mechanisms were proposed, and the important role of silane connection in the composite during the fatigue process was discussed. Silane connection cannot only support the loading stress but also hinder the failure process under loading effectively. For dry specimens, no interfacial failure between the filler and matrix was found. For wet specimens, it is inferred that the synergetic effect of saline solution and high concentrated stress at high stress level could easily destroy the silane connection, which accelerated the fracture process, whereas the synergetic effect of saline solution and silane connection at low stress level could promote the formation of more microcracks on sample surface, which hindered the final fracture.     

Dwell and Normal Cyclic Fatigue Behaviours of Ti60 Alloy

An experimental study of dwell and normal cyclic fatigue behaviours was carried out using specimens from a Ti60 forging with a bimodal microstructure.Apparent decrease in the fatigue life was found under dwell fatigue condition as compared to that under normal cyclic condition.Strain produced in each cycle in dwell fatigued specimens was observed larger than that in its normal cyclic-fatigued counterparts.Interior crack initiation was found in most dwell fatigued specimens as compared to the subsurface crack initiation under normal cyclic fatigue condition.Flat and bright facets were found at crack initiation sites in both cases.The facet density is higher in dwell condition,which is consistent with the crystal orientation and Schmid factors analysis of a grains around secondary cracks using electron back-scattered diffraction(EBSD) methods.Dwell loading favours cleavage in a grains with their basal plane normals aligned no more than 15° to the loading axis,which may account for its lower fatigue life according to the present study.     

Relationship between Grain Size and Fatigue-Creep Interaction Behaviour of Superalloy GH698

The deformation and fracture behaviourof nickel-base superalloy GH698 with differentgrain size have been studied at 700℃ underfatigue-creep interaction conditions. Comparedwith coarse-grained specimens, the grain refiningprocess shows obvious effect on the mechanicalbehaviour of the alloy, i. e. in F and C zones,the fraction of pure fatigue and creep fractureon fracture surfaces is greatly reduced, whichdecreases and increases the fracture life inzones F and C respectively, in FC and C zones,creep deformation is greatly restrained by thealternating stress component, which increasesthe fracture life remarkably. It is also provedthat in spite of the difference in microstru-tures such as grain size, for a constant tem-perature, a unique life equation t_r=A~n canbe used to predict rupture life within the stressregion controlled by the same fracture mode.     

Effect of thermal exposure on the microstructure and creep properties of a fourth-generation Ni-based single crystal superalloy

The effect of thermal exposure on microstructure and creep properties of a fourth-generation nickelbased single crystal superalloy was investigated.The thermal exposure of samples after the full heat treatment was carried out at 1000℃,1100℃ and 1140℃ for 100 h and 200 h.The γ' coarsening,γ' rafting and γ channel widening were observed in samples after thermal exposure.When the thermal exposure time was constant,the morphology of γ' phase in the alloy evolved significantly with increasing aging temperature.The interracial dislocation networks in aged samples after creep ruptured gradually became irregular and sparse with the increase of exposure temperature.When the higher exposure temperature was used,enla rgement of the defect pores was observed in samples,the microcracks were more likely to initiate and propagate at the corner of these pores.After aging at 1000℃ for 100 h,the creep life at 1140℃/137 MPa was slightly longer than that of heat-treated sample,which could be attributed to the slightly coarsened γ' phase,homogenization of refractor elements.In contrast,the creep life of sample exposed at 1140℃ for 100 h was greatly decreased.The decrease of creep life was dominated by the rafting of γ'phase,the irregular interfacial dislocation networks as well as the enlargement of homogenization pores.     

