| In this thesis,the high temperature protective coatings for the fourth-generation single crystal superalloy were investigated including the study of the high cycle fatigue behaviour of the superalloy,the influence of the MCrAlY overlay coating and the PtAl diffusion coating on the oxidation resistance of the alloy,the high cycle fatigue behaviour of the superalloy with two kinds of coatings,and the repair effect of the rejuvenation heat treatment on the damaged superalloy.The effects of the coatings on the microstructure,element distribution and high cycle fatigue behaviour of the fourth-generation single crystal superalloy under thermo-mechanical were studied systematically.The isothermal oxidation experiments of PtAl diffusion coating and two kinds of MCrAlY overlay coatings were carried out at 1140?/200h.The results show that both PtAl coating and MCrAlY coating show excellent oxidation resistance.After 200 hours,the order of oxidation resistance from high to low is PtAl coating,NiCoCrAlYSiHf coating,NiCoCrAlYSi coating.The structure stability from high to low is NiCoCrAlYSi coating,PtAl coating,NiCoCrAlYSiHf coating.In the specimens with PtAl coating,the Al content is relatively high,so it has better oxidation resistance.However,excessive Al element will lead to more serious interdiffusion between the coating and the substrate,and a larger number of TCP phases will form in the samples.The high temperature oxidation behaviour and interdiffusion behaviour of two kinds of MCrAlY coatings are affected by its composition content.The addition of Hf element improves the adhesion between oxide layer and coating,thus improving the oxidation resistance of NiCoCrAlYSiHf coating.However,because NiCoCrAlYSiHf contains more Cr element,a large amount of TCP phase precipitates in the substrate of NiCoCrAlYSiHf sample.In addition,in the case of the same Al content,the difference of diffusion flux of Al elements will affect the volume fraction of ?' phase in the substrate,the solubility of refractory elements in ? phase,and the precipitation tendency of TCP phase.High cycle fatigue tests of single crystal superalloy samples were carried out at 760? and 900?.The results show that temperature,stress amplitude and sample thickness have little effect on the crack propagation behaviour of single crystal alloy samples.The cracks mainly propagate along[111]plane,forming a cross section with 40-60°to the stress axis,while the initiation mechanism of crack is different at different temperatures and stress amplitudes.At 900 ?,when the stress amplitude is low,the experimental time is longer,the surface of the substrate is oxidized seriously and the strength of the local area decreases due to stress concentration and finally leads to the fracture of the sample;when the stress amplitude is high,under the action of stress,the slip lines are easy to be formed inside the sample and the slip step formed after the slip lines move to the surface of the sample.The slip step is the main area of crack source initiation.In addition,the destroyed ?/?' structures under high stress amplitudes are also easy to produce fatigue cracks.When the temperature drops to 760?,the fatigue strength of the sample decreases,which is related to the lower strength of the alloy at low temperature.At 760?,oxidation occurs only at the fracture surface and the oxidation on the surface of the other place of the sample is lighter.The formation of crack initiation is related to the formation of slip steps on the sample surface.High cycle fatigue tests were carried out on the samples with NiCoCrAlYSi coating,NiCoCrAlYHfcoating and PtAl coating.The results show that three kinds of coatings slightly reduce the high cycle fatigue properties of the alloy,mainly due to the formation of cracks on the surface of the coating at the beginning of the fatigue cycle.After the crack propagates to the substrate,the substrate is oxidized,which leads the decrease of local strength,the formation of the stress concentration,the formation of slip lines and secondary cracks along the direction of slip bands.The secondary crack propagation leads to fracture of the sample along[111]surface.The crack propagation behaviour of the coating sample is similar to that of the alloy sample.This means that the application of the coating will only affect the initiation mechanism of the fatigue crack,reduce the fatigue properties of the sample,but have little effect on the crack propagation behaviour.The increase of the thickness of the NiCoCrAlYSi sample will reduce the proportion of the coating affected zone on the sample,improving the high cycle fatigue performance of the sample.With the increase of the experimental temperature,the fatigue performance of NiCoCrAlYHfsamples increases,because the substrate strength increases with the increase of temperature.Compared with the MCrAl Y sample,the oxidation between the coating and the substrate of the PtAl sample is more serious at both 760? and 900?,the oxide will arch the coating,and the slip lines and secondary crack are also formed in the substrate,which leads to the fracture failure of the sample.The similar degree of oxidation occurred in the samples at the two temperatures is the main reason why the temperature has little effect on the PtAl coating sample.The rejuvenation heat treatment of single crystal superalloy after creep damage was carried out.The results show that after the rejuvenation heat treatment,the damaged ?' phase can be restored to the standard cubic shape,the dislocation network in the substrate can be eliminated,the creep properties of the samples can be restored to the initial state,and the total creep life of the sample is improved.The rejuvenation heat treatment can flatten the recrystallization produced during creep.However,the recrystallized grain size is small,so it has little effect on the properties of the alloy.In addition,the morphology of ?' phase in the sample is only related to the content of Al element and is not affected by recrystallization. |
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