Isothermal Low-cycle Fatigue And Thermomechanical Fatigue Behavior Of MCrAlY Coated And Uncoated Nickel Base Superalloy

Nickel base superalloys are widely used for making turbine blades,vanes,and other high-temperature structural components,due to their high yield strength,high creep strength, and good resistance against hot corrosion and oxidation at elevated temperatures.The majority of structural components used in high-temperature applications experience some form of thermomechanical fatigue(TMF) loadings.Some investigations showed that the fatigue life of a real component under thermomechanical fatigue was lower than that under isothermal low-cycle fatigue(IF) at the same maximum temperature and corresponding strain amplitude.Since thermomechanical fatigue test can really simulate the service condition of a high-temperature component in laboratory,it is very important to investigate the damage and failure mechanisms of superalloys working under TMF,which would provide a reliable theoretical basis for design and life prediction of high-temperature structural components.In this paper,the isothermal low-cycle fatigue behavior of two kinds of cast nickel-based superalloys,i.e.M38 and M963,was investigated at 900℃,and the thermomechanical fatigue behavior and life prediction of M963 in temperature range of 450-900℃were also investigated.Results revealed that both M38 and M963 exhibited a good and stable low-cycle fatigue property.After the comparison of their resistance against oxidation,their strength and ductility at 900℃,it was found that M963 has a similar oxidation resistance,an acceptable ductility,and a much higher high-temperature strength than M38.Therefore,M963 was selected as the material for TMF tests.Two types of TMF tests were carded out,i.e.in-phase(IP) and out-of-phase(OP). Results revealed that isothermal fatigue exhibited a longer lifetime than both types of TMF at corresponding mechanical strain amplitude in spite of its higher fraction of plastic strain amplitude.This is because the interaction of different deformation mechanisms in high-temperature half cycle and low-temperature half cycle can cause a much severer damage in TMF than in 900℃IF.Moreover,there was a crossover of the IP and OP mechanical strain-life curves.IP fatigue lives were shorter than OP lives at high mechanical strain amplitudes due to creep damage occurred;while IP lives were greater than OP fatigue lives at low mechanical strain amplitudes,which is because the additional damage caused by the tensile mean stress and the oxidation-fatigue interaction in OP.Cracks initiated intergranularly at high mechanical strain amplitude and interdendritically at low mechanical strain amplitude in IP TMF,while totally transgranular cracking occurred in OP TMF.The Zamrik model and Miller model,which incorporated elastic strain and plastic strain together as damage parameters,can give satisfactory thermomechanical fatigue life prediction by using IF data,while the Ostergren model,which only incorporated plastic strain as TMF damage parameter failed to predict TMF lives.In this paper,a total energy model was proposed to predict TMF lives,result revealed that this model successfully solved the problem that the TMF plastic strain range is so small for high-strength low-ductility superalloys that large errors arise when attempting to either measure it experimentally or predict the TMF life analytically by using the plastic strain range,and a satisfactory prediction result was achieved by using this total energy model.Since the hot-section components are commonly coated with MCrAlY overlay coatings and thermal barrier coatings consisting of a ceramic topcoat and a MCrAlY bond coat in real applications,it is very important to understand the effect of MCrAlY coatings on fatigue property of components.Therefore,two types of spraying techniques were employed to deposit a MCrAlY overlay coating on superalloy M963,i.e.air plasma spray(APS) and high velocity oxyfuel(HVOF).The cracking mechanisms of MCrAlY coatings and the effect of MCrAIY coatings on failure mechanisms and fatigue life of M963 under 900℃IF and 450-900℃TMF were investigated.Furthermore,comparison of the two types of coating was also made.Investigations on the effect of MCrAIY coating on failure mechanisms and fatigue lives of M963 under 900℃IF and IP,OP TMF showed that the coating is like a ductile metal in 900℃IF condition,the coating cracks gradually initiated at the surface imperfections by fatigue and oxidation interactions,but the failure of specimen was not caused by coating cracks,therefore the coating had a beneficial effect on fatigue life;In IP TMF condition,the coating also exhibited a beneficial effect on fatigue life,no crack was found in coating after tests,the failure was caused by coating/substrate interface defects;While under OP TMF conditions,brittle cracking of MCrAlY coating occurred in the first five cycles,and then the cracks penetrated to the coating/substrate interface directly.Early cracking of coating under OP TMF conditions accelerated the crack initiation process in superalloy and thus lead to a reduced fatigue life.In addition,it was observed that the crack density in HVOF coating was smaller than that in APS coating,and there existed many small cracks that did not reach the coating/substrate interface in HVOF coating,this is because the HVOF MCrA1Y coating has a lower porosity and a better cracking resistance than the APS one.