EXPERIENCE WITH THERMOMECHANICAL FATIGUE UNDER SERVICE-TYPE LOADING

The thermomechanical fatigue behaviour of different high temperature alloys has been investigated and is under investigation respectively. The creep-fatigue behaviour of heat resistant steels was investigated by long-term service-type strain cycling tests simulating thermomechanical fatigue (TMF-) loading conditions at the heated surface of e.g. turbine rotors. Single-stage as well as three-stage cycles leads to similar results at the application of the damage accumulation rule. Life prediction which simulates typical combinations of cold starts, warm starts and hot starts has been established successfully for isothermal service-type loading and will be exceeded for thermomechanical loading. Long-term thermomechanical fatigue testing of Thermal Barrier Coating systems show typical delamination damage. An advanced TMF cruciform testing system enables complex multiaxial loading.

SIMULATION OF COMPLEX THERMOMECHANICAL FATIGUE

Two different types of experimental techniques to perform non-isothermal, uniax-ial and biaxial fatigue tests were described. A new miniaturised electrothermal-mechanical test rig was presented and discussed. It enables testing of small specimens under complex thermomechanical loading conditions. In order to cope with the simulation of well defined biaxial proportional and non-proportional loadings with in-phase and out-of-phase superposition of thermal loads a cruciform biaxial fatigue testing machine has been developed. Special design features of both machines, and the specimens tested, as well as typical test results were discussed.

Thermomechanical Fatigue Behaviour of a Nickel Base Superalloy

The isothermal low cycle fatigue (LCF) and thermomechanical fatigue (TMF) behaviour of a Ni-base superalloy was investigated. The results show that temperature plays an important role in both LCF and TMF. The alloy shows the lowest LCF fatigue resistance in the intermediate temperature range (～760℃). For strain-controlled TMF, in-phase (IP) cycling is more damaging than out-phase (OP) cycling. The high tempera- ture exposure in the TMF cycling influences the deformation behaviour at the low temperature. LCF lives at different temperatures, and IP and OP TMF lives are successfully correlated by using the hysteresis parameter Δσ·Δε_p.

Thermomechanical Fatigue Behavior of Powder Metallurgical Nickel Based Superalloy FGH96

The fracture behavior of the thermomechanical fatigue （TMF） of the powder metallurgical nickel based superalloy FGH96 was investigated under in-phase （IP） and out-of-phase （OP） loadings in the temperature range from 550 ℃ to 720 ℃ and the mechanical strain amplitude range from 0.3% to 0.8%. The results show that the FGH96 TMF fracture character is intergranular for the IP samples and transgranular cleavage-like for the OP samples, at the same strain amplitude, the fatigue life is shorter for the IP than that for the OP samples that is related to crack propagation along grain boundary on the IP samples, the γ′ size is larger in the IP than that in the OP sample, which is related to the bulk diffusion processes accelerated by the tensile strain during the high temperature portion of the IP cycle. Dislocation pairs and stacking faults are main microstructures induced by IP TMF, and they are hindered by the grain boundary, which likely resulted in the crack propagation along the grain boundary in the IP samples.

Failure Behavior of Thermal Barrier Coatings on Cylindrical Superalloy Tube Under Thermomechanical Fatigue

Failure behavior of thermal barrier coatings on cylindrical superalloy tube was investigated under thermome- chanical fatigue （TMF）. Two types of TMF tests, i.e. in phase （IP） and out of phase （OP）, were performed in the temperature range of 450-850℃. All tests were carried out under mechanical strain control at a given period of 300 s. The bond coat NiCrAIY was produced by high velocity oxygen fuel （HVOF）, and the top coat 7%Y203-ZrO2 was deposited by air plasma spraying （APS）. The testing results showed that the OP TMF life was longer than the IP TMF one under the same mechanical strain amplitude. Observations of the fractured specimens revealed that the interface damage and cracking behavior in the two phasing conditions were different. In OP loading, the top coat was cracked and detached from the bond coat while no spallation was found in the IP loading.

