The Effect Of Substrate Surface Treatment On Cyclic Oxidation Behavior Of Single Crystal Superalloy And Its Nanocrystalline Coating

In this thesis,nickel based single crystal superalloy N5 was polished,sand blasted and shot peened as the substrate,and nanocrystalline coating was prepared on the substrate surface by magnetron sputtering.The effect of surface treatment on the cyclic oxidation behavior of nickel based single crystal superalloy and its nanocrystalline coating at 1100℃was studied.The phase composition,microstructure and chemical composition of the samples before and after oxidation were characterized by X-ray diffraction（XRD）,scanning electron microscope（SEM）,X-ray energy spectrometer（EDS）and electron backscatter diffraction（EBSD）.The main research results are summarized as follows:The surface condition of N5 single crystal superalloy would not be changed by polishing,while the formation of small angle grain boundaries of N5 single crystal superalloy was promoted by sand blasting and shot peening.The recrystallization was observed beneath the oxide film of sand blasted and shot peened superalloy at high temperature.Ni O,Ni Al₂O₄,Ta O₂ and Al₂O₃ were the main oxides of polished N5 single crystal alloy after cycle oxidation at 1100℃.With the progress of oxidation experiment,Ni O on the alloy surface disappeared,and the oxide film was divided into three layers.The outer layer is Ni Al₂O₄ layer,the middle layer is（Ni Al₂O₄+Al₂O₃）mixed oxide layer,and the inner layer is Al₂O₃ layer.There were a large number of Ta rich oxides in the mixed oxide layer.The surface of sand blasted superalloy was rough and the oxidation morphology was complex.Ni Al₂O₄,Ta O₂ and Al₂O₃were main oxides of sand blasted N5 single crystal after high temperature oxidation.The oxide film was divided into two layers,the outer layer is discontinuous Ni Al₂O₄ layer and the inner layer was Al₂O₃ layer.The oxidation weight gain of shot peened N5 single crystal alloy was the least.Al₂O₃ were only oxide of sand blasted N5 single crystal after high temperature oxidation,and the oxide film gradually became uniform with the progress of oxidation experiment.Theγ?phase depletion region was formed under the oxide film of polished superalloy,while the formation of newγ?phase was observed under the oxide film of shot peened superalloy after cyclic oxidation.Nanocrystalline coatings were prepared on polished and shot peened nickel-based single crystal superalloy substrates by magnetron sputtering.The singleγsolid solution phase was the main phase constitution of nanocrystalline coatings,which gradually changed toγ/γ?phase at 1100℃.The cyclic oxidation kinetics of both of nanocrystalline coatings at 1100℃was similar,a Al₂O₃oxide film could be formed on the surface of nanocrystalline coatings.Both of nanocrystalline coatings showed excellent oxidation resistance and spallation resistance at1100℃.However,the microstructure evolution of the interface between nanocrystalline coating and shot peened superalloy substrate was different from that between nanocrystalline coating and polished superalloy substrate.During high temperature oxidation,the sustained formation ofγ?phase was observed in the nanocrystalline coating near the polished substrate/coating interface,while theγ?phase continued to grow and extend into the superalloy substrate near the shot peened substrate/coating interface.