Microstructure and properties in dissimilar/similar weld joints between DP780 and DP980 steels processed by fiber laser welding

The microstructure and mechanical properties(strength, fatigue and formability) of dissimilar/similar weld joints between DP780 and DP980 steels were studied. The microstructure in fusion zone(FZ) was lath martensite(LM), and alloying elements in the FZ were uniformly distributed. The hardness in the FZ of dissimilar weld joint was similar to the average value(375 HV) of the two similar weld joints. The microstructural evolution in heat affected zone(HAZ) of dissimilar/similar weld joints was as follows:LM(coarse-grained HAZ) →finer LM(fine-grained HAZ) →M-A constituent and ferrite(intercritically HAZ) →tempered martensite(TM) and ferrite(sub-critical HAZ). Lower hardness in intercritically HAZ and sub-critical HAZ(softening zones) was observed compared to base metal(BM) in dissimilar/similar weld joints. The size of softening zone was 0.2–0.3 mm and reduction in hardness was ~7.6%–12.7% of BM in all the weld joints, which did not influence the tensile properties of weld joints such that fracture location was in BM. Formability of dissimilar weld joints was inferior compared to similar weld joints because of the softening zone, non-uniform microstructure and hardness on the two sides of FZ. The effect of microstructure on fatigue life was not influenced due to the presence of welding concavity.     

A comparative study on thermomechanical and low cycle fatigue failures of a single crystal nickel-based superalloy

The cyclic deformation and lifetime behaviors of a single crystal nickel-based superalloy CMSX-4 have been investigated under out-of-phase thermomechanical fatigue (OP TMF) and isothermal low cycle fatigue (LCF) conditions. OP TMF life exhibited less than a half of LCF life although smaller inelastic strain range and lower mean stress level during OP TMF were observed compared to those during LCF. During OP TMF cycling, the maximum tensile strain at the minimum temperature was found to accelerate the surface crack initiation and propagation. Additionally, the multiple groups of parallel twin plates near crack provided a preferential path for crack propagation.     

In‐phase and out‐of‐phase thermomechanical fatigue behavior of 4Cr5MoSiV1 hot work die steel cycling from 400 °C to 700 °C

The hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in‐phase (IP) and out‐of‐phase (OP) thermomechanical fatigue (TMF) tests cycling from 400 °C to 700 °C under full reverse strain‐controlled condition were investigated. Stress‐mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high‐temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi‐cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter.     

High-temperature fatigue behaviour of a second generation near-γ titanium aluminide sheet material under isothermal and thermomechanical conditions

The high-temperature deformation behaviour of a second generation γ-TiAl sheet material with near-γ microstructure was characterised under tensile, creep, isothermal and thermomechanical fatigue (TMF) loading conditions. Test temperature ranged from 500 to 750 °C in isothermal tests and these temperatures were also used as minimum and maximum temperature of in-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests. Under tensile loading, a ductile-to-brittle transition temperature (DBTT) of about 650 °C was observed. At this temperature the material experiences a temperature dependent change in the fracture morphology. Creep tests carried out in the temperature range from 650 to 800 °C under true constant stress conditions revealed a temperature and stress dependence of the Norton stress exponent n and the apparent activation energy for creep Q_app. With increasing temperature, isothermal fatigue life at constant strain amplitude decreased in vacuum, but increased in air indicating an abnormal (inverse) environmental effect. Under IP loading, fatigue is characterised by cyclic softening due to dynamic recrystallisation. OP loading drastically reduces fatigue life and turned out to be an extremely critical loading situation for γ-TiAl alloys.     

Thermisch– mechanisches und isothermes Ermüdungsverhalten der Nickelbasis – Superlegierung IN 792 CC

Thermal-mechanical and isothermal fatigue behaviour of the nickel-base superalloy IN 792 CC Many components used in high temperature applications are exposed to complex thermal-mechanical loadings during operation. For this reason the effect of start-stop-cycles with thermalmechanical fatigue (TMF) as consequence was investigated by means of In-Phase(IP)and Out-of-Phase (OP) TMF tests. The fatigue life of the γ'hardend nickel-base cast superalloy IN 792 CC decreases with increasing maximum temperatures T_max of the TMF cycles, due to the increasing plastic deformations and the increasing mean stress (OP-TMF) or increasing intergranular; damage (IP-TMF), respectively. These relations can be satisfactorily described using the Manson-Coffin-relationship or the damage parameters of Smith-Watson-Topper and Ostergren. By contrast, the influence of different phase shifts between temperature and mechanical loading also cannot be approximately described with one consistant relation between damage parameters and fatigue life. The evaluation of TMF loadings based on results from isothermal LCF-tests with the same frequency and respective mechanical strain leads always to an overrating of the fatigue life, even if the temperature of the isothermal test is the maximum temperature of the TMF cycle. This applies when comparing mechanical loading values as well as when comparing damage parameters.     