Role of grain orientation in the failure of Sn-based solder joints under thermomechanical fatigue

A small Pb-free solder joint exhibits an extremely strong anisotropy due to the body- centered tetragonal （BCT） lattice structure of β-Sn. Grain orientations can signif- icantly influence the failure mode of Pb-free solder joints under thermomechanical fatigue （TMF） due to the coefficient of thermal expansion （CTE） mismatch of β-Sn grains. The research work in this paper focused on the microstructure and damage evolution of Sn3.0Ag0.5Cu BGA packages as well as individual Sn3.5Ag solder joints without constraints introduced by the package structure under TMF tests. The mi- crostructure and damage evolution in cross-sections of solder joints under thermome- chanical shock tests were Characterized using optical microscopy with cross-polarized light and scanning electron microscopy （SEM）, and orientations of Sn grains were determined by orientation imaging microscopy （OIM）. During TMF, obvious recrys- tallization regions were observed with different thermomechanical responses depend- ing on Sn grain orientations. It indicates that substantial stresses can build up at grain boundaries, leading to significant grain boundary sliding. The results show that recrystallized grains prefer to nucleate along pre-existing high-angle grain boundaries and fatigue cracks tend to propagate intergranularly in recrystallized regions, leading to an accelerated damage after recrystallization .

TBCs for Gas Turbines under Thermomechanical Loadings： Failure Behaviour and Life Prediction

The present contribution gives an overview about recent research on a TBC （thermal barrier coating） system consisted of （I） an intermetallic MCrAIY-alloy BC （bond coat） applied by VPS （vacuum plasma spraying） and （2） an YSZ （yttria stabilised zirconia） top coat APS （air plasma sprayed） at Forschungszentrum Juelich, Institute of Energy and Climate Research （IEK-2）. The influence of high temperature dwell time, maximum and minimum temperature on crack growth kinetics during thermal cycling of such plasma sprayed TBCs is investigated using scanning electron microscopy and AE （acoustic emission） analysis. Thermocyclic life in terms of accumulated time at maximum temperature decreases with increasing high temperature dwell time and increases with increasing minimum temperature. AE analysis proves that crack growth mainly occurs during cooling at temperatures below the ductile-to-brittle transition temperature of the BC. Superimposed mechanical load cycles accelerate delamination crack growth and, in case of sufficiently high mechanical loadings, result in premature fatigue failure of the substrate. A life prediction model based on TGO growth kinetics and a fracture mechanics approach has been developed which accounts for the influence of maximum and minimum temperature as well as of high temperature dwell time with good accuracy in an extremely wide parameter range.

Environmentally friendly solders 3-4 beyond Pb-based systems

Based on environmental considerations, global economic pressures, enacted by legislations in several countries, have warranted the elimination of lead from solders used in electronic applications. Sn3.5Ag, SnAgCu, and Sn0.7Cu have emerged among various lead-free candidates as the most promising solder alloys to be utilized in microelectronic industries. However, with the vast development and miniaturization of modern electronic packaging, new requirements such as superior service capabilities have been posed on lead-free solders. In order to improve the comprehensive property of the solder alloys, two possible approaches were adopted in the current research and new materials developed were patented. One approach was involved with the addition of alloying elements to make new ternary or quaternary solder alloys. Proper addition of rare earth element such as La and Ce have rendered solder alloys with improved mechanical properties, especially creep rupture lives of their joints. Another approach, the composite approach, was developed mainly to improve the service temperature capability of the solder alloys. Composite solders fabricated by mechanically incorporating various reinforcement particles to the solder paste have again exhibited enhanced properties without altering the existing processing characteristics. The recent progress and research efforts carried out on lead-free solder materials in Beijing University of Technology were reported. The effects of rare earth addition on the microstructure, processing properties, and mechanical properties were presented. The behaviors of various Sn-3.5Ag based composite solders were also explicated in terms of the roles of reinforcement particles on intermetallic growth, steady-state creep rate, the onset of tertiary creep, as well as the overall creep deformation in the solder joints. Thermomechanical fatigue (TMF) behavior of the solder alloys and composite solders were investigated with different parameters such as ramp rate, dwell time, etc. The damage accumulation features and residual mechanical properties of the thermomechanically-fatigued composite solder joints were compared with non-composite solder joints. To match the lead-free alloys, various types of water soluble no-clean soldering flux have also been developed and their properties were presented.

Fatigue behaviors of Z2CND18.12N stainless steel under thermal-mechanical cycling

Tests under mechanical strain control were performed to investigate the TMF behavior of Z2CND18.12N within the temperature range between 150–550 ？ C. Different strain amplitudes and phase-angles were applied. Total strain controlled low cycle fatigue test was also performed at the peak temperature of TMF cycling. The results show that the cyclic stress response of the material displayed an initial hardening regime followed by a saturation period and then cyclic softening till failure. The TMF cycling leads to the development of significant amounts of mean stress. Some life prediction models were employed to predict the TMF life of Z2CND18.12N, and the results indicate that the energy-based models provide good prediction on the thermal-mechanical fatigue behaviors of this material. An optical microscopic observation shows that the surface crack initiations and crack propagations are typically transgranular mode.