Evolutionary stress cycle behaviour and damage mechanisms in nickel based superalloy under thermomechanical fatigue

Abstract

Both in phase and out of phase thermomechanicalfatigue (TMF)tests were conducted using a computer controlled, servohydraulic uniaxial fatigue system. The TMF behaviour of nickel based superalloy K417 was investigated. The experimental results show that, in comparison with isothermal low cycle fatigue, both in phase and out of phase TMF lead to much reduced fatigue lives, although there is a difference between the in phase and outof phase cases. Creep and oxidation are the main concerns in the lower fatigue life ofin phase TMF.The cyclic hardening atinitial cycles results from the obstacle of γ ′ precipitates to the slipping dislocations and the increase in dislocation density owing to deformation. Shearing of γ ′ precipitates and loss of coherency of γ ′ precipitates with the matrix at high temperatures are the reasons for cyclic softening at later stages of fatigue life.     

Thermomechanical fatigue evaluation and life prediction of 316L(N) stainless steel

An attempt has been made to understand the thermomechanical fatigue (TMF) behaviour of a nitrogen-alloyed type 316L austenitic stainless steel under different temperature domains. Smooth, hollow specimens were subjected to in-phase (IP) and out-of-phase (OP) thermal–mechanical cycling in air under a mechanical strain control mode, at a strain rate of 6.4 × 10^?5 s^?1 and a strain amplitude of ±0.4%. For the sake of comparison, total strain controlled low cycle fatigue (LCF) tests were also performed at the peak temperatures of TMF cycling on similar specimens employing the same strain rate and strain amplitude. Life was found to depend on the thermal/mechanical phasing and temperature. Creep was found to contribute to life reduction in IP tests when the peak temperature of cycling was above 600 °C. A few TMF tests were performed in vacuum in order to assess environmental influence on life. Thermomechanical fatigue cycling led to the development of significant amounts of mean stresses and the stress response was generally higher compared to that of LCF tests at the peak cyclic temperatures. Also, the isothermal tests at the peak temperature of TMF cycling resulted in lower lives compared to those obtained under TMF. An attempt was made to predict the TMF life using the isothermal database and satisfactory predictions were achieved using the Ostergren’s frequency modified damage function (FMDF) approach.     

A critical-plane-based thermomechanical fatigue lifetime prediction model and its application in nickel-based single-crystal turbine blades

In this study, thermomechanical fatigue (TMF) behaviours, failure mechanisms and the lifetime prediction method of a nickel-based single-crystal superalloy with [001] orientation were investigated based on the stress-controlled TMF experiments at different stress/temperature ranges, dwell times and phase angles. The fractographic observations revealed a creep-fatigue failure mechanism for in-phase thermomechanical fatigue (IP TMF) and an oxidation-fatigue failure mechanism for out-of-phase thermomechanical fatigue (OP TMF). According to the observed physical phenomenon of the slip along particular planes during the deformation process, selecting the steady-ratcheting shear-strain rate as the representative physical quantity, a new critical-plane-based lifetime prediction model which was suitable for a variety of experiment conditions was established. The predicted lifetimes for both standard specimens and turbine blades showed good agreements with the experimental data. The strong versatility and the concise mathematic form that made the model have some practical application value.     

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200 °C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200 °C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.     

A study on thermo mechanical fatigue life prediction of Ni-base superalloy

Gas turbine blades are exposed to high-temperature degradation environments due to flames and mechanical loads as a results of high-speed rotation during operation. In addition, blades are exposed to thermo-mechanical fatigue due to frequent start and shutdown. Therefore, it is necessary to evaluate the lifetime of blade materials.In this study, the TMF life of a Ni-base superalloy applied to gas turbine blade was predicted based on LCF and TMF test results. The LCF tests were conducted under various strain ranges based on gas turbine operating conditions. In addition, IP (in-phase) and OP (out of-phase) TMF tests were conducted under various strain ranges.Finally, a fatigue life prediction model was drawn from the LCF and TMF test results. The correlation between the LCF and TMF test results was also evaluated with respect to fatigue life.